deSugar/DsExpr.hs

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

    Desugaring exporessions.
    -}

    {-# LANGUAGE CPP, MultiWayIf #-}

    module DsExpr ( dsExpr, dsLExpr, dsLExprNoLP, dsLocalBinds
                  , dsValBinds, dsLit, dsSyntaxExpr ) where

    #include "HsVersions.h"

    import Match
    import MatchLit
    import DsBinds
    import DsGRHSs
    import DsListComp
    import DsUtils
    import DsArrows
    import DsMonad
    import Name
    import NameEnv
    import FamInstEnv( topNormaliseType )
    import DsMeta
    import HsSyn

    -- NB: The desugarer, which straddles the source and Core worlds, sometimes
    --     needs to see source types
    import TcType
    import TcEvidence
    import TcRnMonad
    import TcHsSyn
    import Type
    import CoreSyn
    import CoreUtils
    import MkCore

    import DynFlags
    import CostCentre
    import Id
    import MkId
    import Module
    import ConLike
    import DataCon
    import TysWiredIn
    import PrelNames
    import BasicTypes
    import Maybes
    import VarEnv
    import SrcLoc
    import Util
    import Bag
    import Outputable
    import PatSyn

    import Control.Monad

    {-
    ************************************************************************
    *                                                                      *
                    dsLocalBinds, dsValBinds
    *                                                                      *
    ************************************************************************
    -}

    dsLocalBinds :: LHsLocalBinds Id -> CoreExpr -> DsM CoreExpr
    dsLocalBinds (L _   EmptyLocalBinds)    body = return body
    dsLocalBinds (L loc (HsValBinds binds)) body = putSrcSpanDs loc $
                                                   dsValBinds binds body
    dsLocalBinds (L _ (HsIPBinds binds))    body = dsIPBinds  binds body

    -------------------------
    -- caller sets location
    dsValBinds :: HsValBinds Id -> CoreExpr -> DsM CoreExpr
    dsValBinds (ValBindsOut binds _) body = foldrM ds_val_bind body binds
    dsValBinds (ValBindsIn {})       _    = panic "dsValBinds ValBindsIn"

    -------------------------
    dsIPBinds :: HsIPBinds Id -> CoreExpr -> DsM CoreExpr
    dsIPBinds (IPBinds ip_binds ev_binds) body
      = do  { ds_binds <- dsTcEvBinds ev_binds
            ; let inner = mkCoreLets ds_binds body
                    -- The dict bindings may not be in
                    -- dependency order; hence Rec
            ; foldrM ds_ip_bind inner ip_binds }
      where
        ds_ip_bind (L _ (IPBind ~(Right n) e)) body
          = do e' <- dsLExpr e
               return (Let (NonRec n e') body)

    -------------------------
    -- caller sets location
    ds_val_bind :: (RecFlag, LHsBinds Id) -> CoreExpr -> DsM CoreExpr
    -- Special case for bindings which bind unlifted variables
    -- We need to do a case right away, rather than building
    -- a tuple and doing selections.
    -- Silently ignore INLINE and SPECIALISE pragmas...
    ds_val_bind (NonRecursive, hsbinds) body
      | [L loc bind] <- bagToList hsbinds
            -- Non-recursive, non-overloaded bindings only come in ones
            -- ToDo: in some bizarre case it's conceivable that there
            --       could be dict binds in the 'binds'.  (See the notes
            --       below.  Then pattern-match would fail.  Urk.)
      , isUnliftedHsBind bind
      = putSrcSpanDs loc $
         -- see Note [Strict binds checks] in DsBinds
        if is_polymorphic bind
        then errDsCoreExpr (poly_bind_err bind)
                -- data Ptr a = Ptr Addr#
                -- f x = let p@(Ptr y) = ... in ...
                -- Here the binding for 'p' is polymorphic, but does
                -- not mix with an unlifted binding for 'y'.  You should
                -- use a bang pattern.  Trac #6078.

        else do { when (looksLazyPatBind bind) $
                  warnIfSetDs Opt_WarnUnbangedStrictPatterns (unlifted_must_be_bang bind)
            -- Complain about a binding that looks lazy
            --    e.g.    let I# y = x in ...
            -- Remember, in checkStrictBinds we are going to do strict
            -- matching, so (for software engineering reasons) we insist
            -- that the strictness is manifest on each binding
            -- However, lone (unboxed) variables are ok

                ; dsUnliftedBind bind body }
      where
        is_polymorphic (AbsBinds { abs_tvs = tvs, abs_ev_vars = evs })
                         = not (null tvs && null evs)
        is_polymorphic (AbsBindsSig { abs_tvs = tvs, abs_ev_vars = evs })
                         = not (null tvs && null evs)
        is_polymorphic _ = False

        unlifted_must_be_bang bind
          = hang (text "Pattern bindings containing unlifted types should use" $$
                  text "an outermost bang pattern:")
               2 (ppr bind)

        poly_bind_err bind
          = hang (text "You can't mix polymorphic and unlifted bindings:")
               2 (ppr bind) $$
            text "Probable fix: add a type signature"

    ds_val_bind (is_rec, binds) _body
      | anyBag (isUnliftedHsBind . unLoc) binds  -- see Note [Strict binds checks] in DsBinds
      = ASSERT( isRec is_rec )
        errDsCoreExpr $
        hang (text "Recursive bindings for unlifted types aren't allowed:")
           2 (vcat (map ppr (bagToList binds)))

    -- Ordinary case for bindings; none should be unlifted
    ds_val_bind (is_rec, binds) body
      = do  { MASSERT( isRec is_rec || isSingletonBag binds )
                   -- we should never produce a non-recursive list of multiple binds

