hsSyn/HsExpr.hs

    {-
    (c) The University of Glasgow 2006
    (c) The GRASP/AQUA Project, Glasgow University, 1992-1998
    -}

    {-# LANGUAGE CPP, DeriveDataTypeable, ScopedTypeVariables #-}
    {-# LANGUAGE StandaloneDeriving #-}
    {-# LANGUAGE FlexibleContexts #-}
    {-# LANGUAGE UndecidableInstances #-} -- Note [Pass sensitive types]
                                          -- in module PlaceHolder
    {-# LANGUAGE ConstraintKinds #-}
    {-# LANGUAGE ExistentialQuantification #-}
    {-# LANGUAGE DeriveFunctor #-}

    -- | Abstract Haskell syntax for expressions.
    module HsExpr where

    #include "HsVersions.h"

    -- friends:
    import HsDecls
    import HsPat
    import HsLit
    import PlaceHolder ( PostTc,PostRn,DataId,DataIdPost,
                         NameOrRdrName,OutputableBndrId )
    import HsTypes
    import HsBinds

    -- others:
    import TcEvidence
    import CoreSyn
    import Var
    import DynFlags ( gopt, GeneralFlag(Opt_PrintExplicitCoercions) )
    import Name
    import NameSet
    import RdrName  ( GlobalRdrEnv )
    import BasicTypes
    import ConLike
    import SrcLoc
    import Util
    import Outputable
    import FastString
    import Type

    -- libraries:
    import Data.Data hiding (Fixity(..))
    import qualified Data.Data as Data (Fixity(..))
    import Data.Maybe (isNothing)

    import GHCi.RemoteTypes ( ForeignRef )
    import qualified Language.Haskell.TH as TH (Q)

    {-
    ************************************************************************
    *                                                                      *
    \subsection{Expressions proper}
    *                                                                      *
    ************************************************************************
    -}

    -- * Expressions proper

    -- | Located Haskell Expression
    type LHsExpr id = Located (HsExpr id)
      -- ^ May have 'ApiAnnotation.AnnKeywordId' : 'ApiAnnotation.AnnComma' when
      --   in a list

      -- For details on above see note [Api annotations] in ApiAnnotation

    -------------------------
    -- | Post-Type checking Expression
    --
    -- PostTcExpr is an evidence expression attached to the syntax tree by the
    -- type checker (c.f. postTcType).
    type PostTcExpr  = HsExpr Id

    -- | Post-Type checking Table
    --
    -- We use a PostTcTable where there are a bunch of pieces of evidence, more
    -- than is convenient to keep individually.
    type PostTcTable = [(Name, PostTcExpr)]

    noPostTcExpr :: PostTcExpr
    noPostTcExpr = HsLit (HsString NoSourceText (fsLit "noPostTcExpr"))

    noPostTcTable :: PostTcTable
    noPostTcTable = []

    -------------------------
    -- | Syntax Expression
    --
    -- SyntaxExpr is like 'PostTcExpr', but it's filled in a little earlier,
    -- by the renamer.  It's used for rebindable syntax.
    --
    -- E.g. @(>>=)@ is filled in before the renamer by the appropriate 'Name' for
    --      @(>>=)@, and then instantiated by the type checker with its type args
    --      etc
    --
    -- This should desugar to
    --
    -- > syn_res_wrap $ syn_expr (syn_arg_wraps[0] arg0)
    -- >                         (syn_arg_wraps[1] arg1) ...
    --
    -- where the actual arguments come from elsewhere in the AST.
    -- This could be defined using @PostRn@ and @PostTc@ and such, but it's
    -- harder to get it all to work out that way. ('noSyntaxExpr' is hard to
    -- write, for example.)
    data SyntaxExpr id = SyntaxExpr { syn_expr      :: HsExpr id
                                    , syn_arg_wraps :: [HsWrapper]
                                    , syn_res_wrap  :: HsWrapper }
    deriving instance (DataId id) => Data (SyntaxExpr id)

    -- | This is used for rebindable-syntax pieces that are too polymorphic
    -- for tcSyntaxOp (trS_fmap and the mzip in ParStmt)
    noExpr :: HsExpr id
    noExpr = HsLit (HsString (SourceText  "noExpr") (fsLit "noExpr"))

    noSyntaxExpr :: SyntaxExpr id -- Before renaming, and sometimes after,
                                  -- (if the syntax slot makes no sense)
    noSyntaxExpr = SyntaxExpr { syn_expr      = HsLit (HsString NoSourceText
                                                            (fsLit "noSyntaxExpr"))
                              , syn_arg_wraps = []
                              , syn_res_wrap  = WpHole }

    -- | Make a 'SyntaxExpr Name' (the "rn" is because this is used in the
    -- renamer), missing its HsWrappers.
    mkRnSyntaxExpr :: Name -> SyntaxExpr Name
    mkRnSyntaxExpr name = SyntaxExpr { syn_expr      = HsVar $ noLoc name
                                     , syn_arg_wraps = []
                                     , syn_res_wrap  = WpHole }
      -- don't care about filling in syn_arg_wraps because we're clearly
      -- not past the typechecker

    instance (OutputableBndrId id) => Outputable (SyntaxExpr id) where
      ppr (SyntaxExpr { syn_expr      = expr
                      , syn_arg_wraps = arg_wraps
                      , syn_res_wrap  = res_wrap })
        = sdocWithDynFlags $ \ dflags ->
          getPprStyle $ \s ->
          if debugStyle s || gopt Opt_PrintExplicitCoercions dflags
          then ppr expr <> braces (pprWithCommas ppr arg_wraps)
                        <> braces (ppr res_wrap)
          else ppr expr

    -- | Command Syntax Table (for Arrow syntax)
    type CmdSyntaxTable id = [(Name, HsExpr id)]
    -- See Note [CmdSyntaxTable]

    {-
    Note [CmdSyntaxtable]
    ~~~~~~~~~~~~~~~~~~~~~
    Used only for arrow-syntax stuff (HsCmdTop), the CmdSyntaxTable keeps
    track of the methods needed for a Cmd.

    * Before the renamer, this list is an empty list

    * After the renamer, it takes the form @[(std_name, HsVar actual_name)]@
      For example, for the 'arr' method
       * normal case:            (GHC.Control.Arrow.arr, HsVar GHC.Control.Arrow.arr)
       * with rebindable syntax: (GHC.Control.Arrow.arr, arr_22)
                 where @arr_22@ is whatever 'arr' is in scope

    * After the type checker, it takes the form [(std_name, <expression>)]
      where <expression> is the evidence for the method.  This evidence is
      instantiated with the class, but is still polymorphic in everything
      else.  For example, in the case of 'arr', the evidence has type
             forall b c. (b->c) -> a b c
      where 'a' is the ambient type of the arrow.  This polymorphism is
      important because the desugarer uses the same evidence at multiple
      different types.

    This is Less Cool than what we normally do for rebindable syntax, which is to
    make fully-instantiated piece of evidence at every use site.  The Cmd way
    is Less Cool because
      * The renamer has to predict which methods are needed.
        See the tedious RnExpr.methodNamesCmd.

      * The desugarer has to know the polymorphic type of the instantiated
        method. This is checked by Inst.tcSyntaxName, but is less flexible
        than the rest of rebindable syntax, where the type is less
        pre-ordained.  (And this flexibility is useful; for example we can
        typecheck do-notation with (>>=) :: m1 a -> (a -> m2 b) -> m2 b.)
    -}

    -- | An unbound variable; used for treating out-of-scope variables as
    -- expression holes
    data UnboundVar
      = OutOfScope OccName GlobalRdrEnv  -- ^ An (unqualified) out-of-scope
                                         -- variable, together with the GlobalRdrEnv
                                         -- with respect to which it is unbound

                                         -- See Note [OutOfScope and GlobalRdrEnv]

      | TrueExprHole OccName             -- ^ A "true" expression hole (_ or _x)

      deriving Data

    instance Outputable UnboundVar where
        ppr = ppr . unboundVarOcc

    unboundVarOcc :: UnboundVar -> OccName
    unboundVarOcc (OutOfScope occ _) = occ
    unboundVarOcc (TrueExprHole occ) = occ

    {-
    Note [OutOfScope and GlobalRdrEnv]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    To understand why we bundle a GlobalRdrEnv with an out-of-scope variable,
    consider the following module:

        module A where

        foo :: ()
        foo = bar

        bat :: [Double]
        bat = [1.2, 3.4]

        $(return [])

        bar = ()
        bad = False

    When A is compiled, the renamer determines that `bar` is not in scope in the
    declaration of `foo` (since `bar` is declared in the following inter-splice
    group).  Once it has finished typechecking the entire module, the typechecker
    then generates the associated error message, which specifies both the type of
    `bar` and a list of possible in-scope alternatives:

        A.hs:6:7: error:
            • Variable not in scope: bar :: ()
            • ‘bar’ (line 13) is not in scope before the splice on line 11
              Perhaps you meant ‘bat’ (line 9)

    When it calls RnEnv.unknownNameSuggestions to identify these alternatives, the
    typechecker must provide a GlobalRdrEnv.  If it provided the current one, which
    contains top-level declarations for the entire module, the error message would
    incorrectly suggest the out-of-scope `bar` and `bad` as possible alternatives
    for `bar` (see Trac #11680).  Instead, the typechecker must use the same
    GlobalRdrEnv the renamer used when it determined that `bar` is out-of-scope.

    To obtain this GlobalRdrEnv, can the typechecker simply use the out-of-scope
    `bar`'s location to either reconstruct it (from the current GlobalRdrEnv) or to
    look it up in some global store?  Unfortunately, no.  The problem is that
    location information is not always sufficient for this task.  This is most
    apparent when dealing with the TH function addTopDecls, which adds its
    declarations to the FOLLOWING inter-splice group.  Consider these declarations:

        ex9 = cat               -- cat is NOT in scope here

        $(do -------------------------------------------------------------
            ds <- [d| f = cab   -- cat and cap are both in scope here
                      cat = ()
                    |]
            addTopDecls ds
            [d| g = cab         -- only cap is in scope here
                cap = True
              |])

        ex10 = cat              -- cat is NOT in scope here

        $(return []) -----------------------------------------------------

        ex11 = cat              -- cat is in scope

    Here, both occurrences of `cab` are out-of-scope, and so the typechecker needs
    the GlobalRdrEnvs which were used when they were renamed.  These GlobalRdrEnvs
    are different (`cat` is present only in the GlobalRdrEnv for f's `cab'), but the
    locations of the two `cab`s are the same (they are both created in the same
    splice).  Thus, we must include some additional information with each `cab` to
    allow the typechecker to obtain the correct GlobalRdrEnv.  Clearly, the simplest
    information to use is the GlobalRdrEnv itself.
    -}

    -- | A Haskell expression.
    data HsExpr id
      = HsVar     (Located id)   -- ^ Variable

                                 -- See Note [Located RdrNames]

      | HsUnboundVar UnboundVar  -- ^ Unbound variable; also used for "holes"
                                 --   (_ or _x).
                                 -- Turned from HsVar to HsUnboundVar by the
                                 --   renamer, when it finds an out-of-scope
                                 --   variable or hole.
                                 -- Turned into HsVar by type checker, to support
                                 --   deferred type errors.

      | HsConLikeOut ConLike     -- ^ After typechecker only; must be different
                                 -- HsVar for pretty printing

      | HsRecFld (AmbiguousFieldOcc id) -- ^ Variable pointing to record selector
                                        -- Not in use after typechecking

      | HsOverLabel (Maybe id) FastString
         -- ^ Overloaded label (Note [Overloaded labels] in GHC.OverloadedLabels)
         --   @Just id@ means @RebindableSyntax@ is in use, and gives the id of the
         --   in-scope 'fromLabel'.
         --   NB: Not in use after typechecking

      | HsIPVar   HsIPName       -- ^ Implicit parameter (not in use after typechecking)
      | HsOverLit (HsOverLit id) -- ^ Overloaded literals

      | HsLit     HsLit          -- ^ Simple (non-overloaded) literals

      | HsLam     (MatchGroup id (LHsExpr id)) -- ^ Lambda abstraction. Currently always a single match
           --
           -- - 'ApiAnnotation.AnnKeywordId' : 'ApiAnnotation.AnnLam',
           --       'ApiAnnotation.AnnRarrow',

           -- For details on above see note [Api annotations] in ApiAnnotation

      | HsLamCase (MatchGroup id (LHsExpr id)) -- ^ Lambda-case
           --
           -- - 'ApiAnnotation.AnnKeywordId' : 'ApiAnnotation.AnnLam',
           --           'ApiAnnotation.AnnCase','ApiAnnotation.AnnOpen',
           --           'ApiAnnotation.AnnClose'

           -- For details on above see note [Api annotations] in ApiAnnotation

      | HsApp     (LHsExpr id) (LHsExpr id) -- ^ Application

      | HsAppType (LHsExpr id) (LHsWcType id) -- ^ Visible type application
           --
           -- Explicit type argument; e.g  f @Int x y
           -- NB: Has wildcards, but no implicit quantification
           --
           -- - 'ApiAnnotation.AnnKeywordId' : 'ApiAnnotation.AnnAt',

      | HsAppTypeOut (LHsExpr id) (LHsWcType Name) -- just for pretty-printing

      -- | Operator applications:
      -- NB Bracketed ops such as (+) come out as Vars.

