deSugar/DsBinds.hs

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

    Pattern-matching bindings (HsBinds and MonoBinds)

    Handles @HsBinds@; those at the top level require different handling,
    in that the @Rec@/@NonRec@/etc structure is thrown away (whereas at
    lower levels it is preserved with @let@/@letrec@s).
    -}

    {-# LANGUAGE CPP #-}

    module DsBinds ( dsTopLHsBinds, dsLHsBinds, decomposeRuleLhs, dsSpec,
                     dsHsWrapper, dsTcEvBinds, dsTcEvBinds_s, dsEvBinds, dsMkUserRule
      ) where

    #include "HsVersions.h"

    import {-# SOURCE #-}   DsExpr( dsLExpr )
    import {-# SOURCE #-}   Match( matchWrapper )

    import DsMonad
    import DsGRHSs
    import DsUtils

    import HsSyn            -- lots of things
    import CoreSyn          -- lots of things
    import Literal          ( Literal(MachStr) )
    import CoreOpt          ( simpleOptExpr )
    import OccurAnal        ( occurAnalyseExpr )
    import MkCore
    import CoreUtils
    import CoreArity ( etaExpand )
    import CoreUnfold
    import CoreFVs
    import Digraph

    import PrelNames
    import TyCon
    import TcEvidence
    import TcType
    import Type
    import Coercion
    import TysWiredIn ( typeNatKind, typeSymbolKind )
    import Id
    import MkId(proxyHashId)
    import Class
    import Name
    import VarSet
    import Rules
    import VarEnv
    import Outputable
    import Module
    import SrcLoc
    import Maybes
    import OrdList
    import Bag
    import BasicTypes
    import DynFlags
    import FastString
    import Util
    import MonadUtils
    import qualified GHC.LanguageExtensions as LangExt
    import Control.Monad

    {-**********************************************************************
    *                                                                      *
               Desugaring a MonoBinds
    *                                                                      *
    **********************************************************************-}

    -- | Desugar top level binds, strict binds are treated like normal
    -- binds since there is no good time to force before first usage.
    dsTopLHsBinds :: LHsBinds Id -> DsM (OrdList (Id,CoreExpr))
    dsTopLHsBinds binds
         -- see Note [Strict binds checks]
      | not (isEmptyBag unlifted_binds) || not (isEmptyBag bang_binds)
      = do { mapBagM_ (top_level_err "bindings for unlifted types") unlifted_binds
           ; mapBagM_ (top_level_err "strict pattern bindings")    bang_binds
           ; return nilOL }

      | otherwise
      = do { (force_vars, prs) <- dsLHsBinds binds
           ; when debugIsOn $
             do { xstrict <- xoptM LangExt.Strict
                ; MASSERT2( null force_vars || xstrict, ppr binds $$ ppr force_vars ) }
                  -- with -XStrict, even top-level vars are listed as force vars.

           ; return (toOL prs) }

      where
        unlifted_binds = filterBag (isUnliftedHsBind . unLoc) binds
        bang_binds     = filterBag (isBangedPatBind  . unLoc) binds

        top_level_err desc (L loc bind)
          = putSrcSpanDs loc $
            errDs (hang (text "Top-level" <+> text desc <+> text "aren't allowed:")
                      2 (ppr bind))

    -- | Desugar all other kind of bindings, Ids of strict binds are returned to
    -- later be forced in the binding group body, see Note [Desugar Strict binds]
    dsLHsBinds :: LHsBinds Id -> DsM ([Id], [(Id,CoreExpr)])
    dsLHsBinds binds
      = do { MASSERT( allBag (not . isUnliftedHsBind . unLoc) binds )
           ; ds_bs <- mapBagM dsLHsBind binds
           ; return (foldBag (\(a, a') (b, b') -> (a ++ b, a' ++ b'))
                             id ([], []) ds_bs) }

    ------------------------
    dsLHsBind :: LHsBind Id
              -> DsM ([Id], [(Id,CoreExpr)])
    dsLHsBind (L loc bind) = do dflags <- getDynFlags
                                putSrcSpanDs loc $ dsHsBind dflags bind

    -- | Desugar a single binding (or group of recursive binds).
    dsHsBind :: DynFlags
             -> HsBind Id
             -> DsM ([Id], [(Id,CoreExpr)])
             -- ^ The Ids of strict binds, to be forced in the body of the
             -- binding group see Note [Desugar Strict binds] and all
             -- bindings and their desugared right hand sides.