            ; (force_vars,prs) <- dsLHsBinds binds
            ; let body' = foldr seqVar body force_vars
            ; ASSERT2( not (any (isUnliftedType . idType . fst) prs), ppr is_rec $$ ppr binds )
              case prs of
                [] -> return body
                _  -> return (Let (Rec prs) body') }
            -- Use a Rec regardless of is_rec.
            -- Why? Because it allows the binds to be all
            -- mixed up, which is what happens in one rare case
            -- Namely, for an AbsBind with no tyvars and no dicts,
            --         but which does have dictionary bindings.
            -- See notes with TcSimplify.inferLoop [NO TYVARS]
            -- It turned out that wrapping a Rec here was the easiest solution
            --
            -- NB The previous case dealt with unlifted bindings, so we
            --    only have to deal with lifted ones now; so Rec is ok

    ------------------
    dsUnliftedBind :: HsBind Id -> CoreExpr -> DsM CoreExpr
    dsUnliftedBind (AbsBinds { abs_tvs = [], abs_ev_vars = []
                   , abs_exports = exports
                   , abs_ev_binds = ev_binds
                   , abs_binds = lbinds }) body
      = do { let body1 = foldr bind_export body exports
                 bind_export export b = bindNonRec (abe_poly export) (Var (abe_mono export)) b
           ; body2 <- foldlBagM (\body lbind -> dsUnliftedBind (unLoc lbind) body)
                                body1 lbinds
           ; ds_binds <- dsTcEvBinds_s ev_binds
           ; return (mkCoreLets ds_binds body2) }

    dsUnliftedBind (AbsBindsSig { abs_tvs         = []
                                , abs_ev_vars     = []
                                , abs_sig_export  = poly
                                , abs_sig_ev_bind = ev_bind
                                , abs_sig_bind    = L _ bind }) body
      = do { ds_binds <- dsTcEvBinds ev_bind
           ; body' <- dsUnliftedBind (bind { fun_id = noLoc poly }) body
           ; return (mkCoreLets ds_binds body') }

    dsUnliftedBind (FunBind { fun_id = L l fun
                            , fun_matches = matches
                            , fun_co_fn = co_fn
                            , fun_tick = tick }) body
                   -- Can't be a bang pattern (that looks like a PatBind)
                   -- so must be simply unboxed
      = do { (args, rhs) <- matchWrapper (mkPrefixFunRhs (L l $ idName fun))
                                         Nothing matches
           ; MASSERT( null args ) -- Functions aren't lifted
           ; MASSERT( isIdHsWrapper co_fn )
           ; let rhs' = mkOptTickBox tick rhs
           ; return (bindNonRec fun rhs' body) }

    dsUnliftedBind (PatBind {pat_lhs = pat, pat_rhs = grhss, pat_rhs_ty = ty }) body
      =     -- let C x# y# = rhs in body
            -- ==> case rhs of C x# y# -> body
        do { rhs <- dsGuarded grhss ty
           ; let upat = unLoc pat
                 eqn = EqnInfo { eqn_pats = [upat],
                                 eqn_rhs = cantFailMatchResult body }
           ; var    <- selectMatchVar upat
           ; result <- matchEquations PatBindRhs [var] [eqn] (exprType body)
           ; return (bindNonRec var rhs result) }

    dsUnliftedBind bind body = pprPanic "dsLet: unlifted" (ppr bind $$ ppr body)

    {-
    ************************************************************************
    *                                                                      *
    \subsection[DsExpr-vars-and-cons]{Variables, constructors, literals}
    *                                                                      *
    ************************************************************************
    -}

    dsLExpr :: LHsExpr Id -> DsM CoreExpr

    dsLExpr (L loc e)
      = putSrcSpanDs loc $
        do { core_expr <- dsExpr e
       -- uncomment this check to test the hsExprType function in TcHsSyn
       --    ; MASSERT2( exprType core_expr `eqType` hsExprType e
       --              , ppr e <+> dcolon <+> ppr (hsExprType e) $$
       --                ppr core_expr <+> dcolon <+> ppr (exprType core_expr) )
           ; return core_expr }

    -- | Variant of 'dsLExpr' that ensures that the result is not levity
    -- polymorphic. This should be used when the resulting expression will
    -- be an argument to some other function.
    -- See Note [Levity polymorphism checking] in DsMonad
    -- See Note [Levity polymorphism invariants] in CoreSyn
    dsLExprNoLP :: LHsExpr Id -> DsM CoreExpr
    dsLExprNoLP (L loc e)
      = putSrcSpanDs loc $
        do { e' <- dsExpr e
           ; dsNoLevPolyExpr e' (text "In the type of expression:" <+> ppr e)
           ; return e' }

    dsExpr :: HsExpr Id -> DsM CoreExpr
    dsExpr (HsPar e)              = dsLExpr e
    dsExpr (ExprWithTySigOut e _) = dsLExpr e
    dsExpr (HsVar (L _ var))      = return (varToCoreExpr var)
                                    -- See Note [Desugaring vars]
    dsExpr (HsUnboundVar {})      = panic "dsExpr: HsUnboundVar" -- Typechecker eliminates them
    dsExpr (HsConLikeOut con)     = return (dsConLike con)
    dsExpr (HsIPVar _)            = panic "dsExpr: HsIPVar"
    dsExpr (HsOverLabel{})        = panic "dsExpr: HsOverLabel"
    dsExpr (HsLit lit)            = dsLit lit
    dsExpr (HsOverLit lit)        = dsOverLit lit

    dsExpr (HsWrap co_fn e)
      = do { e' <- dsExpr e
           ; wrap' <- dsHsWrapper co_fn
           ; dflags <- getDynFlags
           ; let wrapped_e = wrap' e'
           ; warnAboutIdentities dflags e' (exprType wrapped_e)
           ; return wrapped_e }

    dsExpr (NegApp (L loc (HsOverLit lit@(OverLit { ol_val = HsIntegral src i })))
                    neg_expr)
      = do { expr' <- putSrcSpanDs loc $ do
              { dflags <- getDynFlags
              ; warnAboutOverflowedLiterals dflags
                                            (lit { ol_val = HsIntegral src (-i) })
              ; dsOverLit' dflags lit }
           ; dsSyntaxExpr neg_expr [expr'] }