      -- NB We need an expr for the operator in an OpApp/Section since
      -- the typechecker may need to apply the operator to a few types.

      | OpApp       (LHsExpr id)    -- left operand
                    (LHsExpr id)    -- operator
                    (PostRn id Fixity) -- Renamer adds fixity; bottom until then
                    (LHsExpr id)    -- right operand

      -- | Negation operator. Contains the negated expression and the name
      -- of 'negate'
      --
      --  - 'ApiAnnotation.AnnKeywordId' : 'ApiAnnotation.AnnMinus'

      -- For details on above see note [Api annotations] in ApiAnnotation
      | NegApp      (LHsExpr id)
                    (SyntaxExpr id)

      -- | - 'ApiAnnotation.AnnKeywordId' : 'ApiAnnotation.AnnOpen' @'('@,
      --             'ApiAnnotation.AnnClose' @')'@

      -- For details on above see note [Api annotations] in ApiAnnotation
      | HsPar       (LHsExpr id)    -- ^ Parenthesised expr; see Note [Parens in HsSyn]

      | SectionL    (LHsExpr id)    -- operand; see Note [Sections in HsSyn]
                    (LHsExpr id)    -- operator
      | SectionR    (LHsExpr id)    -- operator; see Note [Sections in HsSyn]
                    (LHsExpr id)    -- operand

      -- | Used for explicit tuples and sections thereof
      --
      --  - 'ApiAnnotation.AnnKeywordId' : 'ApiAnnotation.AnnOpen',
      --         'ApiAnnotation.AnnClose'

      -- For details on above see note [Api annotations] in ApiAnnotation
      | ExplicitTuple
            [LHsTupArg id]
            Boxity

      -- | Used for unboxed sum types
      --
      --  - 'ApiAnnotation.AnnKeywordId' : 'ApiAnnotation.AnnOpen' @'(#'@,
      --          'ApiAnnotation.AnnVbar', 'ApiAnnotation.AnnClose' @'#)'@,
      --
      --  There will be multiple 'ApiAnnotation.AnnVbar', (1 - alternative) before
      --  the expression, (arity - alternative) after it
      | ExplicitSum
              ConTag --  Alternative (one-based)
              Arity  --  Sum arity
              (LHsExpr id)
              (PostTc id [Type])   -- the type arguments

      -- | - 'ApiAnnotation.AnnKeywordId' : 'ApiAnnotation.AnnCase',
      --       'ApiAnnotation.AnnOf','ApiAnnotation.AnnOpen' @'{'@,
      --       'ApiAnnotation.AnnClose' @'}'@

      -- For details on above see note [Api annotations] in ApiAnnotation
      | HsCase      (LHsExpr id)
                    (MatchGroup id (LHsExpr id))

      -- | - 'ApiAnnotation.AnnKeywordId' : 'ApiAnnotation.AnnIf',
      --       'ApiAnnotation.AnnSemi',
      --       'ApiAnnotation.AnnThen','ApiAnnotation.AnnSemi',
      --       'ApiAnnotation.AnnElse',

      -- For details on above see note [Api annotations] in ApiAnnotation
      | HsIf        (Maybe (SyntaxExpr id)) -- cond function
                                            -- Nothing => use the built-in 'if'
                                            -- See Note [Rebindable if]
                    (LHsExpr id)    --  predicate
                    (LHsExpr id)    --  then part
                    (LHsExpr id)    --  else part

      -- | Multi-way if
      --
      -- - 'ApiAnnotation.AnnKeywordId' : 'ApiAnnotation.AnnIf'
      --       'ApiAnnotation.AnnOpen','ApiAnnotation.AnnClose',

      -- For details on above see note [Api annotations] in ApiAnnotation
      | HsMultiIf   (PostTc id Type) [LGRHS id (LHsExpr id)]

      -- | let(rec)
      --
      -- - 'ApiAnnotation.AnnKeywordId' : 'ApiAnnotation.AnnLet',
      --       'ApiAnnotation.AnnOpen' @'{'@,
      --       'ApiAnnotation.AnnClose' @'}'@,'ApiAnnotation.AnnIn'

      -- For details on above see note [Api annotations] in ApiAnnotation
      | HsLet       (LHsLocalBinds id)
                    (LHsExpr  id)

      -- | - 'ApiAnnotation.AnnKeywordId' : 'ApiAnnotation.AnnDo',
      --             'ApiAnnotation.AnnOpen', 'ApiAnnotation.AnnSemi',
      --             'ApiAnnotation.AnnVbar',
      --             'ApiAnnotation.AnnClose'

      -- For details on above see note [Api annotations] in ApiAnnotation
      | HsDo        (HsStmtContext Name)     -- The parameterisation is unimportant
                                             -- because in this context we never use
                                             -- the PatGuard or ParStmt variant
                    (Located [ExprLStmt id]) -- "do":one or more stmts
                    (PostTc id Type)         -- Type of the whole expression

      -- | Syntactic list: [a,b,c,...]
      --
      --  - 'ApiAnnotation.AnnKeywordId' : 'ApiAnnotation.AnnOpen' @'['@,
      --              'ApiAnnotation.AnnClose' @']'@

      -- For details on above see note [Api annotations] in ApiAnnotation
      | ExplicitList
                    (PostTc id Type)        -- Gives type of components of list
                    (Maybe (SyntaxExpr id)) -- For OverloadedLists, the fromListN witness
                    [LHsExpr id]

      -- | Syntactic parallel array: [:e1, ..., en:]
      --
      --  - 'ApiAnnotation.AnnKeywordId' : 'ApiAnnotation.AnnOpen' @'[:'@,
      --              'ApiAnnotation.AnnDotdot','ApiAnnotation.AnnComma',
      --              'ApiAnnotation.AnnVbar'
      --              'ApiAnnotation.AnnClose' @':]'@

      -- For details on above see note [Api annotations] in ApiAnnotation
      | ExplicitPArr
                    (PostTc id Type)   -- type of elements of the parallel array
                    [LHsExpr id]

      -- | Record construction
      --
      --  - 'ApiAnnotation.AnnKeywordId' : 'ApiAnnotation.AnnOpen' @'{'@,
      --         'ApiAnnotation.AnnDotdot','ApiAnnotation.AnnClose' @'}'@

      -- For details on above see note [Api annotations] in ApiAnnotation
      | RecordCon
          { rcon_con_name :: Located id         -- The constructor name;
                                                --  not used after type checking
          , rcon_con_like :: PostTc id ConLike  -- The data constructor or pattern synonym
          , rcon_con_expr :: PostTcExpr         -- Instantiated constructor function
          , rcon_flds     :: HsRecordBinds id } -- The fields

      -- | Record update
      --
      --  - 'ApiAnnotation.AnnKeywordId' : 'ApiAnnotation.AnnOpen' @'{'@,
      --         'ApiAnnotation.AnnDotdot','ApiAnnotation.AnnClose' @'}'@

      -- For details on above see note [Api annotations] in ApiAnnotation
      | RecordUpd
          { rupd_expr :: LHsExpr id
          , rupd_flds :: [LHsRecUpdField id]
          , rupd_cons :: PostTc id [ConLike]
                    -- Filled in by the type checker to the
                    -- _non-empty_ list of DataCons that have
                    -- all the upd'd fields

          , rupd_in_tys  :: PostTc id [Type]  -- Argument types of *input* record type
          , rupd_out_tys :: PostTc id [Type]  --              and  *output* record type
                                              -- The original type can be reconstructed
                                              -- with conLikeResTy
          , rupd_wrap :: PostTc id HsWrapper  -- See note [Record Update HsWrapper]
          }
      -- For a type family, the arg types are of the *instance* tycon,
      -- not the family tycon

      -- | Expression with an explicit type signature. @e :: type@
      --
      --  - 'ApiAnnotation.AnnKeywordId' : 'ApiAnnotation.AnnDcolon'

      -- For details on above see note [Api annotations] in ApiAnnotation
      | ExprWithTySig
                    (LHsExpr id)
                    (LHsSigWcType id)

      | ExprWithTySigOut              -- Post typechecking
                    (LHsExpr id)
                    (LHsSigWcType Name)  -- Retain the signature,
                                         -- as HsSigType Name, for
                                         -- round-tripping purposes

      -- | Arithmetic sequence
      --
      --  - 'ApiAnnotation.AnnKeywordId' : 'ApiAnnotation.AnnOpen' @'['@,
      --              'ApiAnnotation.AnnComma','ApiAnnotation.AnnDotdot',
      --              'ApiAnnotation.AnnClose' @']'@

      -- For details on above see note [Api annotations] in ApiAnnotation
      | ArithSeq
                    PostTcExpr
                    (Maybe (SyntaxExpr id))   -- For OverloadedLists, the fromList witness
                    (ArithSeqInfo id)

      -- | Arithmetic sequence for parallel array
      --
      -- > [:e1..e2:] or [:e1, e2..e3:]
      --
      --  - 'ApiAnnotation.AnnKeywordId' : 'ApiAnnotation.AnnOpen' @'[:'@,
      --              'ApiAnnotation.AnnComma','ApiAnnotation.AnnDotdot',
      --              'ApiAnnotation.AnnVbar',
      --              'ApiAnnotation.AnnClose' @':]'@

      -- For details on above see note [Api annotations] in ApiAnnotation
      | PArrSeq
                    PostTcExpr
                    (ArithSeqInfo id)

      -- | - 'ApiAnnotation.AnnKeywordId' : 'ApiAnnotation.AnnOpen' @'{-\# SCC'@,
      --             'ApiAnnotation.AnnVal' or 'ApiAnnotation.AnnValStr',
      --              'ApiAnnotation.AnnClose' @'\#-}'@

      -- For details on above see note [Api annotations] in ApiAnnotation
      | HsSCC       SourceText            -- Note [Pragma source text] in BasicTypes
                    StringLiteral         -- "set cost centre" SCC pragma
                    (LHsExpr id)          -- expr whose cost is to be measured

      -- | - 'ApiAnnotation.AnnKeywordId' : 'ApiAnnotation.AnnOpen' @'{-\# CORE'@,
      --             'ApiAnnotation.AnnVal', 'ApiAnnotation.AnnClose' @'\#-}'@

      -- For details on above see note [Api annotations] in ApiAnnotation
      | HsCoreAnn   SourceText            -- Note [Pragma source text] in BasicTypes
                    StringLiteral         -- hdaume: core annotation
                    (LHsExpr id)

      -----------------------------------------------------------
      -- MetaHaskell Extensions

      -- | - 'ApiAnnotation.AnnKeywordId' : 'ApiAnnotation.AnnOpen',
      --         'ApiAnnotation.AnnOpenE','ApiAnnotation.AnnOpenEQ',
      --         'ApiAnnotation.AnnClose','ApiAnnotation.AnnCloseQ'

      -- For details on above see note [Api annotations] in ApiAnnotation
      | HsBracket    (HsBracket id)

        -- See Note [Pending Splices]
      | HsRnBracketOut
          (HsBracket Name)     -- Output of the renamer is the *original* renamed
                               -- expression, plus
          [PendingRnSplice]    -- _renamed_ splices to be type checked

      | HsTcBracketOut
          (HsBracket Name)     -- Output of the type checker is the *original*
                               -- renamed expression, plus
          [PendingTcSplice]    -- _typechecked_ splices to be
                               -- pasted back in by the desugarer

      -- | - 'ApiAnnotation.AnnKeywordId' : 'ApiAnnotation.AnnOpen',
      --         'ApiAnnotation.AnnClose'

      -- For details on above see note [Api annotations] in ApiAnnotation
      | HsSpliceE  (HsSplice id)

      -----------------------------------------------------------
      -- Arrow notation extension

      -- | @proc@ notation for Arrows
      --
      --  - 'ApiAnnotation.AnnKeywordId' : 'ApiAnnotation.AnnProc',
      --          'ApiAnnotation.AnnRarrow'

      -- For details on above see note [Api annotations] in ApiAnnotation
      | HsProc      (LPat id)               -- arrow abstraction, proc
                    (LHsCmdTop id)          -- body of the abstraction
                                            -- always has an empty stack

      ---------------------------------------
      -- static pointers extension
      -- | - 'ApiAnnotation.AnnKeywordId' : 'ApiAnnotation.AnnStatic',

      -- For details on above see note [Api annotations] in ApiAnnotation
      | HsStatic (PostRn id NameSet) -- Free variables of the body
                 (LHsExpr id)        -- Body

      ---------------------------------------
      -- The following are commands, not expressions proper
      -- They are only used in the parsing stage and are removed
      --    immediately in parser.RdrHsSyn.checkCommand

      -- | - 'ApiAnnotation.AnnKeywordId' : 'ApiAnnotation.Annlarrowtail',
      --          'ApiAnnotation.Annrarrowtail','ApiAnnotation.AnnLarrowtail',
      --          'ApiAnnotation.AnnRarrowtail'

      -- For details on above see note [Api annotations] in ApiAnnotation
      | HsArrApp             -- Arrow tail, or arrow application (f -< arg)
            (LHsExpr id)     -- arrow expression, f
            (LHsExpr id)     -- input expression, arg
            (PostTc id Type) -- type of the arrow expressions f,
                             -- of the form a t t', where arg :: t
            HsArrAppType     -- higher-order (-<<) or first-order (-<)
            Bool             -- True => right-to-left (f -< arg)
                             -- False => left-to-right (arg >- f)

      -- | - 'ApiAnnotation.AnnKeywordId' : 'ApiAnnotation.AnnOpenB' @'(|'@,
      --         'ApiAnnotation.AnnCloseB' @'|)'@

      -- For details on above see note [Api annotations] in ApiAnnotation
      | HsArrForm            -- Command formation,  (| e cmd1 .. cmdn |)
            (LHsExpr id)     -- the operator
                             -- after type-checking, a type abstraction to be
                             -- applied to the type of the local environment tuple
            (Maybe Fixity)   -- fixity (filled in by the renamer), for forms that
                             -- were converted from OpApp's by the renamer
            [LHsCmdTop id]   -- argument commands

      ---------------------------------------
      -- Haskell program coverage (Hpc) Support

      | HsTick
         (Tickish id)
         (LHsExpr id)                       -- sub-expression

      | HsBinTick
         Int                                -- module-local tick number for True
         Int                                -- module-local tick number for False
         (LHsExpr id)                       -- sub-expression

      -- | - 'ApiAnnotation.AnnKeywordId' : 'ApiAnnotation.AnnOpen',
      --       'ApiAnnotation.AnnOpen' @'{-\# GENERATED'@,
      --       'ApiAnnotation.AnnVal','ApiAnnotation.AnnVal',
      --       'ApiAnnotation.AnnColon','ApiAnnotation.AnnVal',
      --       'ApiAnnotation.AnnMinus',
      --       'ApiAnnotation.AnnVal','ApiAnnotation.AnnColon',
      --       'ApiAnnotation.AnnVal',
      --       'ApiAnnotation.AnnClose' @'\#-}'@

      -- For details on above see note [Api annotations] in ApiAnnotation
      | HsTickPragma                      -- A pragma introduced tick
         SourceText                       -- Note [Pragma source text] in BasicTypes
         (StringLiteral,(Int,Int),(Int,Int))
                                          -- external span for this tick
         ((SourceText,SourceText),(SourceText,SourceText))
            -- Source text for the four integers used in the span.
            -- See note [Pragma source text] in BasicTypes
         (LHsExpr id)