    dsHsBind dflags
             (VarBind { var_id = var
                      , var_rhs = expr
                      , var_inline = inline_regardless })
      = do  { core_expr <- dsLExpr expr
                    -- Dictionary bindings are always VarBinds,
                    -- so we only need do this here
            ; let var' | inline_regardless = var `setIdUnfolding` mkCompulsoryUnfolding core_expr
                       | otherwise         = var
            ; let core_bind@(id,_) = makeCorePair dflags var' False 0 core_expr
                  force_var = if xopt LangExt.Strict dflags
                              then [id]
                              else []
            ; return (force_var, [core_bind]) }

    dsHsBind dflags
             b@(FunBind { fun_id = L _ fun, fun_matches = matches
                        , fun_co_fn = co_fn, fun_tick = tick })
     = do   { (args, body) <- matchWrapper
                               (mkPrefixFunRhs (noLoc $ idName fun))
                               Nothing matches
            ; core_wrap <- dsHsWrapper co_fn
            ; let body' = mkOptTickBox tick body
                  rhs   = core_wrap (mkLams args body')
                  core_binds@(id,_) = makeCorePair dflags fun False 0 rhs
                  force_var
                      -- Bindings are strict when -XStrict is enabled
                    | xopt LangExt.Strict dflags
                    , matchGroupArity matches == 0 -- no need to force lambdas
                    = [id]
                    | isBangedBind b
                    = [id]
                    | otherwise
                    = []
            ; --pprTrace "dsHsBind" (ppr fun <+> ppr (idInlinePragma fun) $$ ppr (mg_alts matches) $$ ppr args $$ ppr core_binds) $
               return (force_var, [core_binds]) }

    dsHsBind dflags
             (PatBind { pat_lhs = pat, pat_rhs = grhss, pat_rhs_ty = ty
                      , pat_ticks = (rhs_tick, var_ticks) })
      = do  { body_expr <- dsGuarded grhss ty
            ; let body' = mkOptTickBox rhs_tick body_expr
                  pat'  = decideBangHood dflags pat
            ; (force_var,sel_binds) <- mkSelectorBinds var_ticks pat body'
              -- We silently ignore inline pragmas; no makeCorePair
              -- Not so cool, but really doesn't matter
            ; let force_var' = if isBangedLPat pat'
                               then [force_var]
                               else []
            ; return (force_var', sel_binds) }

            -- A common case: one exported variable, only non-strict binds
            -- Non-recursive bindings come through this way
            -- So do self-recursive bindings
            -- Bindings with complete signatures are AbsBindsSigs, below
    dsHsBind dflags
             (AbsBinds { abs_tvs = tyvars, abs_ev_vars = dicts
                       , abs_exports = [export]
                       , abs_ev_binds = ev_binds, abs_binds = binds })
      | ABE { abe_wrap = wrap, abe_poly = global
            , abe_mono = local, abe_prags = prags } <- export
      , not (xopt LangExt.Strict dflags)             -- Handle strict binds
      , not (anyBag (isBangedBind . unLoc) binds)    --        in the next case
      = -- See Note [AbsBinds wrappers] in HsBinds
        addDictsDs (toTcTypeBag (listToBag dicts)) $
             -- addDictsDs: push type constraints deeper for pattern match check
        do { (force_vars, bind_prs) <- dsLHsBinds binds
           ; let core_bind = Rec bind_prs
           ; ds_binds <- dsTcEvBinds_s ev_binds
           ; core_wrap <- dsHsWrapper wrap -- Usually the identity

           ; let rhs = core_wrap $
                       mkLams tyvars $ mkLams dicts $
                       mkCoreLets ds_binds $
                       mkLet core_bind $
                       Var local
           ; (spec_binds, rules) <- dsSpecs rhs prags

           ; let   global'  = addIdSpecialisations global rules
                   main_bind = makeCorePair dflags global' (isDefaultMethod prags)
                                            (dictArity dicts) rhs

           ; ASSERT(null force_vars)
             return ([], main_bind : fromOL spec_binds) }