    dsExpr (NegApp expr neg_expr)
      = do { expr' <- dsLExpr expr
           ; dsSyntaxExpr neg_expr [expr'] }

    dsExpr (HsLam a_Match)
      = uncurry mkLams <$> matchWrapper LambdaExpr Nothing a_Match

    dsExpr (HsLamCase matches)
      = do { ([discrim_var], matching_code) <- matchWrapper CaseAlt Nothing matches
           ; return $ Lam discrim_var matching_code }

    dsExpr e@(HsApp fun arg)
      = do { fun' <- dsLExpr fun
           ; dsWhenNoErrs (dsLExprNoLP arg)
                          (\arg' -> mkCoreAppDs (text "HsApp" <+> ppr e) fun' arg') }

    dsExpr (HsAppTypeOut e _)
        -- ignore type arguments here; they're in the wrappers instead at this point
      = dsLExpr e

    {-
    Note [Desugaring vars]
    ~~~~~~~~~~~~~~~~~~~~~~
    In one situation we can get a *coercion* variable in a HsVar, namely
    the support method for an equality superclass:
       class (a~b) => C a b where ...
       instance (blah) => C (T a) (T b) where ..
    Then we get
       $dfCT :: forall ab. blah => C (T a) (T b)
       $dfCT ab blah = MkC ($c$p1C a blah) ($cop a blah)

       $c$p1C :: forall ab. blah => (T a ~ T b)
       $c$p1C ab blah = let ...; g :: T a ~ T b = ... } in g

    That 'g' in the 'in' part is an evidence variable, and when
    converting to core it must become a CO.

    Operator sections.  At first it looks as if we can convert
    \begin{verbatim}
            (expr op)
    \end{verbatim}
    to
    \begin{verbatim}
            \x -> op expr x
    \end{verbatim}

    But no!  expr might be a redex, and we can lose laziness badly this
    way.  Consider
    \begin{verbatim}
            map (expr op) xs
    \end{verbatim}
    for example.  So we convert instead to
    \begin{verbatim}
            let y = expr in \x -> op y x
    \end{verbatim}
    If \tr{expr} is actually just a variable, say, then the simplifier
    will sort it out.
    -}

    dsExpr e@(OpApp e1 op _ e2)
      = -- for the type of y, we need the type of op's 2nd argument
        do { op' <- dsLExpr op
           ; dsWhenNoErrs (mapM dsLExprNoLP [e1, e2])
                          (\exprs' -> mkCoreAppsDs (text "opapp" <+> ppr e) op' exprs') }

    dsExpr (SectionL expr op)       -- Desugar (e !) to ((!) e)
      = do { op' <- dsLExpr op
           ; dsWhenNoErrs (dsLExprNoLP expr)
                          (\expr' -> mkCoreAppDs (text "sectionl" <+> ppr expr) op' expr') }

    -- dsLExpr (SectionR op expr)   -- \ x -> op x expr
    dsExpr e@(SectionR op expr) = do
        core_op <- dsLExpr op
        -- for the type of x, we need the type of op's 2nd argument
        let (x_ty:y_ty:_, _) = splitFunTys (exprType core_op)
            -- See comment with SectionL
        y_core <- dsLExpr expr
        dsWhenNoErrs (mapM newSysLocalDsNoLP [x_ty, y_ty])
                     (\[x_id, y_id] -> bindNonRec y_id y_core $
                                       Lam x_id (mkCoreAppsDs (text "sectionr" <+> ppr e)
                                                              core_op [Var x_id, Var y_id]))

    dsExpr (ExplicitTuple tup_args boxity)
      = do { let go (lam_vars, args) (L _ (Missing ty))
                        -- For every missing expression, we need
                        -- another lambda in the desugaring.
                   = do { lam_var <- newSysLocalDsNoLP ty
                        ; return (lam_var : lam_vars, Var lam_var : args) }
                 go (lam_vars, args) (L _ (Present expr))
                        -- Expressions that are present don't generate
                        -- lambdas, just arguments.
                   = do { core_expr <- dsLExprNoLP expr
                        ; return (lam_vars, core_expr : args) }

           ; dsWhenNoErrs (foldM go ([], []) (reverse tup_args))
                    -- The reverse is because foldM goes left-to-right
                          (\(lam_vars, args) -> mkCoreLams lam_vars $
                                                mkCoreTupBoxity boxity args) }

    dsExpr (ExplicitSum alt arity expr types)
      = do { dsWhenNoErrs (dsLExprNoLP expr)
                          (\core_expr -> mkCoreConApps (sumDataCon alt arity)
                                         (map (Type . getRuntimeRep) types ++
                                          map Type types ++
                                          [core_expr]) ) }

    dsExpr (HsSCC _ cc expr@(L loc _)) = do
        dflags <- getDynFlags
        if gopt Opt_SccProfilingOn dflags
          then do
            mod_name <- getModule
            count <- goptM Opt_ProfCountEntries
            uniq <- newUnique
            Tick (ProfNote (mkUserCC (sl_fs cc) mod_name loc uniq) count True)
                   <$> dsLExpr expr
          else dsLExpr expr

    dsExpr (HsCoreAnn _ _ expr)
      = dsLExpr expr

    dsExpr (HsCase discrim matches)
      = do { core_discrim <- dsLExpr discrim
           ; ([discrim_var], matching_code) <- matchWrapper CaseAlt (Just discrim) matches
           ; return (bindNonRec discrim_var core_discrim matching_code) }

    -- Pepe: The binds are in scope in the body but NOT in the binding group
    --       This is to avoid silliness in breakpoints
    dsExpr (HsLet binds body) = do
        body' <- dsLExpr body
        dsLocalBinds binds body'