      ---------------------------------------
      -- These constructors only appear temporarily in the parser.
      -- The renamer translates them into the Right Thing.

      | EWildPat                 -- wildcard

      -- | - 'ApiAnnotation.AnnKeywordId' : 'ApiAnnotation.AnnAt'

      -- For details on above see note [Api annotations] in ApiAnnotation
      | EAsPat      (Located id) -- as pattern
                    (LHsExpr id)

      -- | - 'ApiAnnotation.AnnKeywordId' : 'ApiAnnotation.AnnRarrow'

      -- For details on above see note [Api annotations] in ApiAnnotation
      | EViewPat    (LHsExpr id) -- view pattern
                    (LHsExpr id)

      -- | - 'ApiAnnotation.AnnKeywordId' : 'ApiAnnotation.AnnTilde'

      -- For details on above see note [Api annotations] in ApiAnnotation
      | ELazyPat    (LHsExpr id) -- ~ pattern

      ---------------------------------------
      -- Finally, HsWrap appears only in typechecker output

      |  HsWrap     HsWrapper    -- TRANSLATION
                    (HsExpr id)

    deriving instance (DataId id) => Data (HsExpr id)

    -- | Located Haskell Tuple Argument
    --
    -- 'HsTupArg' is used for tuple sections
    -- @(,a,)@ is represented by
    -- @ExplicitTuple [Missing ty1, Present a, Missing ty3]@
    -- Which in turn stands for @(\x:ty1 \y:ty2. (x,a,y))@
    type LHsTupArg id = Located (HsTupArg id)
    -- | - 'ApiAnnotation.AnnKeywordId' : 'ApiAnnotation.AnnComma'

    -- For details on above see note [Api annotations] in ApiAnnotation

    -- | Haskell Tuple Argument
    data HsTupArg id
      = Present (LHsExpr id)     -- ^ The argument
      | Missing (PostTc id Type) -- ^ The argument is missing, but this is its type
    deriving instance (DataId id) => Data (HsTupArg id)

    tupArgPresent :: LHsTupArg id -> Bool
    tupArgPresent (L _ (Present {})) = True
    tupArgPresent (L _ (Missing {})) = False

    {-
    Note [Parens in HsSyn]
    ~~~~~~~~~~~~~~~~~~~~~~
    HsPar (and ParPat in patterns, HsParTy in types) is used as follows

      * HsPar is required; the pretty printer does not add parens.

      * HsPars are respected when rearranging operator fixities.
        So   a * (b + c)  means what it says (where the parens are an HsPar)

      * For ParPat and HsParTy the pretty printer does add parens but this should be
        a no-op for ParsedSource, based on the pretty printer round trip feature
        introduced in
        https://phabricator.haskell.org/rGHC499e43824bda967546ebf95ee33ec1f84a114a7c

      * ParPat and HsParTy are pretty printed as '( .. )' regardless of whether or
        not they are strictly necssary. This should be addressed when #13238 is
        completed, to be treated the same as HsPar.

    Note [Sections in HsSyn]
    ~~~~~~~~~~~~~~~~~~~~~~~~
    Sections should always appear wrapped in an HsPar, thus
             HsPar (SectionR ...)
    The parser parses sections in a wider variety of situations
    (See Note [Parsing sections]), but the renamer checks for those
    parens.  This invariant makes pretty-printing easier; we don't need
    a special case for adding the parens round sections.

    Note [Rebindable if]
    ~~~~~~~~~~~~~~~~~~~~
    The rebindable syntax for 'if' is a bit special, because when
    rebindable syntax is *off* we do not want to treat
       (if c then t else e)
    as if it was an application (ifThenElse c t e).  Why not?
    Because we allow an 'if' to return *unboxed* results, thus
      if blah then 3# else 4#
    whereas that would not be possible using a all to a polymorphic function
    (because you can't call a polymorphic function at an unboxed type).

    So we use Nothing to mean "use the old built-in typing rule".

    Note [Record Update HsWrapper]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    There is a wrapper in RecordUpd which is used for the *required*
    constraints for pattern synonyms. This wrapper is created in the
    typechecking and is then directly used in the desugaring without
    modification.

    For example, if we have the record pattern synonym P,
      pattern P :: (Show a) => a -> Maybe a
      pattern P{x} = Just x

      foo = (Just True) { x = False }
    then `foo` desugars to something like
      foo = case Just True of
              P x -> P False
    hence we need to provide the correct dictionaries to P's matcher on
    the RHS so that we can build the expression.

    Note [Located RdrNames]
    ~~~~~~~~~~~~~~~~~~~~~~~
    A number of syntax elements have seemingly redundant locations attached to them.
    This is deliberate, to allow transformations making use of the API Annotations
    to easily correlate a Located Name in the RenamedSource with a Located RdrName
    in the ParsedSource.

    There are unfortunately enough differences between the ParsedSource and the
    RenamedSource that the API Annotations cannot be used directly with
    RenamedSource, so this allows a simple mapping to be used based on the location.
    -}

    instance (OutputableBndrId id) => Outputable (HsExpr id) where
        ppr expr = pprExpr expr

    -----------------------
    -- pprExpr, pprLExpr, pprBinds call pprDeeper;
    -- the underscore versions do not
    pprLExpr :: (OutputableBndrId id) => LHsExpr id -> SDoc
    pprLExpr (L _ e) = pprExpr e

    pprExpr :: (OutputableBndrId id) => HsExpr id -> SDoc
    pprExpr e | isAtomicHsExpr e || isQuietHsExpr e =            ppr_expr e
              | otherwise                           = pprDeeper (ppr_expr e)

    isQuietHsExpr :: HsExpr id -> Bool
    -- Parentheses do display something, but it gives little info and
    -- if we go deeper when we go inside them then we get ugly things
    -- like (...)
    isQuietHsExpr (HsPar _)          = True
    -- applications don't display anything themselves
    isQuietHsExpr (HsApp _ _)        = True
    isQuietHsExpr (HsAppType _ _)    = True
    isQuietHsExpr (HsAppTypeOut _ _) = True
    isQuietHsExpr (OpApp _ _ _ _)    = True
    isQuietHsExpr _ = False

    pprBinds :: (OutputableBndrId idL, OutputableBndrId idR)
             => HsLocalBindsLR idL idR -> SDoc
    pprBinds b = pprDeeper (ppr b)

    -----------------------
    ppr_lexpr :: (OutputableBndrId id) => LHsExpr id -> SDoc
    ppr_lexpr e = ppr_expr (unLoc e)

    ppr_expr :: forall id. (OutputableBndrId id) => HsExpr id -> SDoc
    ppr_expr (HsVar (L _ v))  = pprPrefixOcc v
    ppr_expr (HsUnboundVar uv)= pprPrefixOcc (unboundVarOcc uv)
    ppr_expr (HsConLikeOut c) = pprPrefixOcc c
    ppr_expr (HsIPVar v)      = ppr v
    ppr_expr (HsOverLabel _ l)= char '#' <> ppr l
    ppr_expr (HsLit lit)      = ppr lit
    ppr_expr (HsOverLit lit)  = ppr lit
    ppr_expr (HsPar e)        = parens (ppr_lexpr e)

    ppr_expr (HsCoreAnn stc (StringLiteral sta s) e)
      = vcat [pprWithSourceText stc (text "{-# CORE")
              <+> pprWithSourceText sta (doubleQuotes $ ftext s) <+> text "#-}"
             , ppr_lexpr e]

    ppr_expr e@(HsApp {})        = ppr_apps e []
    ppr_expr e@(HsAppType {})    = ppr_apps e []
    ppr_expr e@(HsAppTypeOut {}) = ppr_apps e []

    ppr_expr (OpApp e1 op _ e2)
      | Just pp_op <- should_print_infix (unLoc op)
      = pp_infixly pp_op
      | otherwise
      = pp_prefixly

      where
        should_print_infix (HsVar (L _ v)) = Just (pprInfixOcc v)
        should_print_infix (HsConLikeOut c)= Just (pprInfixOcc (conLikeName c))
        should_print_infix (HsRecFld f)    = Just (pprInfixOcc f)
        should_print_infix (HsUnboundVar h@TrueExprHole{})
                                           = Just (pprInfixOcc (unboundVarOcc h))
        should_print_infix EWildPat        = Just (text "`_`")
        should_print_infix (HsWrap _ e)    = should_print_infix e
        should_print_infix _               = Nothing

        pp_e1 = pprDebugParendExpr e1   -- In debug mode, add parens
        pp_e2 = pprDebugParendExpr e2   -- to make precedence clear

        pp_prefixly
          = hang (ppr op) 2 (sep [pp_e1, pp_e2])

        pp_infixly pp_op
          = hang pp_e1 2 (sep [pp_op, nest 2 pp_e2])

    ppr_expr (NegApp e _) = char '-' <+> pprDebugParendExpr e

    ppr_expr (SectionL expr op)
      = case unLoc op of
          HsVar (L _ v)  -> pp_infixly v
          HsConLikeOut c -> pp_infixly (conLikeName c)
          _              -> pp_prefixly
      where
        pp_expr = pprDebugParendExpr expr

        pp_prefixly = hang (hsep [text " \\ x_ ->", ppr op])
                           4 (hsep [pp_expr, text "x_ )"])
        pp_infixly v = (sep [pp_expr, pprInfixOcc v])

    ppr_expr (SectionR op expr)
      = case unLoc op of
          HsVar (L _ v)  -> pp_infixly v
          HsConLikeOut c -> pp_infixly (conLikeName c)
          _              -> pp_prefixly
      where
        pp_expr = pprDebugParendExpr expr

        pp_prefixly = hang (hsep [text "( \\ x_ ->", ppr op, text "x_"])
                           4 (pp_expr <> rparen)
        pp_infixly v = sep [pprInfixOcc v, pp_expr]

    ppr_expr (ExplicitTuple exprs boxity)
      = tupleParens (boxityTupleSort boxity) (fcat (ppr_tup_args $ map unLoc exprs))
      where
        ppr_tup_args []               = []
        ppr_tup_args (Present e : es) = (ppr_lexpr e <> punc es) : ppr_tup_args es
        ppr_tup_args (Missing _ : es) = punc es : ppr_tup_args es

        punc (Present {} : _) = comma <> space
        punc (Missing {} : _) = comma
        punc []               = empty

    ppr_expr (ExplicitSum alt arity expr _)
      = text "(#" <+> ppr_bars (alt - 1) <+> ppr expr <+> ppr_bars (arity - alt) <+> text "#)"
      where
        ppr_bars n = hsep (replicate n (char '|'))

    ppr_expr (HsLam matches)
      = pprMatches matches

    ppr_expr (HsLamCase matches)
      = sep [ sep [text "\\case"],
              nest 2 (pprMatches matches) ]

    ppr_expr (HsCase expr matches@(MG { mg_alts = L _ [_] }))
      = sep [ sep [text "case", nest 4 (ppr expr), ptext (sLit "of {")],
              nest 2 (pprMatches matches) <+> char '}']
    ppr_expr (HsCase expr matches)
      = sep [ sep [text "case", nest 4 (ppr expr), ptext (sLit "of")],
              nest 2 (pprMatches matches) ]

    ppr_expr (HsIf _ e1 e2 e3)
      = sep [hsep [text "if", nest 2 (ppr e1), ptext (sLit "then")],
             nest 4 (ppr e2),
             text "else",
             nest 4 (ppr e3)]

    ppr_expr (HsMultiIf _ alts)
      = hang (text "if") 3  (vcat (map ppr_alt alts))
      where ppr_alt (L _ (GRHS guards expr)) =
              hang vbar 2 (ppr_one one_alt)
              where
                ppr_one [] = panic "ppr_exp HsMultiIf"
                ppr_one (h:t) = hang h 2 (sep t)
                one_alt = [ interpp'SP guards
                          , text "->" <+> pprDeeper (ppr expr) ]

    -- special case: let ... in let ...
    ppr_expr (HsLet (L _ binds) expr@(L _ (HsLet _ _)))
      = sep [hang (text "let") 2 (hsep [pprBinds binds, ptext (sLit "in")]),
             ppr_lexpr expr]

    ppr_expr (HsLet (L _ binds) expr)
      = sep [hang (text "let") 2 (pprBinds binds),
             hang (text "in")  2 (ppr expr)]

    ppr_expr (HsDo do_or_list_comp (L _ stmts) _) = pprDo do_or_list_comp stmts

    ppr_expr (ExplicitList _ _ exprs)
      = brackets (pprDeeperList fsep (punctuate comma (map ppr_lexpr exprs)))

    ppr_expr (ExplicitPArr _ exprs)
      = paBrackets (pprDeeperList fsep (punctuate comma (map ppr_lexpr exprs)))

    ppr_expr (RecordCon { rcon_con_name = con_id, rcon_flds = rbinds })
      = hang (ppr con_id) 2 (ppr rbinds)

    ppr_expr (RecordUpd { rupd_expr = L _ aexp, rupd_flds = rbinds })
      = hang (ppr aexp) 2 (braces (fsep (punctuate comma (map ppr rbinds))))

    ppr_expr (ExprWithTySig expr sig)
      = hang (nest 2 (ppr_lexpr expr) <+> dcolon)
             4 (ppr sig)
    ppr_expr (ExprWithTySigOut expr sig)
      = hang (nest 2 (ppr_lexpr expr) <+> dcolon)
             4 (ppr sig)

    ppr_expr (ArithSeq _ _ info) = brackets (ppr info)
    ppr_expr (PArrSeq  _ info) = paBrackets (ppr info)

    ppr_expr EWildPat       = char '_'
    ppr_expr (ELazyPat e)   = char '~' <> ppr e
    ppr_expr (EAsPat v e)   = ppr v <> char '@' <> ppr e
    ppr_expr (EViewPat p e) = ppr p <+> text "->" <+> ppr e

    ppr_expr (HsSCC st (StringLiteral stl lbl) expr)
      = sep [ pprWithSourceText st (text "{-# SCC")
             -- no doublequotes if stl empty, for the case where the SCC was written
             -- without quotes.
              <+> pprWithSourceText stl (ftext lbl) <+> text "#-}",
              ppr expr ]