            -- Another common case: no tyvars, no dicts
            -- In this case we can have a much simpler desugaring
    dsHsBind dflags
             (AbsBinds { abs_tvs = [], abs_ev_vars = []
                       , abs_exports = exports
                       , abs_ev_binds = ev_binds, abs_binds = binds })
      = do { (force_vars, bind_prs) <- dsLHsBinds binds
           ; let mk_bind (ABE { abe_wrap = wrap
                              , abe_poly = global
                              , abe_mono = local
                              , abe_prags = prags })
                  = do { core_wrap <- dsHsWrapper wrap
                       ; return (makeCorePair dflags global
                                              (isDefaultMethod prags)
                                              0 (core_wrap (Var local))) }
           ; main_binds <- mapM mk_bind exports

           ; ds_binds <- dsTcEvBinds_s ev_binds
           ; return (force_vars, flattenBinds ds_binds ++ bind_prs ++ main_binds) }

    dsHsBind dflags
             (AbsBinds { abs_tvs = tyvars, abs_ev_vars = dicts
                       , abs_exports = exports, abs_ev_binds = ev_binds
                       , abs_binds = binds })
             -- See Note [Desugaring AbsBinds]
      = addDictsDs (toTcTypeBag (listToBag dicts)) $
             -- addDictsDs: push type constraints deeper for pattern match check
         do { (local_force_vars, bind_prs) <- dsLHsBinds binds
            ; let core_bind = Rec [ makeCorePair dflags (add_inline lcl_id) False 0 rhs
                                  | (lcl_id, rhs) <- bind_prs ]
                    -- Monomorphic recursion possible, hence Rec
                  new_force_vars = get_new_force_vars local_force_vars
                  locals       = map abe_mono exports
                  all_locals   = locals ++ new_force_vars
                  tup_expr     = mkBigCoreVarTup all_locals
                  tup_ty       = exprType tup_expr
            ; ds_binds <- dsTcEvBinds_s ev_binds
            ; let poly_tup_rhs = mkLams tyvars $ mkLams dicts $
                                 mkCoreLets ds_binds $
                                 mkLet core_bind $
                                 tup_expr

            ; poly_tup_id <- newSysLocalDs (exprType poly_tup_rhs)

            -- Find corresponding global or make up a new one: sometimes
            -- we need to make new export to desugar strict binds, see
            -- Note [Desugar Strict binds]
            ; (exported_force_vars, extra_exports) <- get_exports local_force_vars

            ; let mk_bind (ABE { abe_wrap = wrap
                               , abe_poly = global
                               , abe_mono = local, abe_prags = spec_prags })
                             -- See Note [AbsBinds wrappers] in HsBinds
                    = do { tup_id  <- newSysLocalDs tup_ty
                         ; core_wrap <- dsHsWrapper wrap
                         ; let rhs = core_wrap $ mkLams tyvars $ mkLams dicts $
                                     mkTupleSelector all_locals local tup_id $
                                     mkVarApps (Var poly_tup_id) (tyvars ++ dicts)
                               rhs_for_spec = Let (NonRec poly_tup_id poly_tup_rhs) rhs
                         ; (spec_binds, rules) <- dsSpecs rhs_for_spec spec_prags
                         ; let global' = (global `setInlinePragma` defaultInlinePragma)
                                                 `addIdSpecialisations` rules
                               -- Kill the INLINE pragma because it applies to
                               -- the user written (local) function.  The global
                               -- Id is just the selector.  Hmm.
                         ; return ((global', rhs) : fromOL spec_binds) }

            ; export_binds_s <- mapM mk_bind (exports ++ extra_exports)

            ; return (exported_force_vars
                     ,(poly_tup_id, poly_tup_rhs) :
                       concat export_binds_s) }
      where
        inline_env :: IdEnv Id   -- Maps a monomorphic local Id to one with
                                 -- the inline pragma from the source
                                 -- The type checker put the inline pragma
                                 -- on the *global* Id, so we need to transfer it
        inline_env
          = mkVarEnv [ (lcl_id, setInlinePragma lcl_id prag)
                     | ABE { abe_mono = lcl_id, abe_poly = gbl_id } <- exports
                     , let prag = idInlinePragma gbl_id ]

        add_inline :: Id -> Id    -- tran
        add_inline lcl_id = lookupVarEnv inline_env lcl_id
                            `orElse` lcl_id

        global_env :: IdEnv Id -- Maps local Id to its global exported Id
        global_env =
          mkVarEnv [ (local, global)
                   | ABE { abe_mono = local, abe_poly = global } <- exports
                   ]