    -- We need the `ListComp' form to use `deListComp' (rather than the "do" form)
    -- because the interpretation of `stmts' depends on what sort of thing it is.
    --
    dsExpr (HsDo ListComp     (L _ stmts) res_ty) = dsListComp stmts res_ty
    dsExpr (HsDo PArrComp     (L _ stmts) _)      = dsPArrComp (map unLoc stmts)
    dsExpr (HsDo DoExpr       (L _ stmts) _)      = dsDo stmts
    dsExpr (HsDo GhciStmtCtxt (L _ stmts) _)      = dsDo stmts
    dsExpr (HsDo MDoExpr      (L _ stmts) _)      = dsDo stmts
    dsExpr (HsDo MonadComp    (L _ stmts) _)      = dsMonadComp stmts

    dsExpr (HsIf mb_fun guard_expr then_expr else_expr)
      = do { pred <- dsLExpr guard_expr
           ; b1 <- dsLExpr then_expr
           ; b2 <- dsLExpr else_expr
           ; case mb_fun of
               Just fun -> dsSyntaxExpr fun [pred, b1, b2]
               Nothing  -> return $ mkIfThenElse pred b1 b2 }

    dsExpr (HsMultiIf res_ty alts)
      | null alts
      = mkErrorExpr

      | otherwise
      = do { match_result <- liftM (foldr1 combineMatchResults)
                                   (mapM (dsGRHS IfAlt res_ty) alts)
           ; error_expr   <- mkErrorExpr
           ; extractMatchResult match_result error_expr }
      where
        mkErrorExpr = mkErrorAppDs nON_EXHAUSTIVE_GUARDS_ERROR_ID res_ty
                                   (text "multi-way if")

    {-
    \noindent
    \underline{\bf Various data construction things}
                 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    -}

    dsExpr (ExplicitList elt_ty wit xs)
      = dsExplicitList elt_ty wit xs

    -- We desugar [:x1, ..., xn:] as
    --   singletonP x1 +:+ ... +:+ singletonP xn
    --
    dsExpr (ExplicitPArr ty []) = do
        emptyP <- dsDPHBuiltin emptyPVar
        return (Var emptyP `App` Type ty)
    dsExpr (ExplicitPArr ty xs) = do
        singletonP <- dsDPHBuiltin singletonPVar
        appP       <- dsDPHBuiltin appPVar
        xs'        <- mapM dsLExprNoLP xs
        let unary  fn x   = mkApps (Var fn) [Type ty, x]
            binary fn x y = mkApps (Var fn) [Type ty, x, y]

        return . foldr1 (binary appP) $ map (unary singletonP) xs'

    dsExpr (ArithSeq expr witness seq)
      = case witness of
         Nothing -> dsArithSeq expr seq
         Just fl -> do { newArithSeq <- dsArithSeq expr seq
                       ; dsSyntaxExpr fl [newArithSeq] }

    dsExpr (PArrSeq expr (FromTo from to))
      = mkApps <$> dsExpr expr <*> mapM dsLExprNoLP [from, to]

    dsExpr (PArrSeq expr (FromThenTo from thn to))
      = mkApps <$> dsExpr expr <*> mapM dsLExprNoLP [from, thn, to]

    dsExpr (PArrSeq _ _)
      = panic "DsExpr.dsExpr: Infinite parallel array!"
        -- the parser shouldn't have generated it and the renamer and typechecker
        -- shouldn't have let it through

    {-
    Static Pointers
    ~~~~~~~~~~~~~~~

    See Note [Grand plan for static forms] in StaticPtrTable for an overview.

        g = ... static f ...
    ==>
        g = ... makeStatic loc f ...
    -}

    dsExpr (HsStatic _ expr@(L loc _)) = do
        expr_ds <- dsLExprNoLP expr
        let ty = exprType expr_ds
        makeStaticId <- dsLookupGlobalId makeStaticName

        dflags <- getDynFlags
        let (line, col) = case loc of
               RealSrcSpan r -> ( srcLocLine $ realSrcSpanStart r
                                , srcLocCol  $ realSrcSpanStart r
                                )
               _             -> (0, 0)
            srcLoc = mkCoreConApps (tupleDataCon Boxed 2)
                         [ Type intTy              , Type intTy
                         , mkIntExprInt dflags line, mkIntExprInt dflags col
                         ]

        putSrcSpanDs loc $ return $
          mkCoreApps (Var makeStaticId) [ Type ty, srcLoc, expr_ds ]

    {-
    \noindent
    \underline{\bf Record construction and update}
                 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    For record construction we do this (assuming T has three arguments)
    \begin{verbatim}
            T { op2 = e }
    ==>
            let err = /\a -> recConErr a
            T (recConErr t1 "M.hs/230/op1")
              e
              (recConErr t1 "M.hs/230/op3")
    \end{verbatim}
    @recConErr@ then converts its argument string into a proper message
    before printing it as
    \begin{verbatim}
            M.hs, line 230: missing field op1 was evaluated
    \end{verbatim}

    We also handle @C{}@ as valid construction syntax for an unlabelled
    constructor @C@, setting all of @C@'s fields to bottom.
    -}

    dsExpr (RecordCon { rcon_con_expr = con_expr, rcon_flds = rbinds
                      , rcon_con_like = con_like })
      = do { con_expr' <- dsExpr con_expr
           ; let
                 (arg_tys, _) = tcSplitFunTys (exprType con_expr')
                 -- A newtype in the corner should be opaque;
                 -- hence TcType.tcSplitFunTys

                 mk_arg (arg_ty, fl)
                   = case findField (rec_flds rbinds) (flSelector fl) of
                       (rhs:rhss) -> ASSERT( null rhss )
                                     dsLExprNoLP rhs
                       []         -> mkErrorAppDs rEC_CON_ERROR_ID arg_ty (ppr (flLabel fl))
                 unlabelled_bottom arg_ty = mkErrorAppDs rEC_CON_ERROR_ID arg_ty Outputable.empty

                 labels = conLikeFieldLabels con_like

           ; con_args <- if null labels
                         then mapM unlabelled_bottom arg_tys
                         else mapM mk_arg (zipEqual "dsExpr:RecordCon" arg_tys labels)

           ; return (mkCoreApps con_expr' con_args) }

    {-
    Record update is a little harder. Suppose we have the decl:
    \begin{verbatim}
            data T = T1 {op1, op2, op3 :: Int}
                   | T2 {op4, op2 :: Int}
                   | T3
    \end{verbatim}
    Then we translate as follows:
    \begin{verbatim}
            r { op2 = e }
    ===>
            let op2 = e in
            case r of
              T1 op1 _ op3 -> T1 op1 op2 op3
              T2 op4 _     -> T2 op4 op2
              other        -> recUpdError "M.hs/230"
    \end{verbatim}
    It's important that we use the constructor Ids for @T1@, @T2@ etc on the
    RHSs, and do not generate a Core constructor application directly, because the constructor
    might do some argument-evaluation first; and may have to throw away some
    dictionaries.