    ppr_expr (HsWrap co_fn e)
      = pprHsWrapper co_fn (\parens -> if parens then pprExpr e
                                                 else pprExpr e)

    ppr_expr (HsSpliceE s)         = pprSplice s
    ppr_expr (HsBracket b)         = pprHsBracket b
    ppr_expr (HsRnBracketOut e []) = ppr e
    ppr_expr (HsRnBracketOut e ps) = ppr e $$ text "pending(rn)" <+> ppr ps
    ppr_expr (HsTcBracketOut e []) = ppr e
    ppr_expr (HsTcBracketOut e ps) = ppr e $$ text "pending(tc)" <+> ppr ps

    ppr_expr (HsProc pat (L _ (HsCmdTop cmd _ _ _)))
      = hsep [text "proc", ppr pat, ptext (sLit "->"), ppr cmd]

    ppr_expr (HsStatic _ e)
      = hsep [text "static", ppr e]

    ppr_expr (HsTick tickish exp)
      = pprTicks (ppr exp) $
        ppr tickish <+> ppr_lexpr exp
    ppr_expr (HsBinTick tickIdTrue tickIdFalse exp)
      = pprTicks (ppr exp) $
        hcat [text "bintick<",
              ppr tickIdTrue,
              text ",",
              ppr tickIdFalse,
              text ">(",
              ppr exp, text ")"]
    ppr_expr (HsTickPragma _ externalSrcLoc _ exp)
      = pprTicks (ppr exp) $
        hcat [text "tickpragma<",
              pprExternalSrcLoc externalSrcLoc,
              text ">(",
              ppr exp,
              text ")"]

    ppr_expr (HsArrApp arrow arg _ HsFirstOrderApp True)
      = hsep [ppr_lexpr arrow, larrowt, ppr_lexpr arg]
    ppr_expr (HsArrApp arrow arg _ HsFirstOrderApp False)
      = hsep [ppr_lexpr arg, arrowt, ppr_lexpr arrow]
    ppr_expr (HsArrApp arrow arg _ HsHigherOrderApp True)
      = hsep [ppr_lexpr arrow, larrowtt, ppr_lexpr arg]
    ppr_expr (HsArrApp arrow arg _ HsHigherOrderApp False)
      = hsep [ppr_lexpr arg, arrowtt, ppr_lexpr arrow]

    ppr_expr (HsArrForm (L _ (HsVar (L _ v))) (Just _) [arg1, arg2])
      = sep [pprCmdArg (unLoc arg1), hsep [pprInfixOcc v, pprCmdArg (unLoc arg2)]]
    ppr_expr (HsArrForm (L _ (HsConLikeOut c)) (Just _) [arg1, arg2])
      = sep [pprCmdArg (unLoc arg1), hsep [pprInfixOcc (conLikeName c), pprCmdArg (unLoc arg2)]]
    ppr_expr (HsArrForm op _ args)
      = hang (text "(|" <+> ppr_lexpr op)
             4 (sep (map (pprCmdArg.unLoc) args) <+> text "|)")
    ppr_expr (HsRecFld f) = ppr f

    -- We must tiresomely make the "id" parameter to the LHsWcType existential
    -- because it's different in the HsAppType case and the HsAppTypeOut case
    -- | Located Haskell Wildcard Type Expression
    data LHsWcTypeX = forall id. (OutputableBndrId id) => LHsWcTypeX (LHsWcType id)

    ppr_apps :: (OutputableBndrId id) => HsExpr id
             -> [Either (LHsExpr id) LHsWcTypeX]
             -> SDoc
    ppr_apps (HsApp (L _ fun) arg)        args
      = ppr_apps fun (Left arg : args)
    ppr_apps (HsAppType (L _ fun) arg)    args
      = ppr_apps fun (Right (LHsWcTypeX arg) : args)
    ppr_apps (HsAppTypeOut (L _ fun) arg) args
      = ppr_apps fun (Right (LHsWcTypeX arg) : args)
    ppr_apps fun args = hang (ppr_expr fun) 2 (sep (map pp args))
      where
        pp (Left arg)                             = ppr arg
        pp (Right (LHsWcTypeX (HsWC { hswc_body = L _ arg })))
          = char '@' <> pprParendHsType arg

    pprExternalSrcLoc :: (StringLiteral,(Int,Int),(Int,Int)) -> SDoc
    pprExternalSrcLoc (StringLiteral _ src,(n1,n2),(n3,n4))
      = ppr (src,(n1,n2),(n3,n4))

    {-
    HsSyn records exactly where the user put parens, with HsPar.
    So generally speaking we print without adding any parens.
    However, some code is internally generated, and in some places
    parens are absolutely required; so for these places we use
    pprParendLExpr (but don't print double parens of course).

    For operator applications we don't add parens, because the operator
    fixities should do the job, except in debug mode (-dppr-debug) so we
    can see the structure of the parse tree.
    -}

    pprDebugParendExpr :: (OutputableBndrId id) => LHsExpr id -> SDoc
    pprDebugParendExpr expr
      = getPprStyle (\sty ->
        if debugStyle sty then pprParendLExpr expr
                          else pprLExpr      expr)

    pprParendLExpr :: (OutputableBndrId id) => LHsExpr id -> SDoc
    pprParendLExpr (L _ e) = pprParendExpr e

    pprParendExpr :: (OutputableBndrId id) => HsExpr id -> SDoc
    pprParendExpr expr
      | hsExprNeedsParens expr = parens (pprExpr expr)
      | otherwise              = pprExpr expr
            -- Using pprLExpr makes sure that we go 'deeper'
            -- I think that is usually (always?) right

    hsExprNeedsParens :: HsExpr id -> Bool
    -- True of expressions for which '(e)' and 'e'
    -- mean the same thing
    hsExprNeedsParens (ArithSeq {})       = False
    hsExprNeedsParens (PArrSeq {})        = False
    hsExprNeedsParens (HsLit {})          = False
    hsExprNeedsParens (HsOverLit {})      = False
    hsExprNeedsParens (HsVar {})          = False
    hsExprNeedsParens (HsUnboundVar {})   = False
    hsExprNeedsParens (HsConLikeOut {})   = False
    hsExprNeedsParens (HsIPVar {})        = False
    hsExprNeedsParens (HsOverLabel {})    = False
    hsExprNeedsParens (ExplicitTuple {})  = False
    hsExprNeedsParens (ExplicitList {})   = False
    hsExprNeedsParens (ExplicitPArr {})   = False
    hsExprNeedsParens (HsPar {})          = False
    hsExprNeedsParens (HsBracket {})      = False
    hsExprNeedsParens (HsRnBracketOut {}) = False
    hsExprNeedsParens (HsTcBracketOut {}) = False
    hsExprNeedsParens (HsDo sc _ _)
           | isListCompExpr sc            = False
    hsExprNeedsParens (HsRecFld{})        = False
    hsExprNeedsParens (RecordCon{})       = False
    hsExprNeedsParens (HsSpliceE{})       = False
    hsExprNeedsParens (RecordUpd{})       = False
    hsExprNeedsParens (HsWrap _ e)        = hsExprNeedsParens e
    hsExprNeedsParens _ = True

    isAtomicHsExpr :: HsExpr id -> Bool
    -- True of a single token
    isAtomicHsExpr (HsVar {})        = True
    isAtomicHsExpr (HsConLikeOut {}) = True
    isAtomicHsExpr (HsLit {})        = True
    isAtomicHsExpr (HsOverLit {})    = True
    isAtomicHsExpr (HsIPVar {})      = True
    isAtomicHsExpr (HsOverLabel {})  = True
    isAtomicHsExpr (HsUnboundVar {}) = True
    isAtomicHsExpr (HsWrap _ e)      = isAtomicHsExpr e
    isAtomicHsExpr (HsPar e)         = isAtomicHsExpr (unLoc e)
    isAtomicHsExpr (HsRecFld{})      = True
    isAtomicHsExpr _                 = False

    {-
    ************************************************************************
    *                                                                      *
    \subsection{Commands (in arrow abstractions)}
    *                                                                      *
    ************************************************************************

    We re-use HsExpr to represent these.
    -}

    -- | Located Haskell Command (for arrow syntax)
    type LHsCmd id = Located (HsCmd id)

    -- | Haskell Command (e.g. a "statement" in an Arrow proc block)
    data HsCmd id
      -- | - 'ApiAnnotation.AnnKeywordId' : 'ApiAnnotation.Annlarrowtail',
      --          'ApiAnnotation.Annrarrowtail','ApiAnnotation.AnnLarrowtail',
      --          'ApiAnnotation.AnnRarrowtail'

      -- For details on above see note [Api annotations] in ApiAnnotation
      = HsCmdArrApp          -- Arrow tail, or arrow application (f -< arg)
            (LHsExpr id)     -- arrow expression, f
            (LHsExpr id)     -- input expression, arg
            (PostTc id Type) -- type of the arrow expressions f,
                             -- of the form a t t', where arg :: t
            HsArrAppType     -- higher-order (-<<) or first-order (-<)
            Bool             -- True => right-to-left (f -< arg)
                             -- False => left-to-right (arg >- f)

      -- | - 'ApiAnnotation.AnnKeywordId' : 'ApiAnnotation.AnnOpenB' @'(|'@,
      --         'ApiAnnotation.AnnCloseB' @'|)'@

      -- For details on above see note [Api annotations] in ApiAnnotation
      | HsCmdArrForm         -- Command formation,  (| e cmd1 .. cmdn |)
            (LHsExpr id)     -- The operator.
                             -- After type-checking, a type abstraction to be
                             -- applied to the type of the local environment tuple
            LexicalFixity    -- Whether the operator appeared prefix or infix when
                             -- parsed.
            (Maybe Fixity)   -- fixity (filled in by the renamer), for forms that
                             -- were converted from OpApp's by the renamer
            [LHsCmdTop id]   -- argument commands

      | HsCmdApp    (LHsCmd id)
                    (LHsExpr id)

      | HsCmdLam    (MatchGroup id (LHsCmd id))     -- kappa
           -- ^ - 'ApiAnnotation.AnnKeywordId' : 'ApiAnnotation.AnnLam',
           --       'ApiAnnotation.AnnRarrow',

           -- For details on above see note [Api annotations] in ApiAnnotation

      | HsCmdPar    (LHsCmd id)                     -- parenthesised command
        -- ^ - 'ApiAnnotation.AnnKeywordId' : 'ApiAnnotation.AnnOpen' @'('@,
        --             'ApiAnnotation.AnnClose' @')'@

        -- For details on above see note [Api annotations] in ApiAnnotation

      | HsCmdCase   (LHsExpr id)
                    (MatchGroup id (LHsCmd id))     -- bodies are HsCmd's
        -- ^ - 'ApiAnnotation.AnnKeywordId' : 'ApiAnnotation.AnnCase',
        --       'ApiAnnotation.AnnOf','ApiAnnotation.AnnOpen' @'{'@,
        --       'ApiAnnotation.AnnClose' @'}'@

        -- For details on above see note [Api annotations] in ApiAnnotation

      | HsCmdIf     (Maybe (SyntaxExpr id))         -- cond function
                    (LHsExpr id)                    -- predicate
                    (LHsCmd id)                     -- then part
                    (LHsCmd id)                     -- else part
        -- ^ - 'ApiAnnotation.AnnKeywordId' : 'ApiAnnotation.AnnIf',
        --       'ApiAnnotation.AnnSemi',
        --       'ApiAnnotation.AnnThen','ApiAnnotation.AnnSemi',
        --       'ApiAnnotation.AnnElse',

        -- For details on above see note [Api annotations] in ApiAnnotation

      | HsCmdLet    (LHsLocalBinds id)      -- let(rec)
                    (LHsCmd  id)
        -- ^ - 'ApiAnnotation.AnnKeywordId' : 'ApiAnnotation.AnnLet',
        --       'ApiAnnotation.AnnOpen' @'{'@,
        --       'ApiAnnotation.AnnClose' @'}'@,'ApiAnnotation.AnnIn'

        -- For details on above see note [Api annotations] in ApiAnnotation

      | HsCmdDo     (Located [CmdLStmt id])
                    (PostTc id Type)                -- Type of the whole expression
        -- ^ - 'ApiAnnotation.AnnKeywordId' : 'ApiAnnotation.AnnDo',
        --             'ApiAnnotation.AnnOpen', 'ApiAnnotation.AnnSemi',
        --             'ApiAnnotation.AnnVbar',
        --             'ApiAnnotation.AnnClose'

        -- For details on above see note [Api annotations] in ApiAnnotation

      | HsCmdWrap   HsWrapper
                    (HsCmd id)     -- If   cmd :: arg1 --> res
                                   --      wrap :: arg1 "->" arg2
                                   -- Then (HsCmdWrap wrap cmd) :: arg2 --> res
    deriving instance (DataId id) => Data (HsCmd id)

    -- | Haskell Array Application Type
    data HsArrAppType = HsHigherOrderApp | HsFirstOrderApp
      deriving Data

    {- | Top-level command, introducing a new arrow.
    This may occur inside a proc (where the stack is empty) or as an
    argument of a command-forming operator.
    -}

    -- | Located Haskell Top-level Command
    type LHsCmdTop id = Located (HsCmdTop id)

    -- | Haskell Top-level Command
    data HsCmdTop id
      = HsCmdTop (LHsCmd id)
                 (PostTc id Type)   -- Nested tuple of inputs on the command's stack
                 (PostTc id Type)   -- return type of the command
                 (CmdSyntaxTable id) -- See Note [CmdSyntaxTable]
    deriving instance (DataId id) => Data (HsCmdTop id)

    instance (OutputableBndrId id) => Outputable (HsCmd id) where
        ppr cmd = pprCmd cmd

    -----------------------
    -- pprCmd and pprLCmd call pprDeeper;
    -- the underscore versions do not
    pprLCmd :: (OutputableBndrId id) => LHsCmd id -> SDoc
    pprLCmd (L _ c) = pprCmd c

    pprCmd :: (OutputableBndrId id) => HsCmd id -> SDoc
    pprCmd c | isQuietHsCmd c =            ppr_cmd c
             | otherwise      = pprDeeper (ppr_cmd c)

    isQuietHsCmd :: HsCmd id -> Bool
    -- Parentheses do display something, but it gives little info and
    -- if we go deeper when we go inside them then we get ugly things
    -- like (...)
    isQuietHsCmd (HsCmdPar _) = True
    -- applications don't display anything themselves
    isQuietHsCmd (HsCmdApp _ _) = True
    isQuietHsCmd _ = False