        -- find variables that are not exported
        get_new_force_vars lcls =
          foldr (\lcl acc -> case lookupVarEnv global_env lcl of
                               Just _ -> acc
                               Nothing -> lcl:acc)
                [] lcls

        -- find exports or make up new exports for force variables
        get_exports :: [Id] -> DsM ([Id], [ABExport Id])
        get_exports lcls =
          foldM (\(glbls, exports) lcl ->
                  case lookupVarEnv global_env lcl of
                    Just glbl -> return (glbl:glbls, exports)
                    Nothing   -> do export <- mk_export lcl
                                    let glbl = abe_poly export
                                    return (glbl:glbls, export:exports))
                ([],[]) lcls

        mk_export local =
          do global <- newSysLocalDs
                         (exprType (mkLams tyvars (mkLams dicts (Var local))))
             return (ABE {abe_poly = global
                         ,abe_mono = local
                         ,abe_wrap = WpHole
                         ,abe_prags = SpecPrags []})

    -- AbsBindsSig is a combination of AbsBinds and FunBind
    dsHsBind dflags (AbsBindsSig { abs_tvs = tyvars, abs_ev_vars = dicts
                                 , abs_sig_export  = global
                                 , abs_sig_prags   = prags
                                 , abs_sig_ev_bind = ev_bind
                                 , abs_sig_bind    = bind })
      | L bind_loc FunBind { fun_matches = matches
                           , fun_co_fn   = co_fn
                           , fun_tick    = tick } <- bind
      = putSrcSpanDs bind_loc $
        addDictsDs (toTcTypeBag (listToBag dicts)) $
                 -- addDictsDs: push type constraints deeper for pattern match check
        do { (args, body) <- matchWrapper
                               (mkPrefixFunRhs (noLoc $ idName global))
                               Nothing matches
           ; core_wrap <- dsHsWrapper co_fn
           ; let body'   = mkOptTickBox tick body
                 fun_rhs = core_wrap (mkLams args body')
                 force_vars
                   | xopt LangExt.Strict dflags
                   , matchGroupArity matches == 0 -- no need to force lambdas
                   = [global]
                   | isBangedBind (unLoc bind)
                   = [global]
                   | otherwise
                   = []

           ; ds_binds <- dsTcEvBinds ev_bind
           ; let rhs = mkLams tyvars $
                       mkLams dicts $
                       mkCoreLets ds_binds $
                       fun_rhs

           ; (spec_binds, rules) <- dsSpecs rhs prags
           ; let global' = addIdSpecialisations global rules
                 main_bind = makeCorePair dflags global' (isDefaultMethod prags)
                                          (dictArity dicts) rhs

           ; return (force_vars, main_bind : fromOL spec_binds) }

      | otherwise
      = pprPanic "dsHsBind: AbsBindsSig" (ppr bind)

    dsHsBind _ (PatSynBind{}) = panic "dsHsBind: PatSynBind"

    -- | This is where we apply INLINE and INLINABLE pragmas. All we need to
    -- do is to attach the unfolding information to the Id.
    --
    -- Other decisions about whether to inline are made in
    -- `calcUnfoldingGuidance` but the decision about whether to then expose
    -- the unfolding in the interface file is made in `TidyPgm.addExternal`
    -- using this information.
    ------------------------
    makeCorePair :: DynFlags -> Id -> Bool -> Arity -> CoreExpr -> (Id, CoreExpr)
    makeCorePair dflags gbl_id is_default_method dict_arity rhs
      | is_default_method                 -- Default methods are *always* inlined
      = (gbl_id `setIdUnfolding` mkCompulsoryUnfolding rhs, rhs)

      | otherwise
      = case inlinePragmaSpec inline_prag of
              EmptyInlineSpec -> (gbl_id, rhs)
              NoInline        -> (gbl_id, rhs)
              Inlinable       -> (gbl_id `setIdUnfolding` inlinable_unf, rhs)
              Inline          -> inline_pair

      where
        inline_prag   = idInlinePragma gbl_id
        inlinable_unf = mkInlinableUnfolding dflags rhs
        inline_pair
           | Just arity <- inlinePragmaSat inline_prag
            -- Add an Unfolding for an INLINE (but not for NOINLINE)
            -- And eta-expand the RHS; see Note [Eta-expanding INLINE things]
           , let real_arity = dict_arity + arity
            -- NB: The arity in the InlineRule takes account of the dictionaries
           = ( gbl_id `setIdUnfolding` mkInlineUnfoldingWithArity real_arity rhs
             , etaExpand real_arity rhs)

           | otherwise
           = pprTrace "makeCorePair: arity missing" (ppr gbl_id) $
             (gbl_id `setIdUnfolding` mkInlineUnfolding rhs, rhs)

    dictArity :: [Var] -> Arity
    -- Don't count coercion variables in arity
    dictArity dicts = count isId dicts