    Note [Update for GADTs]
    ~~~~~~~~~~~~~~~~~~~~~~~
    Consider
       data T a b where
         T1 :: { f1 :: a } -> T a Int

    Then the wrapper function for T1 has type
       $WT1 :: a -> T a Int
    But if x::T a b, then
       x { f1 = v } :: T a b   (not T a Int!)
    So we need to cast (T a Int) to (T a b).  Sigh.

    -}

    dsExpr expr@(RecordUpd { rupd_expr = record_expr, rupd_flds = fields
                           , rupd_cons = cons_to_upd
                           , rupd_in_tys = in_inst_tys, rupd_out_tys = out_inst_tys
                           , rupd_wrap = dict_req_wrap } )
      | null fields
      = dsLExpr record_expr
      | otherwise
      = ASSERT2( notNull cons_to_upd, ppr expr )

        do  { record_expr' <- dsLExpr record_expr
            ; field_binds' <- mapM ds_field fields
            ; let upd_fld_env :: NameEnv Id -- Maps field name to the LocalId of the field binding
                  upd_fld_env = mkNameEnv [(f,l) | (f,l,_) <- field_binds']

            -- It's important to generate the match with matchWrapper,
            -- and the right hand sides with applications of the wrapper Id
            -- so that everything works when we are doing fancy unboxing on the
            -- constructor arguments.
            ; alts <- mapM (mk_alt upd_fld_env) cons_to_upd
            ; ([discrim_var], matching_code)
                    <- matchWrapper RecUpd Nothing (MG { mg_alts = noLoc alts
                                                       , mg_arg_tys = [in_ty]
                                                       , mg_res_ty = out_ty, mg_origin = FromSource })
                                                       -- FromSource is not strictly right, but we
                                                       -- want incomplete pattern-match warnings

            ; return (add_field_binds field_binds' $
                      bindNonRec discrim_var record_expr' matching_code) }
      where
        ds_field :: LHsRecUpdField Id -> DsM (Name, Id, CoreExpr)
          -- Clone the Id in the HsRecField, because its Name is that
          -- of the record selector, and we must not make that a local binder
          -- else we shadow other uses of the record selector
          -- Hence 'lcl_id'.  Cf Trac #2735
        ds_field (L _ rec_field) = do { rhs <- dsLExpr (hsRecFieldArg rec_field)
                                      ; let fld_id = unLoc (hsRecUpdFieldId rec_field)
                                      ; lcl_id <- newSysLocalDs (idType fld_id)
                                      ; return (idName fld_id, lcl_id, rhs) }

        add_field_binds [] expr = expr
        add_field_binds ((_,b,r):bs) expr = bindNonRec b r (add_field_binds bs expr)

            -- Awkwardly, for families, the match goes
            -- from instance type to family type
        (in_ty, out_ty) =
          case (head cons_to_upd) of
            RealDataCon data_con ->
              let tycon = dataConTyCon data_con in
              (mkTyConApp tycon in_inst_tys, mkFamilyTyConApp tycon out_inst_tys)
            PatSynCon pat_syn ->
              ( patSynInstResTy pat_syn in_inst_tys
              , patSynInstResTy pat_syn out_inst_tys)
        mk_alt upd_fld_env con
          = do { let (univ_tvs, ex_tvs, eq_spec,
                      prov_theta, _req_theta, arg_tys, _) = conLikeFullSig con
                     subst = zipTvSubst univ_tvs in_inst_tys

                    -- I'm not bothering to clone the ex_tvs
               ; eqs_vars   <- mapM newPredVarDs (substTheta subst (eqSpecPreds eq_spec))
               ; theta_vars <- mapM newPredVarDs (substTheta subst prov_theta)
               ; arg_ids    <- newSysLocalsDs (substTysUnchecked subst arg_tys)
               ; let field_labels = conLikeFieldLabels con
                     val_args = zipWithEqual "dsExpr:RecordUpd" mk_val_arg
                                             field_labels arg_ids
                     mk_val_arg fl pat_arg_id
                         = nlHsVar (lookupNameEnv upd_fld_env (flSelector fl) `orElse` pat_arg_id)

                     inst_con = noLoc $ HsWrap wrap (HsConLikeOut con)
                            -- Reconstruct with the WrapId so that unpacking happens
                     -- The order here is because of the order in `TcPatSyn`.
                     wrap = mkWpEvVarApps theta_vars                                <.>
                            dict_req_wrap                                           <.>
                            mkWpTyApps    (mkTyVarTys ex_tvs)                       <.>
                            mkWpTyApps    [ ty
                                          | (tv, ty) <- univ_tvs `zip` out_inst_tys
                                          , not (tv `elemVarEnv` wrap_subst) ]
                     rhs = foldl (\a b -> nlHsApp a b) inst_con val_args