    -----------------------
    ppr_lcmd :: (OutputableBndrId id) => LHsCmd id -> SDoc
    ppr_lcmd c = ppr_cmd (unLoc c)

    ppr_cmd :: forall id. (OutputableBndrId id) => HsCmd id -> SDoc
    ppr_cmd (HsCmdPar c) = parens (ppr_lcmd c)

    ppr_cmd (HsCmdApp c e)
      = let (fun, args) = collect_args c [e] in
        hang (ppr_lcmd fun) 2 (sep (map ppr args))
      where
        collect_args (L _ (HsCmdApp fun arg)) args = collect_args fun (arg:args)
        collect_args fun args = (fun, args)

    ppr_cmd (HsCmdLam matches)
      = pprMatches matches

    ppr_cmd (HsCmdCase expr matches)
      = sep [ sep [text "case", nest 4 (ppr expr), ptext (sLit "of")],
              nest 2 (pprMatches matches) ]

    ppr_cmd (HsCmdIf _ e ct ce)
      = sep [hsep [text "if", nest 2 (ppr e), ptext (sLit "then")],
             nest 4 (ppr ct),
             text "else",
             nest 4 (ppr ce)]

    -- special case: let ... in let ...
    ppr_cmd (HsCmdLet (L _ binds) cmd@(L _ (HsCmdLet _ _)))
      = sep [hang (text "let") 2 (hsep [pprBinds binds, ptext (sLit "in")]),
             ppr_lcmd cmd]

    ppr_cmd (HsCmdLet (L _ binds) cmd)
      = sep [hang (text "let") 2 (pprBinds binds),
             hang (text "in")  2 (ppr cmd)]

    ppr_cmd (HsCmdDo (L _ stmts) _)  = pprDo ArrowExpr stmts

    ppr_cmd (HsCmdWrap w cmd)
      = pprHsWrapper w (\_ -> parens (ppr_cmd cmd))
    ppr_cmd (HsCmdArrApp arrow arg _ HsFirstOrderApp True)
      = hsep [ppr_lexpr arrow, larrowt, ppr_lexpr arg]
    ppr_cmd (HsCmdArrApp arrow arg _ HsFirstOrderApp False)
      = hsep [ppr_lexpr arg, arrowt, ppr_lexpr arrow]
    ppr_cmd (HsCmdArrApp arrow arg _ HsHigherOrderApp True)
      = hsep [ppr_lexpr arrow, larrowtt, ppr_lexpr arg]
    ppr_cmd (HsCmdArrApp arrow arg _ HsHigherOrderApp False)
      = hsep [ppr_lexpr arg, arrowtt, ppr_lexpr arrow]

    ppr_cmd (HsCmdArrForm (L _ (HsVar (L _ v))) _ (Just _) [arg1, arg2])
      = hang (pprCmdArg (unLoc arg1)) 4 (sep [ pprInfixOcc v
                                             , pprCmdArg (unLoc arg2)])
    ppr_cmd (HsCmdArrForm (L _ (HsVar (L _ v))) Infix _    [arg1, arg2])
      = hang (pprCmdArg (unLoc arg1)) 4 (sep [ pprInfixOcc v
                                             , pprCmdArg (unLoc arg2)])
    ppr_cmd (HsCmdArrForm (L _ (HsConLikeOut c)) _ (Just _) [arg1, arg2])
      = hang (pprCmdArg (unLoc arg1)) 4 (sep [ pprInfixOcc (conLikeName c)
                                             , pprCmdArg (unLoc arg2)])
    ppr_cmd (HsCmdArrForm (L _ (HsConLikeOut c)) Infix _    [arg1, arg2])
      = hang (pprCmdArg (unLoc arg1)) 4 (sep [ pprInfixOcc (conLikeName c)
                                             , pprCmdArg (unLoc arg2)])
    ppr_cmd (HsCmdArrForm op _ _ args)
      = hang (text "(|" <> ppr_lexpr op)
             4 (sep (map (pprCmdArg.unLoc) args) <> text "|)")

    pprCmdArg :: (OutputableBndrId id) => HsCmdTop id -> SDoc
    pprCmdArg (HsCmdTop cmd _ _ _)
      = ppr_lcmd cmd

    instance (OutputableBndrId id) => Outputable (HsCmdTop id) where
        ppr = pprCmdArg

    {-
    ************************************************************************
    *                                                                      *
    \subsection{Record binds}
    *                                                                      *
    ************************************************************************
    -}

    -- | Haskell Record Bindings
    type HsRecordBinds id = HsRecFields id (LHsExpr id)

    {-
    ************************************************************************
    *                                                                      *
    \subsection{@Match@, @GRHSs@, and @GRHS@ datatypes}
    *                                                                      *
    ************************************************************************

    @Match@es are sets of pattern bindings and right hand sides for
    functions, patterns or case branches. For example, if a function @g@
    is defined as:
    \begin{verbatim}
    g (x,y) = y
    g ((x:ys),y) = y+1,
    \end{verbatim}
    then \tr{g} has two @Match@es: @(x,y) = y@ and @((x:ys),y) = y+1@.

    It is always the case that each element of an @[Match]@ list has the
    same number of @pats@s inside it.  This corresponds to saying that
    a function defined by pattern matching must have the same number of
    patterns in each equation.
    -}

    data MatchGroup id body
      = MG { mg_alts    :: Located [LMatch id body]  -- The alternatives
           , mg_arg_tys :: [PostTc id Type]  -- Types of the arguments, t1..tn
           , mg_res_ty  :: PostTc id Type    -- Type of the result, tr
           , mg_origin  :: Origin }
         -- The type is the type of the entire group
         --      t1 -> ... -> tn -> tr
         -- where there are n patterns
    deriving instance (Data body,DataId id) => Data (MatchGroup id body)

    -- | Located Match
    type LMatch id body = Located (Match id body)
    -- ^ May have 'ApiAnnotation.AnnKeywordId' : 'ApiAnnotation.AnnSemi' when in a
    --   list

    -- For details on above see note [Api annotations] in ApiAnnotation
    data Match id body
      = Match {
            m_ctxt :: HsMatchContext (NameOrRdrName id),
              -- See note [m_ctxt in Match]
            m_pats :: [LPat id], -- The patterns
            m_type :: (Maybe (LHsType id)),
                                     -- A type signature for the result of the match
                                     -- Nothing after typechecking
                                     -- NB: No longer supported
            m_grhss :: (GRHSs id body)
      }
    deriving instance (Data body,DataId id) => Data (Match id body)

    instance (OutputableBndrId idR, Outputable body)
                => Outputable (Match idR body) where
      ppr = pprMatch

    {-
    Note [m_ctxt in Match]
    ~~~~~~~~~~~~~~~~~~~~~~

    A Match can occur in a number of contexts, such as a FunBind, HsCase, HsLam and
    so on.

    In order to simplify tooling processing and pretty print output, the provenance
    is captured in an HsMatchContext.

    This is particularly important for the API Annotations for a multi-equation
    FunBind.

    The parser initially creates a FunBind with a single Match in it for
    every function definition it sees.

    These are then grouped together by getMonoBind into a single FunBind,
    where all the Matches are combined.

    In the process, all the original FunBind fun_id's bar one are
    discarded, including the locations.

    This causes a problem for source to source conversions via API
    Annotations, so the original fun_ids and infix flags are preserved in
    the Match, when it originates from a FunBind.

    Example infix function definition requiring individual API Annotations

        (&&&  ) [] [] =  []
        xs    &&&   [] =  xs
        (  &&&  ) [] ys =  ys

    -}

    isInfixMatch :: Match id body -> Bool
    isInfixMatch match = case m_ctxt match of
      FunRhs {mc_fixity = Infix} -> True
      _                          -> False

    isEmptyMatchGroup :: MatchGroup id body -> Bool
    isEmptyMatchGroup (MG { mg_alts = ms }) = null $ unLoc ms

    -- | Is there only one RHS in this list of matches?
    isSingletonMatchGroup :: [LMatch id body] -> Bool
    isSingletonMatchGroup matches
      | [L _ match] <- matches
      , Match { m_grhss = GRHSs { grhssGRHSs = [_] } } <- match
      = True
      | otherwise
      = False

    matchGroupArity :: MatchGroup id body -> Arity
    -- Precondition: MatchGroup is non-empty
    -- This is called before type checking, when mg_arg_tys is not set
    matchGroupArity (MG { mg_alts = alts })
      | L _ (alt1:_) <- alts = length (hsLMatchPats alt1)
      | otherwise        = panic "matchGroupArity"

    hsLMatchPats :: LMatch id body -> [LPat id]
    hsLMatchPats (L _ (Match _ pats _ _)) = pats

    -- | Guarded Right-Hand Sides
    --
    -- GRHSs are used both for pattern bindings and for Matches
    --
    --  - 'ApiAnnotation.AnnKeywordId' : 'ApiAnnotation.AnnVbar',
    --        'ApiAnnotation.AnnEqual','ApiAnnotation.AnnWhere',
    --        'ApiAnnotation.AnnOpen','ApiAnnotation.AnnClose'
    --        'ApiAnnotation.AnnRarrow','ApiAnnotation.AnnSemi'

    -- For details on above see note [Api annotations] in ApiAnnotation
    data GRHSs id body
      = GRHSs {
          grhssGRHSs :: [LGRHS id body],      -- ^ Guarded RHSs
          grhssLocalBinds :: LHsLocalBinds id -- ^ The where clause
        }
    deriving instance (Data body,DataId id) => Data (GRHSs id body)

    -- | Located Guarded Right-Hand Side
    type LGRHS id body = Located (GRHS id body)

    -- | Guarded Right Hand Side.
    data GRHS id body = GRHS [GuardLStmt id] -- Guards
                             body            -- Right hand side
    deriving instance (Data body,DataId id) => Data (GRHS id body)

    -- We know the list must have at least one @Match@ in it.

    pprMatches :: (OutputableBndrId idR, Outputable body)
               => MatchGroup idR body -> SDoc
    pprMatches MG { mg_alts = matches }
        = vcat (map pprMatch (map unLoc (unLoc matches)))
          -- Don't print the type; it's only a place-holder before typechecking

    -- Exported to HsBinds, which can't see the defn of HsMatchContext
    pprFunBind :: (OutputableBndrId idR, Outputable body)
               => MatchGroup idR body -> SDoc
    pprFunBind matches = pprMatches matches

    -- Exported to HsBinds, which can't see the defn of HsMatchContext
    pprPatBind :: forall bndr id body. (OutputableBndrId bndr,
                                        OutputableBndrId id,
                                        Outputable body)
               => LPat bndr -> GRHSs id body -> SDoc
    pprPatBind pat (grhss)
     = sep [ppr pat, nest 2 (pprGRHSs (PatBindRhs :: HsMatchContext id) grhss)]

    pprMatch :: (OutputableBndrId idR, Outputable body) => Match idR body -> SDoc
    pprMatch match
      = sep [ sep (herald : map (nest 2 . pprParendLPat) other_pats)
            , nest 2 ppr_maybe_ty
            , nest 2 (pprGRHSs ctxt (m_grhss match)) ]
      where
        ctxt = m_ctxt match
        (herald, other_pats)
            = case ctxt of
                FunRhs {mc_fun=L _ fun, mc_fixity=fixity, mc_strictness=strictness}
                    | strictness == SrcStrict -> ASSERT(null $ m_pats match)
                                                 (char '!'<>pprPrefixOcc fun, m_pats match)
                            -- a strict variable binding
                    | fixity == Prefix -> (pprPrefixOcc fun, m_pats match)
                            -- f x y z = e
                            -- Not pprBndr; the AbsBinds will
                            -- have printed the signature

                    | null pats2 -> (pp_infix, [])
                            -- x &&& y = e

                    | otherwise -> (parens pp_infix, pats2)
                            -- (x &&& y) z = e
                    where
                      pp_infix = pprParendLPat pat1 <+> pprInfixOcc fun <+> pprParendLPat pat2

                LambdaExpr -> (char '\\', m_pats match)

                _  -> ASSERT2( null pats1, ppr ctxt $$ ppr pat1 $$ ppr pats1 )
                      (ppr pat1, [])        -- No parens around the single pat

        (pat1:pats1) = m_pats match
        (pat2:pats2) = pats1
        ppr_maybe_ty = case m_type match of
                            Just ty -> dcolon <+> ppr ty
                            Nothing -> empty

    pprGRHSs :: (OutputableBndrId idR, Outputable body)
             => HsMatchContext idL -> GRHSs idR body -> SDoc
    pprGRHSs ctxt (GRHSs grhss (L _ binds))
      = vcat (map (pprGRHS ctxt . unLoc) grhss)
      -- Print the "where" even if the contents of the binds is empty. Only
      -- EmptyLocalBinds means no "where" keyword
     $$ ppUnless (eqEmptyLocalBinds binds)
          (text "where" $$ nest 4 (pprBinds binds))

    pprGRHS :: (OutputableBndrId idR, Outputable body)
            => HsMatchContext idL -> GRHS idR body -> SDoc
    pprGRHS ctxt (GRHS [] body)
     =  pp_rhs ctxt body

    pprGRHS ctxt (GRHS guards body)
     = sep [vbar <+> interpp'SP guards, pp_rhs ctxt body]

    pp_rhs :: Outputable body => HsMatchContext idL -> body -> SDoc
    pp_rhs ctxt rhs = matchSeparator ctxt <+> pprDeeper (ppr rhs)

    {-
    ************************************************************************
    *                                                                      *
    \subsection{Do stmts and list comprehensions}
    *                                                                      *
    ************************************************************************
    -}

    -- | Located @do@ block Statement
    type LStmt id body = Located (StmtLR id id body)

    -- | Located Statement with separate Left and Right id's
    type LStmtLR idL idR body = Located (StmtLR idL idR body)

    -- | @do@ block Statement
    type Stmt id body = StmtLR id id body

    -- | Command Located Statement
    type CmdLStmt   id = LStmt id (LHsCmd  id)

    -- | Command Statement
    type CmdStmt    id = Stmt  id (LHsCmd  id)