    {-
    Note [Desugaring AbsBinds]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~
    In the general AbsBinds case we desugar the binding to this:

           tup a (d:Num a) = let fm = ...gm...
                                 gm = ...fm...
                             in (fm,gm)
           f a d = case tup a d of { (fm,gm) -> fm }
           g a d = case tup a d of { (fm,gm) -> fm }

    Note [Rules and inlining]
    ~~~~~~~~~~~~~~~~~~~~~~~~~
    Common special case: no type or dictionary abstraction
    This is a bit less trivial than you might suppose
    The naive way woudl be to desguar to something like
            f_lcl = ...f_lcl...     -- The "binds" from AbsBinds
            M.f = f_lcl             -- Generated from "exports"
    But we don't want that, because if M.f isn't exported,
    it'll be inlined unconditionally at every call site (its rhs is
    trivial).  That would be ok unless it has RULES, which would
    thereby be completely lost.  Bad, bad, bad.

    Instead we want to generate
            M.f = ...f_lcl...
            f_lcl = M.f
    Now all is cool. The RULES are attached to M.f (by SimplCore),
    and f_lcl is rapidly inlined away.

    This does not happen in the same way to polymorphic binds,
    because they desugar to
            M.f = /\a. let f_lcl = ...f_lcl... in f_lcl
    Although I'm a bit worried about whether full laziness might
    float the f_lcl binding out and then inline M.f at its call site

    Note [Specialising in no-dict case]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Even if there are no tyvars or dicts, we may have specialisation pragmas.
    Class methods can generate
          AbsBinds [] [] [( ... spec-prag]
             { AbsBinds [tvs] [dicts] ...blah }
    So the overloading is in the nested AbsBinds. A good example is in GHC.Float:

      class  (Real a, Fractional a) => RealFrac a  where
        round :: (Integral b) => a -> b

      instance  RealFrac Float  where
        {-# SPECIALIZE round :: Float -> Int #-}

    The top-level AbsBinds for $cround has no tyvars or dicts (because the
    instance does not).  But the method is locally overloaded!

    Note [Abstracting over tyvars only]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    When abstracting over type variable only (not dictionaries), we don't really need to
    built a tuple and select from it, as we do in the general case. Instead we can take

            AbsBinds [a,b] [ ([a,b], fg, fl, _),
                             ([b],   gg, gl, _) ]
                    { fl = e1
                      gl = e2
                       h = e3 }

    and desugar it to

            fg = /\ab. let B in e1
            gg = /\b. let a = () in let B in S(e2)
            h  = /\ab. let B in e3

    where B is the *non-recursive* binding
            fl = fg a b
            gl = gg b
            h  = h a b    -- See (b); note shadowing!

    Notice (a) g has a different number of type variables to f, so we must
                 use the mkArbitraryType thing to fill in the gaps.
                 We use a type-let to do that.

             (b) The local variable h isn't in the exports, and rather than
                 clone a fresh copy we simply replace h by (h a b), where
                 the two h's have different types!  Shadowing happens here,
                 which looks confusing but works fine.

             (c) The result is *still* quadratic-sized if there are a lot of
                 small bindings.  So if there are more than some small
                 number (10), we filter the binding set B by the free
                 variables of the particular RHS.  Tiresome.

    Why got to this trouble?  It's a common case, and it removes the
    quadratic-sized tuple desugaring.  Less clutter, hopefully faster
    compilation, especially in a case where there are a *lot* of
    bindings.

    Note [Eta-expanding INLINE things]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Consider
       foo :: Eq a => a -> a
       {-# INLINE foo #-}
       foo x = ...

    If (foo d) ever gets floated out as a common sub-expression (which can
    happen as a result of method sharing), there's a danger that we never
    get to do the inlining, which is a Terribly Bad thing given that the
    user said "inline"!