                            -- Tediously wrap the application in a cast
                            -- Note [Update for GADTs]
                     wrapped_rhs =
                      case con of
                        RealDataCon data_con ->
                          let
                            wrap_co =
                              mkTcTyConAppCo Nominal
                                (dataConTyCon data_con)
                                [ lookup tv ty
                                  | (tv,ty) <- univ_tvs `zip` out_inst_tys ]
                            lookup univ_tv ty =
                              case lookupVarEnv wrap_subst univ_tv of
                                Just co' -> co'
                                Nothing  -> mkTcReflCo Nominal ty
                            in if null eq_spec
                                 then rhs
                                 else mkLHsWrap (mkWpCastN wrap_co) rhs
                        -- eq_spec is always null for a PatSynCon
                        PatSynCon _ -> rhs

                     wrap_subst =
                      mkVarEnv [ (tv, mkTcSymCo (mkTcCoVarCo eq_var))
                               | (spec, eq_var) <- eq_spec `zip` eqs_vars
                               , let tv = eqSpecTyVar spec ]

                     req_wrap = dict_req_wrap <.> mkWpTyApps in_inst_tys

                     pat = noLoc $ ConPatOut { pat_con = noLoc con
                                             , pat_tvs = ex_tvs
                                             , pat_dicts = eqs_vars ++ theta_vars
                                             , pat_binds = emptyTcEvBinds
                                             , pat_args = PrefixCon $ map nlVarPat arg_ids
                                             , pat_arg_tys = in_inst_tys
                                             , pat_wrap = req_wrap }
               ; return (mkSimpleMatch RecUpd [pat] wrapped_rhs) }

    -- Here is where we desugar the Template Haskell brackets and escapes

    -- Template Haskell stuff

    dsExpr (HsRnBracketOut _ _) = panic "dsExpr HsRnBracketOut"
    dsExpr (HsTcBracketOut x ps) = dsBracket x ps
    dsExpr (HsSpliceE s)  = pprPanic "dsExpr:splice" (ppr s)

    -- Arrow notation extension
    dsExpr (HsProc pat cmd) = dsProcExpr pat cmd

    -- Hpc Support

    dsExpr (HsTick tickish e) = do
      e' <- dsLExpr e
      return (Tick tickish e')

    -- There is a problem here. The then and else branches
    -- have no free variables, so they are open to lifting.
    -- We need someway of stopping this.
    -- This will make no difference to binary coverage
    -- (did you go here: YES or NO), but will effect accurate
    -- tick counting.

    dsExpr (HsBinTick ixT ixF e) = do
      e2 <- dsLExpr e
      do { ASSERT(exprType e2 `eqType` boolTy)
           mkBinaryTickBox ixT ixF e2
         }

    dsExpr (HsTickPragma _ _ _ expr) = do
      dflags <- getDynFlags
      if gopt Opt_Hpc dflags
        then panic "dsExpr:HsTickPragma"
        else dsLExpr expr

    -- HsSyn constructs that just shouldn't be here:
    dsExpr (ExprWithTySig {})  = panic "dsExpr:ExprWithTySig"
    dsExpr (HsBracket     {})  = panic "dsExpr:HsBracket"
    dsExpr (HsArrApp      {})  = panic "dsExpr:HsArrApp"
    dsExpr (HsArrForm     {})  = panic "dsExpr:HsArrForm"
    dsExpr (EWildPat      {})  = panic "dsExpr:EWildPat"
    dsExpr (EAsPat        {})  = panic "dsExpr:EAsPat"
    dsExpr (EViewPat      {})  = panic "dsExpr:EViewPat"
    dsExpr (ELazyPat      {})  = panic "dsExpr:ELazyPat"
    dsExpr (HsAppType     {})  = panic "dsExpr:HsAppType" -- removed by typechecker
    dsExpr (HsDo          {})  = panic "dsExpr:HsDo"
    dsExpr (HsRecFld      {})  = panic "dsExpr:HsRecFld"

    ------------------------------
    dsSyntaxExpr :: SyntaxExpr Id -> [CoreExpr] -> DsM CoreExpr
    dsSyntaxExpr (SyntaxExpr { syn_expr      = expr
                             , syn_arg_wraps = arg_wraps
                             , syn_res_wrap  = res_wrap })
                 arg_exprs
      = do { fun            <- dsExpr expr
           ; core_arg_wraps <- mapM dsHsWrapper arg_wraps
           ; core_res_wrap  <- dsHsWrapper res_wrap
           ; let wrapped_args = zipWith ($) core_arg_wraps arg_exprs
           ; dsWhenNoErrs (zipWithM_ dsNoLevPolyExpr wrapped_args [ mk_doc n | n <- [1..] ])
                          (\_ -> core_res_wrap (mkApps fun wrapped_args)) }
      where
        mk_doc n = text "In the" <+> speakNth n <+> text "argument of" <+> quotes (ppr expr)

    findField :: [LHsRecField Id arg] -> Name -> [arg]
    findField rbinds sel
      = [hsRecFieldArg fld | L _ fld <- rbinds
                           , sel == idName (unLoc $ hsRecFieldId fld) ]

    {-
    %--------------------------------------------------------------------

    Note [Desugaring explicit lists]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Explicit lists are desugared in a cleverer way to prevent some
    fruitless allocations.  Essentially, whenever we see a list literal
    [x_1, ..., x_n] we generate the corresponding expression in terms of
    build:

    Explicit lists (literals) are desugared to allow build/foldr fusion when
    beneficial. This is a bit of a trade-off,

     * build/foldr fusion can generate far larger code than the corresponding
       cons-chain (e.g. see #11707)

     * even when it doesn't produce more code, build can still fail to fuse,
       requiring that the simplifier do more work to bring the expression
       back into cons-chain form; this costs compile time

     * when it works, fusion can be a significant win. Allocations are reduced
       by up to 25% in some nofib programs. Specifically,

            Program           Size    Allocs   Runtime  CompTime
            rewrite          +0.0%    -26.3%      0.02     -1.8%
               ansi          -0.3%    -13.8%      0.00     +0.0%
               lift          +0.0%     -8.7%      0.00     -2.3%

    At the moment we use a simple heuristic to determine whether build will be
    fruitful: for small lists we assume the benefits of fusion will be worthwhile;
    for long lists we assume that the benefits will be outweighted by the cost of
    code duplication. This magic length threshold is @maxBuildLength@. Also, fusion
    won't work at all if rewrite rules are disabled, so we don't use the build-based
    desugaring in this case.