    -- | Expression Located Statement
    type ExprLStmt  id = LStmt id (LHsExpr id)

    -- | Expression Statement
    type ExprStmt   id = Stmt  id (LHsExpr id)

    -- | Guard Located Statement
    type GuardLStmt id = LStmt id (LHsExpr id)

    -- | Guard Statement
    type GuardStmt  id = Stmt  id (LHsExpr id)

    -- | Ghci Located Statemnt
    type GhciLStmt  id = LStmt id (LHsExpr id)

    -- | Ghci Statement
    type GhciStmt   id = Stmt  id (LHsExpr id)

    -- The SyntaxExprs in here are used *only* for do-notation and monad
    -- comprehensions, which have rebindable syntax. Otherwise they are unused.
    -- | API Annotations when in qualifier lists or guards
    --  - 'ApiAnnotation.AnnKeywordId' : 'ApiAnnotation.AnnVbar',
    --         'ApiAnnotation.AnnComma','ApiAnnotation.AnnThen',
    --         'ApiAnnotation.AnnBy','ApiAnnotation.AnnBy',
    --         'ApiAnnotation.AnnGroup','ApiAnnotation.AnnUsing'

    -- For details on above see note [Api annotations] in ApiAnnotation
    data StmtLR idL idR body -- body should always be (LHs**** idR)
      = LastStmt  -- Always the last Stmt in ListComp, MonadComp, PArrComp,
                  -- and (after the renamer) DoExpr, MDoExpr
                  -- Not used for GhciStmtCtxt, PatGuard, which scope over other stuff
              body
              Bool               -- True <=> return was stripped by ApplicativeDo
              (SyntaxExpr idR)   -- The return operator, used only for
                                 -- MonadComp For ListComp, PArrComp, we
                                 -- use the baked-in 'return' For DoExpr,
                                 -- MDoExpr, we don't apply a 'return' at
                                 -- all See Note [Monad Comprehensions] |
                                 -- - 'ApiAnnotation.AnnKeywordId' :
                                 -- 'ApiAnnotation.AnnLarrow'

      -- For details on above see note [Api annotations] in ApiAnnotation
      | BindStmt (LPat idL)
                 body
                 (SyntaxExpr idR) -- The (>>=) operator; see Note [The type of bind in Stmts]
                 (SyntaxExpr idR) -- The fail operator
                 -- The fail operator is noSyntaxExpr
                 -- if the pattern match can't fail

                 (PostTc idR Type)  -- result type of the function passed to bind;
                                    -- that is, S in (>>=) :: Q -> (R -> S) -> T

      -- | 'ApplicativeStmt' represents an applicative expression built with
      -- <$> and <*>.  It is generated by the renamer, and is desugared into the
      -- appropriate applicative expression by the desugarer, but it is intended
      -- to be invisible in error messages.
      --
      -- For full details, see Note [ApplicativeDo] in RnExpr
      --
      | ApplicativeStmt
                 [ ( SyntaxExpr idR
                   , ApplicativeArg idL idR) ]
                          -- [(<$>, e1), (<*>, e2), ..., (<*>, en)]
                 (Maybe (SyntaxExpr idR))  -- 'join', if necessary
                 (PostTc idR Type)     -- Type of the body

      | BodyStmt body              -- See Note [BodyStmt]
                 (SyntaxExpr idR)  -- The (>>) operator
                 (SyntaxExpr idR)  -- The `guard` operator; used only in MonadComp
                                   -- See notes [Monad Comprehensions]
                 (PostTc idR Type) -- Element type of the RHS (used for arrows)

      -- | - 'ApiAnnotation.AnnKeywordId' : 'ApiAnnotation.AnnLet'
      --          'ApiAnnotation.AnnOpen' @'{'@,'ApiAnnotation.AnnClose' @'}'@,

      -- For details on above see note [Api annotations] in ApiAnnotation
      | LetStmt  (LHsLocalBindsLR idL idR)

      -- ParStmts only occur in a list/monad comprehension
      | ParStmt  [ParStmtBlock idL idR]
                 (HsExpr idR)               -- Polymorphic `mzip` for monad comprehensions
                 (SyntaxExpr idR)           -- The `>>=` operator
                                            -- See notes [Monad Comprehensions]
                 (PostTc idR Type)          -- S in (>>=) :: Q -> (R -> S) -> T
                -- After renaming, the ids are the binders
                -- bound by the stmts and used after themp

      | TransStmt {
          trS_form  :: TransForm,
          trS_stmts :: [ExprLStmt idL],   -- Stmts to the *left* of the 'group'
                                          -- which generates the tuples to be grouped

          trS_bndrs :: [(idR, idR)],      -- See Note [TransStmt binder map]

          trS_using :: LHsExpr idR,
          trS_by :: Maybe (LHsExpr idR),  -- "by e" (optional)
            -- Invariant: if trS_form = GroupBy, then grp_by = Just e

          trS_ret :: SyntaxExpr idR,      -- The monomorphic 'return' function for
                                          -- the inner monad comprehensions
          trS_bind :: SyntaxExpr idR,     -- The '(>>=)' operator
          trS_bind_arg_ty :: PostTc idR Type,  -- R in (>>=) :: Q -> (R -> S) -> T
          trS_fmap :: HsExpr idR          -- The polymorphic 'fmap' function for desugaring
                                          -- Only for 'group' forms
                                          -- Just a simple HsExpr, because it's
                                          -- too polymorphic for tcSyntaxOp
        }                                 -- See Note [Monad Comprehensions]

      -- Recursive statement (see Note [How RecStmt works] below)
      -- | - 'ApiAnnotation.AnnKeywordId' : 'ApiAnnotation.AnnRec'

      -- For details on above see note [Api annotations] in ApiAnnotation
      | RecStmt
         { recS_stmts :: [LStmtLR idL idR body]

            -- The next two fields are only valid after renaming
         , recS_later_ids :: [idR] -- The ids are a subset of the variables bound by the
                                   -- stmts that are used in stmts that follow the RecStmt

         , recS_rec_ids :: [idR]   -- Ditto, but these variables are the "recursive" ones,
                                   -- that are used before they are bound in the stmts of
                                   -- the RecStmt.
            -- An Id can be in both groups
            -- Both sets of Ids are (now) treated monomorphically
            -- See Note [How RecStmt works] for why they are separate

            -- Rebindable syntax
         , recS_bind_fn :: SyntaxExpr idR -- The bind function
         , recS_ret_fn  :: SyntaxExpr idR -- The return function
         , recS_mfix_fn :: SyntaxExpr idR -- The mfix function
         , recS_bind_ty :: PostTc idR Type  -- S in (>>=) :: Q -> (R -> S) -> T

            -- These fields are only valid after typechecking
         , recS_later_rets :: [PostTcExpr] -- (only used in the arrow version)
         , recS_rec_rets :: [PostTcExpr] -- These expressions correspond 1-to-1
                                         -- with recS_later_ids and recS_rec_ids,
                                         -- and are the expressions that should be
                                         -- returned by the recursion.
                                         -- They may not quite be the Ids themselves,
                                         -- because the Id may be *polymorphic*, but
                                         -- the returned thing has to be *monomorphic*,
                                         -- so they may be type applications

          , recS_ret_ty :: PostTc idR Type -- The type of
                                           -- do { stmts; return (a,b,c) }
                                       -- With rebindable syntax the type might not
                                       -- be quite as simple as (m (tya, tyb, tyc)).
          }
    deriving instance (Data body, DataId idL, DataId idR)
      => Data (StmtLR idL idR body)

    data TransForm   -- The 'f' below is the 'using' function, 'e' is the by function
      = ThenForm     -- then f               or    then f by e             (depending on trS_by)
      | GroupForm    -- then group using f   or    then group by e using f (depending on trS_by)
      deriving Data

    -- | Parenthesised Statement Block
    data ParStmtBlock idL idR
      = ParStmtBlock
            [ExprLStmt idL]
            [idR]              -- The variables to be returned
            (SyntaxExpr idR)   -- The return operator
    deriving instance (DataId idL, DataId idR) => Data (ParStmtBlock idL idR)

    -- | Applicative Argument
    data ApplicativeArg idL idR
      = ApplicativeArgOne            -- pat <- expr (pat must be irrefutable)
          (LPat idL)
          (LHsExpr idL)
      | ApplicativeArgMany           -- do { stmts; return vars }
          [ExprLStmt idL]            -- stmts
          (HsExpr idL)               -- return (v1,..,vn), or just (v1,..,vn)
          (LPat idL)                 -- (v1,...,vn)
    deriving instance (DataId idL, DataId idR) => Data (ApplicativeArg idL idR)

    {-
    Note [The type of bind in Stmts]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Some Stmts, notably BindStmt, keep the (>>=) bind operator.
    We do NOT assume that it has type
        (>>=) :: m a -> (a -> m b) -> m b
    In some cases (see Trac #303, #1537) it might have a more
    exotic type, such as
        (>>=) :: m i j a -> (a -> m j k b) -> m i k b
    So we must be careful not to make assumptions about the type.
    In particular, the monad may not be uniform throughout.

    Note [TransStmt binder map]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~
    The [(idR,idR)] in a TransStmt behaves as follows:

      * Before renaming: []

      * After renaming:
              [ (x27,x27), ..., (z35,z35) ]
        These are the variables
           bound by the stmts to the left of the 'group'
           and used either in the 'by' clause,
                    or     in the stmts following the 'group'
        Each item is a pair of identical variables.

      * After typechecking:
              [ (x27:Int, x27:[Int]), ..., (z35:Bool, z35:[Bool]) ]
        Each pair has the same unique, but different *types*.

    Note [BodyStmt]
    ~~~~~~~~~~~~~~~
    BodyStmts are a bit tricky, because what they mean
    depends on the context.  Consider the following contexts:

            A do expression of type (m res_ty)
            ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
            * BodyStmt E any_ty:   do { ....; E; ... }
                    E :: m any_ty
              Translation: E >> ...

            A list comprehensions of type [elt_ty]
            ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
            * BodyStmt E Bool:   [ .. | .... E ]
                            [ .. | ..., E, ... ]
                            [ .. | .... | ..., E | ... ]
                    E :: Bool
              Translation: if E then fail else ...

            A guard list, guarding a RHS of type rhs_ty
            ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
            * BodyStmt E BooParStmtBlockl:   f x | ..., E, ... = ...rhs...
                    E :: Bool
              Translation: if E then fail else ...

            A monad comprehension of type (m res_ty)
            ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
            * BodyStmt E Bool:   [ .. | .... E ]
                    E :: Bool
              Translation: guard E >> ...

    Array comprehensions are handled like list comprehensions.

    Note [How RecStmt works]
    ~~~~~~~~~~~~~~~~~~~~~~~~
    Example:
       HsDo [ BindStmt x ex

            , RecStmt { recS_rec_ids   = [a, c]
                      , recS_stmts     = [ BindStmt b (return (a,c))
                                         , LetStmt a = ...b...
                                         , BindStmt c ec ]
                      , recS_later_ids = [a, b]

            , return (a b) ]

    Here, the RecStmt binds a,b,c; but
      - Only a,b are used in the stmts *following* the RecStmt,
      - Only a,c are used in the stmts *inside* the RecStmt
            *before* their bindings

    Why do we need *both* rec_ids and later_ids?  For monads they could be
    combined into a single set of variables, but not for arrows.  That
    follows from the types of the respective feedback operators:

            mfix :: MonadFix m => (a -> m a) -> m a
            loop :: ArrowLoop a => a (b,d) (c,d) -> a b c

    * For mfix, the 'a' covers the union of the later_ids and the rec_ids
    * For 'loop', 'c' is the later_ids and 'd' is the rec_ids

    Note [Typing a RecStmt]
    ~~~~~~~~~~~~~~~~~~~~~~~
    A (RecStmt stmts) types as if you had written

      (v1,..,vn, _, ..., _) <- mfix (\~(_, ..., _, r1, ..., rm) ->
                                     do { stmts
                                        ; return (v1,..vn, r1, ..., rm) })

    where v1..vn are the later_ids
          r1..rm are the rec_ids

    Note [Monad Comprehensions]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Monad comprehensions require separate functions like 'return' and
    '>>=' for desugaring. These functions are stored in the statements
    used in monad comprehensions. For example, the 'return' of the 'LastStmt'
    expression is used to lift the body of the monad comprehension:

      [ body | stmts ]
       =>
      stmts >>= \bndrs -> return body

    In transform and grouping statements ('then ..' and 'then group ..') the
    'return' function is required for nested monad comprehensions, for example:

      [ body | stmts, then f, rest ]
       =>
      f [ env | stmts ] >>= \bndrs -> [ body | rest ]

    BodyStmts require the 'Control.Monad.guard' function for boolean
    expressions:

      [ body | exp, stmts ]
       =>
      guard exp >> [ body | stmts ]

    Parallel statements require the 'Control.Monad.Zip.mzip' function:

      [ body | stmts1 | stmts2 | .. ]
       =>
      mzip stmts1 (mzip stmts2 (..)) >>= \(bndrs1, (bndrs2, ..)) -> return body

    In any other context than 'MonadComp', the fields for most of these
    'SyntaxExpr's stay bottom.
    -}

    instance (OutputableBndrId idL) => Outputable (ParStmtBlock idL idR) where
      ppr (ParStmtBlock stmts _ _) = interpp'SP stmts

    instance (OutputableBndrId idL, OutputableBndrId idR, Outputable body)
             => Outputable (StmtLR idL idR body) where
        ppr stmt = pprStmt stmt

    pprStmt :: forall idL idR body . (OutputableBndrId idL, OutputableBndrId idR,
                                      Outputable body)
            => (StmtLR idL idR body) -> SDoc
    pprStmt (LastStmt expr ret_stripped _)
      = ifPprDebug (text "[last]") <+>
           (if ret_stripped then text "return" else empty) <+>
           ppr expr
    pprStmt (BindStmt pat expr _ _ _) = hsep [ppr pat, larrow, ppr expr]
    pprStmt (LetStmt (L _ binds))     = hsep [text "let", pprBinds binds]
    pprStmt (BodyStmt expr _ _ _)     = ppr expr
    pprStmt (ParStmt stmtss _ _ _)    = sep (punctuate (text " | ") (map ppr stmtss))

    pprStmt (TransStmt { trS_stmts = stmts, trS_by = by, trS_using = using, trS_form = form })
      = sep $ punctuate comma (map ppr stmts ++ [pprTransStmt by using form])

    pprStmt (RecStmt { recS_stmts = segment, recS_rec_ids = rec_ids
                     , recS_later_ids = later_ids })
      = text "rec" <+>
        vcat [ ppr_do_stmts segment
             , ifPprDebug (vcat [ text "rec_ids=" <> ppr rec_ids
                                , text "later_ids=" <> ppr later_ids])]

    pprStmt (ApplicativeStmt args mb_join _)
      = getPprStyle $ \style ->
          if userStyle style
             then pp_for_user
             else pp_debug
      where
      -- make all the Applicative stuff invisible in error messages by
      -- flattening the whole ApplicativeStmt nest back to a sequence
      -- of statements.
       pp_for_user = vcat $ concatMap flattenArg args