    We used to have a more complex heuristic which would try to break the list into
    "static" and "dynamic" parts and only build-desugar the dynamic part.
    Unfortunately, determining "static-ness" reliably is a bit tricky and the
    heuristic at times produced surprising behavior (see #11710) so it was dropped.
    -}

    {- | The longest list length which we will desugar using @build@.

    This is essentially a magic number and its setting is unfortunate rather
    arbitrary. The idea here, as mentioned in Note [Desugaring explicit lists],
    is to avoid deforesting large static data into large(r) code. Ideally we'd
    want a smaller threshold with larger consumers and vice-versa, but we have no
    way of knowing what will be consuming our list in the desugaring impossible to
    set generally correctly.

    The effect of reducing this number will be that 'build' fusion is applied
    less often. From a runtime performance perspective, applying 'build' more
    liberally on "moderately" sized lists should rarely hurt and will often it can
    only expose further optimization opportunities; if no fusion is possible it will
    eventually get rule-rewritten back to a list). We do, however, pay in compile
    time.
    -}
    maxBuildLength :: Int
    maxBuildLength = 32

    dsExplicitList :: Type -> Maybe (SyntaxExpr Id) -> [LHsExpr Id]
                   -> DsM CoreExpr
    -- See Note [Desugaring explicit lists]
    dsExplicitList elt_ty Nothing xs
      = do { dflags <- getDynFlags
           ; xs' <- mapM dsLExprNoLP xs
           ; if length xs' > maxBuildLength
                    -- Don't generate builds if the list is very long.
             || length xs' == 0
                    -- Don't generate builds when the [] constructor will do
             || not (gopt Opt_EnableRewriteRules dflags)  -- Rewrite rules off
                    -- Don't generate a build if there are no rules to eliminate it!
                    -- See Note [Desugaring RULE left hand sides] in Desugar
             then return $ mkListExpr elt_ty xs'
             else mkBuildExpr elt_ty (mk_build_list xs') }
      where
        mk_build_list xs' (cons, _) (nil, _)
          = return (foldr (App . App (Var cons)) (Var nil) xs')

    dsExplicitList elt_ty (Just fln) xs
      = do { list <- dsExplicitList elt_ty Nothing xs
           ; dflags <- getDynFlags
           ; dsSyntaxExpr fln [mkIntExprInt dflags (length xs), list] }

    dsArithSeq :: PostTcExpr -> (ArithSeqInfo Id) -> DsM CoreExpr
    dsArithSeq expr (From from)
      = App <$> dsExpr expr <*> dsLExprNoLP from
    dsArithSeq expr (FromTo from to)
      = do dflags <- getDynFlags
           warnAboutEmptyEnumerations dflags from Nothing to
           expr' <- dsExpr expr
           from' <- dsLExprNoLP from
           to'   <- dsLExprNoLP to
           return $ mkApps expr' [from', to']
    dsArithSeq expr (FromThen from thn)
      = mkApps <$> dsExpr expr <*> mapM dsLExprNoLP [from, thn]
    dsArithSeq expr (FromThenTo from thn to)
      = do dflags <- getDynFlags
           warnAboutEmptyEnumerations dflags from (Just thn) to
           expr' <- dsExpr expr
           from' <- dsLExprNoLP from
           thn'  <- dsLExprNoLP thn
           to'   <- dsLExprNoLP to
           return $ mkApps expr' [from', thn', to']

    {-
    Desugar 'do' and 'mdo' expressions (NOT list comprehensions, they're
    handled in DsListComp).  Basically does the translation given in the
    Haskell 98 report:
    -}

    dsDo :: [ExprLStmt Id] -> DsM CoreExpr
    dsDo stmts
      = goL stmts
      where
        goL [] = panic "dsDo"
        goL (L loc stmt:lstmts) = putSrcSpanDs loc (go loc stmt lstmts)

        go _ (LastStmt body _ _) stmts
          = ASSERT( null stmts ) dsLExpr body
            -- The 'return' op isn't used for 'do' expressions

        go _ (BodyStmt rhs then_expr _ _) stmts
          = do { rhs2 <- dsLExpr rhs
               ; warnDiscardedDoBindings rhs (exprType rhs2)
               ; rest <- goL stmts
               ; dsSyntaxExpr then_expr [rhs2, rest] }

        go _ (LetStmt binds) stmts
          = do { rest <- goL stmts
               ; dsLocalBinds binds rest }

        go _ (BindStmt pat rhs bind_op fail_op res1_ty) stmts
          = do  { body     <- goL stmts
                ; rhs'     <- dsLExpr rhs
                ; var   <- selectSimpleMatchVarL pat
                ; match <- matchSinglePat (Var var) (StmtCtxt DoExpr) pat
                                          res1_ty (cantFailMatchResult body)
                ; match_code <- handle_failure pat match fail_op
                ; dsSyntaxExpr bind_op [rhs', Lam var match_code] }

        go _ (ApplicativeStmt args mb_join body_ty) stmts
          = do {
                 let
                   (pats, rhss) = unzip (map (do_arg . snd) args)

                   do_arg (ApplicativeArgOne pat expr) =
                     (pat, dsLExpr expr)
                   do_arg (ApplicativeArgMany stmts ret pat) =
                     (pat, dsDo (stmts ++ [noLoc $ mkLastStmt (noLoc ret)]))

                   arg_tys = map hsLPatType pats

               ; rhss' <- sequence rhss

               ; let body' = noLoc $ HsDo DoExpr (noLoc stmts) body_ty

               ; let fun = L noSrcSpan $ HsLam $
                       MG { mg_alts = noLoc [mkSimpleMatch LambdaExpr pats
                                                           body']
                          , mg_arg_tys = arg_tys
                          , mg_res_ty = body_ty
                          , mg_origin = Generated }

               ; fun' <- dsLExpr fun
               ; let mk_ap_call l (op,r) = dsSyntaxExpr op [l,r]
               ; expr <- foldlM mk_ap_call fun' (zip (map fst args) rhss')
               ; case mb_join of
                   Nothing -> return expr
                   Just join_op -> dsSyntaxExpr join_op [expr] }

        go loc (RecStmt { recS_stmts = rec_stmts, recS_later_ids = later_ids
                        , recS_rec_ids = rec_ids, recS_ret_fn = return_op
                        , recS_mfix_fn = mfix_op, recS_bind_fn = bind_op
                        , recS_bind_ty = bind_ty
                        , recS_rec_rets = rec_rets, recS_ret_ty = body_ty }) stmts
          = goL (new_bind_stmt : stmts)  -- rec_ids can be empty; eg  rec { print 'x' }
          where
            new_bind_stmt = L loc $ BindStmt (mkBigLHsPatTupId later_pats)
                                             mfix_app bind_op
                                             noSyntaxExpr  -- Tuple cannot fail
                                             bind_ty

            tup_ids      = rec_ids ++ filterOut (`elem` rec_ids) later_ids
            tup_ty       = mkBigCoreTupTy (map idType tup_ids) -- Deals with singleton case
            rec_tup_pats = map nlVarPat tup_ids
            later_pats   = rec_tup_pats
            rets         = map noLoc rec_rets
            mfix_app     = nlHsSyntaxApps mfix_op [mfix_arg]
            mfix_arg     = noLoc $ HsLam
                               (MG { mg_alts = noLoc [mkSimpleMatch
                                                        LambdaExpr
                                                        [mfix_pat] body]
                                   , mg_arg_tys = [tup_ty], mg_res_ty = body_ty
                                   , mg_origin = Generated })
            mfix_pat     = noLoc $ LazyPat $ mkBigLHsPatTupId rec_tup_pats
            body         = noLoc $ HsDo
                                    DoExpr (noLoc (rec_stmts ++ [ret_stmt])) body_ty
            ret_app      = nlHsSyntaxApps return_op [mkBigLHsTupId rets]
            ret_stmt     = noLoc $ mkLastStmt ret_app
                         -- This LastStmt will be desugared with dsDo,
                         -- which ignores the return_op in the LastStmt,
                         -- so we must apply the return_op explicitly

        go _ (ParStmt   {}) _ = panic "dsDo ParStmt"
        go _ (TransStmt {}) _ = panic "dsDo TransStmt"

    handle_failure :: LPat Id -> MatchResult -> SyntaxExpr Id -> DsM CoreExpr
        -- In a do expression, pattern-match failure just calls
        -- the monadic 'fail' rather than throwing an exception
    handle_failure pat match fail_op
      | matchCanFail match
      = do { dflags <- getDynFlags
           ; fail_msg <- mkStringExpr (mk_fail_msg dflags pat)
           ; fail_expr <- dsSyntaxExpr fail_op [fail_msg]
           ; extractMatchResult match fail_expr }
      | otherwise
      = extractMatchResult match (error "It can't fail")

    mk_fail_msg :: DynFlags -> Located e -> String
    mk_fail_msg dflags pat = "Pattern match failure in do expression at " ++
                             showPpr dflags (getLoc pat)

    {-
    ************************************************************************
    *                                                                      *
       Desugaring ConLikes
    *                                                                      *
    ************************************************************************
    -}

    dsConLike :: ConLike -> CoreExpr
    dsConLike (RealDataCon dc) = Var (dataConWrapId dc)
    dsConLike (PatSynCon ps) = case patSynBuilder ps of
      Just (id, add_void)
        | add_void  -> mkCoreApp (text "dsConLike" <+> ppr ps) (Var id) (Var voidPrimId)
        | otherwise -> Var id
      _ -> pprPanic "dsConLike" (ppr ps)

    {-
    ************************************************************************
    *                                                                      *
    \subsection{Errors and contexts}
    *                                                                      *
    ************************************************************************
    -}

    -- Warn about certain types of values discarded in monadic bindings (#3263)
    warnDiscardedDoBindings :: LHsExpr Id -> Type -> DsM ()
    warnDiscardedDoBindings rhs rhs_ty
      | Just (m_ty, elt_ty) <- tcSplitAppTy_maybe rhs_ty
      = do { warn_unused <- woptM Opt_WarnUnusedDoBind
           ; warn_wrong <- woptM Opt_WarnWrongDoBind
           ; when (warn_unused || warn_wrong) $
        do { fam_inst_envs <- dsGetFamInstEnvs
           ; let norm_elt_ty = topNormaliseType fam_inst_envs elt_ty

               -- Warn about discarding non-() things in 'monadic' binding
           ; if warn_unused && not (isUnitTy norm_elt_ty)
             then warnDs (Reason Opt_WarnUnusedDoBind)
                         (badMonadBind rhs elt_ty)
             else

               -- Warn about discarding m a things in 'monadic' binding of the same type,
               -- but only if we didn't already warn due to Opt_WarnUnusedDoBind
               when warn_wrong $
                    do { case tcSplitAppTy_maybe norm_elt_ty of
                             Just (elt_m_ty, _)
                                | m_ty `eqType` topNormaliseType fam_inst_envs elt_m_ty
                                -> warnDs (Reason Opt_WarnWrongDoBind)
                                          (badMonadBind rhs elt_ty)
                             _ -> return () } } }

      | otherwise   -- RHS does have type of form (m ty), which is weird
      = return ()   -- but at lesat this warning is irrelevant

    badMonadBind :: LHsExpr Id -> Type -> SDoc
    badMonadBind rhs elt_ty
      = vcat [ hang (text "A do-notation statement discarded a result of type")
                  2 (quotes (ppr elt_ty))
             , hang (text "Suppress this warning by saying")
                  2 (quotes $ text "_ <-" <+> ppr rhs)
             ]