       -- ppr directly rather than transforming here, because we need to
       -- inject a "return" which is hard when we're polymorphic in the id
       -- type.
       flattenStmt :: ExprLStmt idL -> [SDoc]
       flattenStmt (L _ (ApplicativeStmt args _ _)) = concatMap flattenArg args
       flattenStmt stmt = [ppr stmt]

       flattenArg (_, ApplicativeArgOne pat expr) =
         [ppr (BindStmt pat expr noSyntaxExpr noSyntaxExpr (panic "pprStmt")
                 :: ExprStmt idL)]
       flattenArg (_, ApplicativeArgMany stmts _ _) =
         concatMap flattenStmt stmts

       pp_debug =
         let
             ap_expr = sep (punctuate (text " |") (map pp_arg args))
         in
           if isNothing mb_join
              then ap_expr
              else text "join" <+> parens ap_expr

       pp_arg (_, ApplicativeArgOne pat expr) =
         ppr (BindStmt pat expr noSyntaxExpr noSyntaxExpr (panic "pprStmt")
                :: ExprStmt idL)
       pp_arg (_, ApplicativeArgMany stmts return pat) =
         ppr pat <+>
         text "<-" <+>
         ppr (HsDo DoExpr (noLoc
                    (stmts ++ [noLoc (LastStmt (noLoc return) False noSyntaxExpr)]))
               (error "pprStmt"))

    pprTransformStmt :: (OutputableBndrId id)
                     => [id] -> LHsExpr id -> Maybe (LHsExpr id) -> SDoc
    pprTransformStmt bndrs using by
      = sep [ text "then" <+> ifPprDebug (braces (ppr bndrs))
            , nest 2 (ppr using)
            , nest 2 (pprBy by)]

    pprTransStmt :: Outputable body => Maybe body -> body -> TransForm -> SDoc
    pprTransStmt by using ThenForm
      = sep [ text "then", nest 2 (ppr using), nest 2 (pprBy by)]
    pprTransStmt by using GroupForm
      = sep [ text "then group", nest 2 (pprBy by), nest 2 (ptext (sLit "using") <+> ppr using)]

    pprBy :: Outputable body => Maybe body -> SDoc
    pprBy Nothing  = empty
    pprBy (Just e) = text "by" <+> ppr e

    pprDo :: (OutputableBndrId id, Outputable body)
          => HsStmtContext any -> [LStmt id body] -> SDoc
    pprDo DoExpr        stmts = text "do"  <+> ppr_do_stmts stmts
    pprDo GhciStmtCtxt  stmts = text "do"  <+> ppr_do_stmts stmts
    pprDo ArrowExpr     stmts = text "do"  <+> ppr_do_stmts stmts
    pprDo MDoExpr       stmts = text "mdo" <+> ppr_do_stmts stmts
    pprDo ListComp      stmts = brackets    $ pprComp stmts
    pprDo PArrComp      stmts = paBrackets  $ pprComp stmts
    pprDo MonadComp     stmts = brackets    $ pprComp stmts
    pprDo _             _     = panic "pprDo" -- PatGuard, ParStmtCxt

    ppr_do_stmts :: (OutputableBndrId idL, OutputableBndrId idR, Outputable body)
                 => [LStmtLR idL idR body] -> SDoc
    -- Print a bunch of do stmts
    ppr_do_stmts stmts = pprDeeperList vcat (map ppr stmts)

    pprComp :: (OutputableBndrId id, Outputable body) => [LStmt id body] -> SDoc
    pprComp quals     -- Prints:  body | qual1, ..., qualn
      | Just (initStmts, L _ (LastStmt body _ _)) <- snocView quals
      = if null initStmts
           -- If there are no statements in a list comprehension besides the last
           -- one, we simply treat it like a normal list. This does arise
           -- occasionally in code that GHC generates, e.g., in implementations of
           -- 'range' for derived 'Ix' instances for product datatypes with exactly
           -- one constructor (e.g., see Trac #12583).
           then ppr body
           else hang (ppr body <+> vbar) 2 (pprQuals initStmts)
      | otherwise
      = pprPanic "pprComp" (pprQuals quals)

    pprQuals :: (OutputableBndrId id, Outputable body) => [LStmt id body] -> SDoc
    -- Show list comprehension qualifiers separated by commas
    pprQuals quals = interpp'SP quals

    {-
    ************************************************************************
    *                                                                      *
                    Template Haskell quotation brackets
    *                                                                      *
    ************************************************************************
    -}

    -- | Haskell Splice
    data HsSplice id
       = HsTypedSplice       --  $$z  or $$(f 4)
            SpliceDecoration -- Whether $$( ) variant found, for pretty printing
            id               -- A unique name to identify this splice point
            (LHsExpr id)     -- See Note [Pending Splices]

       | HsUntypedSplice     --  $z  or $(f 4)
            SpliceDecoration -- Whether $( ) variant found, for pretty printing
            id               -- A unique name to identify this splice point
            (LHsExpr id)     -- See Note [Pending Splices]

       | HsQuasiQuote        -- See Note [Quasi-quote overview] in TcSplice
            id               -- Splice point
            id               -- Quoter
            SrcSpan          -- The span of the enclosed string
            FastString       -- The enclosed string

       | HsSpliced  -- See Note [Delaying modFinalizers in untyped splices] in
                    -- RnSplice.
                    -- This is the result of splicing a splice. It is produced by
                    -- the renamer and consumed by the typechecker. It lives only
                    -- between the two.
            ThModFinalizers     -- TH finalizers produced by the splice.
            (HsSplicedThing id) -- The result of splicing
      deriving Typeable
    deriving instance (DataId id) => Data (HsSplice id)

    -- | A splice can appear with various decorations wrapped around it. This data
    -- type captures explicitly how it was originally written, for use in the pretty
    -- printer.
    data SpliceDecoration
      = HasParens -- ^ $( splice ) or $$( splice )
      | HasDollar -- ^ $splice or $$splice
      | NoParens  -- ^ bare splice
      deriving (Data, Eq, Show)

    instance Outputable SpliceDecoration where
      ppr x = text $ show x

    isTypedSplice :: HsSplice id -> Bool
    isTypedSplice (HsTypedSplice {}) = True
    isTypedSplice _                  = False   -- Quasi-quotes are untyped splices

    -- | Finalizers produced by a splice with
    -- 'Language.Haskell.TH.Syntax.addModFinalizer'
    --
    -- See Note [Delaying modFinalizers in untyped splices] in RnSplice. For how
    -- this is used.
    --
    newtype ThModFinalizers = ThModFinalizers [ForeignRef (TH.Q ())]

    -- A Data instance which ignores the argument of 'ThModFinalizers'.
    instance Data ThModFinalizers where
      gunfold _ z _ = z $ ThModFinalizers []
      toConstr  a   = mkConstr (dataTypeOf a) "ThModFinalizers" [] Data.Prefix
      dataTypeOf a  = mkDataType "HsExpr.ThModFinalizers" [toConstr a]

    -- | Haskell Spliced Thing
    --
    -- Values that can result from running a splice.
    data HsSplicedThing id
        = HsSplicedExpr (HsExpr id) -- ^ Haskell Spliced Expression
        | HsSplicedTy   (HsType id) -- ^ Haskell Spliced Type
        | HsSplicedPat  (Pat id)    -- ^ Haskell Spliced Pattern
      deriving Typeable

    deriving instance (DataId id) => Data (HsSplicedThing id)

    -- See Note [Pending Splices]
    type SplicePointName = Name

    -- | Pending Renamer Splice
    data PendingRnSplice
      = PendingRnSplice UntypedSpliceFlavour SplicePointName (LHsExpr Name)
      deriving Data

    data UntypedSpliceFlavour
      = UntypedExpSplice
      | UntypedPatSplice
      | UntypedTypeSplice
      | UntypedDeclSplice
      deriving Data

    -- | Pending Type-checker Splice
    data PendingTcSplice
      = PendingTcSplice SplicePointName (LHsExpr Id)
      deriving Data

    {-
    Note [Pending Splices]
    ~~~~~~~~~~~~~~~~~~~~~~
    When we rename an untyped bracket, we name and lift out all the nested
    splices, so that when the typechecker hits the bracket, it can
    typecheck those nested splices without having to walk over the untyped
    bracket code.  So for example
        [| f $(g x) |]
    looks like

        HsBracket (HsApp (HsVar "f") (HsSpliceE _ (g x)))

    which the renamer rewrites to

        HsRnBracketOut (HsApp (HsVar f) (HsSpliceE sn (g x)))
                       [PendingRnSplice UntypedExpSplice sn (g x)]

    * The 'sn' is the Name of the splice point, the SplicePointName

    * The PendingRnExpSplice gives the splice that splice-point name maps to;
      and the typechecker can now conveniently find these sub-expressions

    * The other copy of the splice, in the second argument of HsSpliceE
                                    in the renamed first arg of HsRnBracketOut
      is used only for pretty printing

    There are four varieties of pending splices generated by the renamer,
    distinguished by their UntypedSpliceFlavour

     * Pending expression splices (UntypedExpSplice), e.g.,
           [|$(f x) + 2|]

       UntypedExpSplice is also used for
         * quasi-quotes, where the pending expression expands to
              $(quoter "...blah...")
           (see RnSplice.makePending, HsQuasiQuote case)

         * cross-stage lifting, where the pending expression expands to
              $(lift x)
           (see RnSplice.checkCrossStageLifting)

     * Pending pattern splices (UntypedPatSplice), e.g.,
           [| \$(f x) -> x |]

     * Pending type splices (UntypedTypeSplice), e.g.,
           [| f :: $(g x) |]

     * Pending declaration (UntypedDeclSplice), e.g.,
           [| let $(f x) in ... |]

    There is a fifth variety of pending splice, which is generated by the type
    checker:

      * Pending *typed* expression splices, (PendingTcSplice), e.g.,
            [||1 + $$(f 2)||]

    It would be possible to eliminate HsRnBracketOut and use HsBracketOut for the
    output of the renamer. However, when pretty printing the output of the renamer,
    e.g., in a type error message, we *do not* want to print out the pending
    splices. In contrast, when pretty printing the output of the type checker, we
    *do* want to print the pending splices. So splitting them up seems to make
    sense, although I hate to add another constructor to HsExpr.
    -}

    instance (OutputableBndrId id) => Outputable (HsSplicedThing id) where
      ppr (HsSplicedExpr e) = ppr_expr e
      ppr (HsSplicedTy   t) = ppr t
      ppr (HsSplicedPat  p) = ppr p

    instance (OutputableBndrId id) => Outputable (HsSplice id) where
      ppr s = pprSplice s

    pprPendingSplice :: (OutputableBndrId id)
                     => SplicePointName -> LHsExpr id -> SDoc
    pprPendingSplice n e = angleBrackets (ppr n <> comma <+> ppr e)

    pprSpliceDecl ::  (OutputableBndrId id)
              => HsSplice id -> SpliceExplicitFlag -> SDoc
    pprSpliceDecl e@HsQuasiQuote{} _ = pprSplice e
    pprSpliceDecl e ExplicitSplice   = text "$(" <> ppr_splice_decl e <> text ")"
    pprSpliceDecl e ImplicitSplice   = ppr_splice_decl e

    ppr_splice_decl :: (OutputableBndrId id) => HsSplice id -> SDoc
    ppr_splice_decl (HsUntypedSplice _ n e) = ppr_splice empty n e empty
    ppr_splice_decl e = pprSplice e

    pprSplice :: (OutputableBndrId id) => HsSplice id -> SDoc
    pprSplice (HsTypedSplice HasParens  n e)
      = ppr_splice (text "$$(") n e (text ")")
    pprSplice (HsTypedSplice HasDollar n e)
      = ppr_splice (text "$$") n e empty
    pprSplice (HsTypedSplice NoParens n e)
      = ppr_splice empty n e empty
    pprSplice (HsUntypedSplice HasParens  n e)
      = ppr_splice (text "$(") n e (text ")")
    pprSplice (HsUntypedSplice HasDollar n e)
      = ppr_splice (text "$")  n e empty
    pprSplice (HsUntypedSplice NoParens n e)
      = ppr_splice empty  n e empty
    pprSplice (HsQuasiQuote n q _ s)      = ppr_quasi n q s
    pprSplice (HsSpliced _ thing)         = ppr thing

    ppr_quasi :: OutputableBndr id => id -> id -> FastString -> SDoc
    ppr_quasi n quoter quote = ifPprDebug (brackets (ppr n)) <>
                               char '[' <> ppr quoter <> vbar <>
                               ppr quote <> text "|]"

    ppr_splice :: (OutputableBndrId id)
               => SDoc -> id -> LHsExpr id -> SDoc -> SDoc
    ppr_splice herald n e trail
        = herald <> ifPprDebug (brackets (ppr n)) <> ppr e <> trail

    -- | Haskell Bracket
    data HsBracket id = ExpBr (LHsExpr id)   -- [|  expr  |]
                      | PatBr (LPat id)      -- [p| pat   |]
                      | DecBrL [LHsDecl id]  -- [d| decls |]; result of parser
                      | DecBrG (HsGroup id)  -- [d| decls |]; result of renamer
                      | TypBr (LHsType id)   -- [t| type  |]
                      | VarBr Bool id        -- True: 'x, False: ''T
                                             -- (The Bool flag is used only in pprHsBracket)
                      | TExpBr (LHsExpr id)  -- [||  expr  ||]
    deriving instance (DataId id) => Data (HsBracket id)

    isTypedBracket :: HsBracket id -> Bool
    isTypedBracket (TExpBr {}) = True
    isTypedBracket _           = False

    instance (OutputableBndrId id) => Outputable (HsBracket id) where
      ppr = pprHsBracket

    pprHsBracket :: (OutputableBndrId id) => HsBracket id -> SDoc
    pprHsBracket (ExpBr e)   = thBrackets empty (ppr e)
    pprHsBracket (PatBr p)   = thBrackets (char 'p') (ppr p)
    pprHsBracket (DecBrG gp) = thBrackets (char 'd') (ppr gp)
    pprHsBracket (DecBrL ds) = thBrackets (char 'd') (vcat (map ppr ds))
    pprHsBracket (TypBr t)   = thBrackets (char 't') (ppr t)
    pprHsBracket (VarBr True n)
      = char '\'' <> pprPrefixOcc n
    pprHsBracket (VarBr False n)
      = text "''" <> pprPrefixOcc n
    pprHsBracket (TExpBr e)  = thTyBrackets (ppr e)

    thBrackets :: SDoc -> SDoc -> SDoc
    thBrackets pp_kind pp_body = char '[' <> pp_kind <> vbar <+>
                                 pp_body <+> text "|]"

    thTyBrackets :: SDoc -> SDoc
    thTyBrackets pp_body = text "[||" <+> pp_body <+> ptext (sLit "||]")

    instance Outputable PendingRnSplice where
      ppr (PendingRnSplice _ n e) = pprPendingSplice n e

    instance Outputable PendingTcSplice where
      ppr (PendingTcSplice n e) = pprPendingSplice n e

    {-
    ************************************************************************
    *                                                                      *
    \subsection{Enumerations and list comprehensions}
    *                                                                      *
    ************************************************************************
    -}

    -- | Arithmetic Sequence Information
    data ArithSeqInfo id
      = From            (LHsExpr id)
      | FromThen        (LHsExpr id)
                        (LHsExpr id)
      | FromTo          (LHsExpr id)
                        (LHsExpr id)
      | FromThenTo      (LHsExpr id)
                        (LHsExpr id)
                        (LHsExpr id)
    deriving instance (DataId id) => Data (ArithSeqInfo id)

    instance (OutputableBndrId id)
             => Outputable (ArithSeqInfo id) where
        ppr (From e1)             = hcat [ppr e1, pp_dotdot]
        ppr (FromThen e1 e2)      = hcat [ppr e1, comma, space, ppr e2, pp_dotdot]
        ppr (FromTo e1 e3)        = hcat [ppr e1, pp_dotdot, ppr e3]
        ppr (FromThenTo e1 e2 e3)
          = hcat [ppr e1, comma, space, ppr e2, pp_dotdot, ppr e3]

    pp_dotdot :: SDoc
    pp_dotdot = text " .. "

    {-
    ************************************************************************
    *                                                                      *
    \subsection{HsMatchCtxt}
    *                                                                      *
    ************************************************************************
    -}

    -- | Haskell Match Context
    --
    -- Context of a pattern match. This is more subtle than it would seem. See Note
    -- [Varieties of pattern matches].
    data HsMatchContext id -- Not an extensible tag
      = FunRhs { mc_fun :: Located id -- ^ function binder of @f@
               , mc_fixity :: LexicalFixity -- ^ fixing of @f@
               , mc_strictness :: SrcStrictness
                 -- ^ was the pattern banged? See
                 -- Note [Varieties of binding pattern matches]
               }
                                    -- ^A pattern matching on an argument of a
                                    -- function binding
      | LambdaExpr                  -- ^Patterns of a lambda
      | CaseAlt                     -- ^Patterns and guards on a case alternative
      | IfAlt                       -- ^Guards of a multi-way if alternative
      | ProcExpr                    -- ^Patterns of a proc
      | PatBindRhs                  -- ^A pattern binding  eg [y] <- e = e

      | RecUpd                      -- ^Record update [used only in DsExpr to
                                    --    tell matchWrapper what sort of
                                    --    runtime error message to generate]

      | StmtCtxt (HsStmtContext id) -- ^Pattern of a do-stmt, list comprehension,
                                    -- pattern guard, etc

      | ThPatSplice            -- ^A Template Haskell pattern splice
      | ThPatQuote             -- ^A Template Haskell pattern quotation [p| (a,b) |]
      | PatSyn                 -- ^A pattern synonym declaration
      deriving Functor
    deriving instance (DataIdPost id) => Data (HsMatchContext id)

    instance OutputableBndr id => Outputable (HsMatchContext id) where
      ppr m@(FunRhs{})          = text "FunRhs" <+> ppr (mc_fun m) <+> ppr (mc_fixity m)
      ppr LambdaExpr            = text "LambdaExpr"
      ppr CaseAlt               = text "CaseAlt"
      ppr IfAlt                 = text "IfAlt"
      ppr ProcExpr              = text "ProcExpr"
      ppr PatBindRhs            = text "PatBindRhs"
      ppr RecUpd                = text "RecUpd"
      ppr (StmtCtxt _)          = text "StmtCtxt _"
      ppr ThPatSplice           = text "ThPatSplice"
      ppr ThPatQuote            = text "ThPatQuote"
      ppr PatSyn                = text "PatSyn"

    isPatSynCtxt :: HsMatchContext id -> Bool
    isPatSynCtxt ctxt =
      case ctxt of
        PatSyn -> True
        _      -> False

    -- | Haskell Statement Context
    data HsStmtContext id
      = ListComp
      | MonadComp
      | PArrComp                         -- ^Parallel array comprehension

      | DoExpr                           -- ^do { ... }
      | MDoExpr                          -- ^mdo { ... }  ie recursive do-expression
      | ArrowExpr                        -- ^do-notation in an arrow-command context

      | GhciStmtCtxt                     -- ^A command-line Stmt in GHCi pat <- rhs
      | PatGuard (HsMatchContext id)     -- ^Pattern guard for specified thing
      | ParStmtCtxt (HsStmtContext id)   -- ^A branch of a parallel stmt
      | TransStmtCtxt (HsStmtContext id) -- ^A branch of a transform stmt
      deriving Functor
    deriving instance (DataIdPost id) => Data (HsStmtContext id)

    isListCompExpr :: HsStmtContext id -> Bool
    -- Uses syntax [ e | quals ]
    isListCompExpr ListComp          = True
    isListCompExpr PArrComp          = True
    isListCompExpr MonadComp         = True
    isListCompExpr (ParStmtCtxt c)   = isListCompExpr c
    isListCompExpr (TransStmtCtxt c) = isListCompExpr c
    isListCompExpr _                 = False

    isMonadCompExpr :: HsStmtContext id -> Bool
    isMonadCompExpr MonadComp            = True
    isMonadCompExpr (ParStmtCtxt ctxt)   = isMonadCompExpr ctxt
    isMonadCompExpr (TransStmtCtxt ctxt) = isMonadCompExpr ctxt
    isMonadCompExpr _                    = False

    -- | Should pattern match failure in a 'HsStmtContext' be desugared using
    -- 'MonadFail'?
    isMonadFailStmtContext :: HsStmtContext id -> Bool
    isMonadFailStmtContext MonadComp    = True
    isMonadFailStmtContext DoExpr       = True
    isMonadFailStmtContext MDoExpr      = True
    isMonadFailStmtContext GhciStmtCtxt = True
    isMonadFailStmtContext _            = False

    matchSeparator :: HsMatchContext id -> SDoc
    matchSeparator (FunRhs {})  = text "="
    matchSeparator CaseAlt      = text "->"
    matchSeparator IfAlt        = text "->"
    matchSeparator LambdaExpr   = text "->"
    matchSeparator ProcExpr     = text "->"
    matchSeparator PatBindRhs   = text "="
    matchSeparator (StmtCtxt _) = text "<-"
    matchSeparator RecUpd       = text "=" -- This can be printed by the pattern
                                           -- match checker trace
    matchSeparator ThPatSplice  = panic "unused"
    matchSeparator ThPatQuote   = panic "unused"
    matchSeparator PatSyn       = panic "unused"

    pprMatchContext :: (Outputable (NameOrRdrName id),Outputable id)
                    => HsMatchContext id -> SDoc
    pprMatchContext ctxt
      | want_an ctxt = text "an" <+> pprMatchContextNoun ctxt
      | otherwise    = text "a"  <+> pprMatchContextNoun ctxt
      where
        want_an (FunRhs {}) = True  -- Use "an" in front
        want_an ProcExpr    = True
        want_an _           = False

    pprMatchContextNoun :: (Outputable (NameOrRdrName id),Outputable id)
                        => HsMatchContext id -> SDoc
    pprMatchContextNoun (FunRhs {mc_fun=L _ fun})
                                        = text "equation for"
                                          <+> quotes (ppr fun)
    pprMatchContextNoun CaseAlt         = text "case alternative"
    pprMatchContextNoun IfAlt           = text "multi-way if alternative"
    pprMatchContextNoun RecUpd          = text "record-update construct"
    pprMatchContextNoun ThPatSplice     = text "Template Haskell pattern splice"
    pprMatchContextNoun ThPatQuote      = text "Template Haskell pattern quotation"
    pprMatchContextNoun PatBindRhs      = text "pattern binding"
    pprMatchContextNoun LambdaExpr      = text "lambda abstraction"
    pprMatchContextNoun ProcExpr        = text "arrow abstraction"
    pprMatchContextNoun (StmtCtxt ctxt) = text "pattern binding in"
                                          $$ pprStmtContext ctxt
    pprMatchContextNoun PatSyn          = text "pattern synonym declaration"

    -----------------
    pprAStmtContext, pprStmtContext :: (Outputable id,
                                        Outputable (NameOrRdrName id))
                                    => HsStmtContext id -> SDoc
    pprAStmtContext ctxt = article <+> pprStmtContext ctxt
      where
        pp_an = text "an"
        pp_a  = text "a"
        article = case ctxt of
                      MDoExpr       -> pp_an
                      PArrComp      -> pp_an
                      GhciStmtCtxt  -> pp_an
                      _             -> pp_a

    -----------------
    pprStmtContext GhciStmtCtxt    = text "interactive GHCi command"
    pprStmtContext DoExpr          = text "'do' block"
    pprStmtContext MDoExpr         = text "'mdo' block"
    pprStmtContext ArrowExpr       = text "'do' block in an arrow command"
    pprStmtContext ListComp        = text "list comprehension"
    pprStmtContext MonadComp       = text "monad comprehension"
    pprStmtContext PArrComp        = text "array comprehension"
    pprStmtContext (PatGuard ctxt) = text "pattern guard for" $$ pprMatchContext ctxt

    -- Drop the inner contexts when reporting errors, else we get
    --     Unexpected transform statement
    --     in a transformed branch of
    --          transformed branch of
    --          transformed branch of monad comprehension
    pprStmtContext (ParStmtCtxt c) =
      sdocWithPprDebug $ \dbg -> if dbg
        then sep [text "parallel branch of", pprAStmtContext c]
        else pprStmtContext c
    pprStmtContext (TransStmtCtxt c) =
      sdocWithPprDebug $ \dbg -> if dbg
        then sep [text "transformed branch of", pprAStmtContext c]
        else pprStmtContext c

    instance (Outputable id, Outputable (NameOrRdrName id))
          => Outputable (HsStmtContext id) where
        ppr = pprStmtContext

    -- Used to generate the string for a *runtime* error message
    matchContextErrString :: Outputable id
                          => HsMatchContext id -> SDoc
    matchContextErrString (FunRhs{mc_fun=L _ fun})   = text "function" <+> ppr fun
    matchContextErrString CaseAlt                    = text "case"
    matchContextErrString IfAlt                      = text "multi-way if"
    matchContextErrString PatBindRhs                 = text "pattern binding"
    matchContextErrString RecUpd                     = text "record update"
    matchContextErrString LambdaExpr                 = text "lambda"
    matchContextErrString ProcExpr                   = text "proc"
    matchContextErrString ThPatSplice                = panic "matchContextErrString"  -- Not used at runtime
    matchContextErrString ThPatQuote                 = panic "matchContextErrString"  -- Not used at runtime
    matchContextErrString PatSyn                     = panic "matchContextErrString"  -- Not used at runtime
    matchContextErrString (StmtCtxt (ParStmtCtxt c))   = matchContextErrString (StmtCtxt c)
    matchContextErrString (StmtCtxt (TransStmtCtxt c)) = matchContextErrString (StmtCtxt c)
    matchContextErrString (StmtCtxt (PatGuard _))      = text "pattern guard"
    matchContextErrString (StmtCtxt GhciStmtCtxt)      = text "interactive GHCi command"
    matchContextErrString (StmtCtxt DoExpr)            = text "'do' block"
    matchContextErrString (StmtCtxt ArrowExpr)         = text "'do' block"
    matchContextErrString (StmtCtxt MDoExpr)           = text "'mdo' block"
    matchContextErrString (StmtCtxt ListComp)          = text "list comprehension"
    matchContextErrString (StmtCtxt MonadComp)         = text "monad comprehension"
    matchContextErrString (StmtCtxt PArrComp)          = text "array comprehension"

    pprMatchInCtxt :: (OutputableBndrId idR,
                       Outputable (NameOrRdrName (NameOrRdrName idR)),
                       Outputable body)
                   => Match idR body -> SDoc
    pprMatchInCtxt match  = hang (text "In" <+> pprMatchContext (m_ctxt match)
                                            <> colon)
                                 4 (pprMatch match)

    pprStmtInCtxt :: (OutputableBndrId idL, OutputableBndrId idR,
                      Outputable body)
                   => HsStmtContext idL -> StmtLR idL idR body -> SDoc
    pprStmtInCtxt ctxt (LastStmt e _ _)
      | isListCompExpr ctxt      -- For [ e | .. ], do not mutter about "stmts"
      = hang (text "In the expression:") 2 (ppr e)

    pprStmtInCtxt ctxt stmt
      = hang (text "In a stmt of" <+> pprAStmtContext ctxt <> colon)
           2 (ppr_stmt stmt)
      where
        -- For Group and Transform Stmts, don't print the nested stmts!
        ppr_stmt (TransStmt { trS_by = by, trS_using = using
                            , trS_form = form }) = pprTransStmt by using form
        ppr_stmt stmt = pprStmt stmt