    To avoid this we pre-emptively eta-expand the definition, so that foo
    has the arity with which it is declared in the source code.  In this
    example it has arity 2 (one for the Eq and one for x). Doing this
    should mean that (foo d) is a PAP and we don't share it.

    Note [Nested arities]
    ~~~~~~~~~~~~~~~~~~~~~
    For reasons that are not entirely clear, method bindings come out looking like
    this:

      AbsBinds [] [] [$cfromT <= [] fromT]
        $cfromT [InlPrag=INLINE] :: T Bool -> Bool
        { AbsBinds [] [] [fromT <= [] fromT_1]
            fromT :: T Bool -> Bool
            { fromT_1 ((TBool b)) = not b } } }

    Note the nested AbsBind.  The arity for the InlineRule on $cfromT should be
    gotten from the binding for fromT_1.

    It might be better to have just one level of AbsBinds, but that requires more
    thought!

    Note [Desugar Strict binds]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~

    Desugaring strict variable bindings looks as follows (core below ==>)

      let !x = rhs
      in  body
    ==>
      let x = rhs
      in x `seq` body -- seq the variable

    and if it is a pattern binding the desugaring looks like

      let !pat = rhs
      in body
    ==>
      let x = rhs -- bind the rhs to a new variable
          pat = x
      in x `seq` body -- seq the new variable

    if there is no variable in the pattern desugaring looks like

      let False = rhs
      in body
    ==>
      let x = case rhs of {False -> (); _ -> error "Match failed"}
      in x `seq` body

    In order to force the Ids in the binding group they are passed around
    in the dsHsBind family of functions, and later seq'ed in DsExpr.ds_val_bind.

    Consider a recursive group like this

      letrec
         f : g = rhs[f,g]
      in <body>

    Without `Strict`, we get a translation like this:

      let t = /\a. letrec tm = rhs[fm,gm]
                          fm = case t of fm:_ -> fm
                          gm = case t of _:gm -> gm
                    in
                    (fm,gm)

      in let f = /\a. case t a of (fm,_) -> fm
      in let g = /\a. case t a of (_,gm) -> gm
      in <body>

    Here `tm` is the monomorphic binding for `rhs`.

    With `Strict`, we want to force `tm`, but NOT `fm` or `gm`.
    Alas, `tm` isn't in scope in the `in <body>` part.

    The simplest thing is to return it in the polymorphic
    tuple `t`, thus:

      let t = /\a. letrec tm = rhs[fm,gm]
                          fm = case t of fm:_ -> fm
                          gm = case t of _:gm -> gm
                    in
                    (tm, fm, gm)

      in let f = /\a. case t a of (_,fm,_) -> fm
      in let g = /\a. case t a of (_,_,gm) -> gm
      in let tm = /\a. case t a of (tm,_,_) -> tm
      in tm `seq` <body>

    See https://ghc.haskell.org/trac/ghc/wiki/StrictPragma for a more
    detailed explanation of the desugaring of strict bindings.

    Note [Strict binds checks]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~
    There are several checks around properly formed strict bindings. They
    all link to this Note. These checks must be here in the desugarer because
    we cannot know whether or not a type is unlifted until after zonking, due
    to levity polymorphism. These checks all used to be handled in the typechecker
    in checkStrictBinds (before Jan '17).

    We define an "unlifted bind" to be any bind that binds an unlifted id. Note that

      x :: Char
      (# True, x #) = blah

    is *not* an unlifted bind. Unlifted binds are detected by HsUtils.isUnliftedHsBind.

    Define a "banged bind" to have a top-level bang. Detected by HsPat.isBangedPatBind.
    Define a "strict bind" to be either an unlifted bind or a banged bind.

    The restrictions are:
      1. Strict binds may not be top-level. Checked in dsTopLHsBinds.

      2. Unlifted binds must also be banged. (There is no trouble to compile an unbanged
         unlifted bind, but an unbanged bind looks lazy, and we don't want users to be
         surprised by the strictness of an unlifted bind.) Checked in first clause
         of DsExpr.ds_val_bind.

      3. Unlifted binds may not have polymorphism (#6078). (That is, no quantified type
         variables or constraints.) Checked in first clause
         of DsExpr.ds_val_bind.

      4. Unlifted binds may not be recursive. Checked in second clause of ds_val_bind.

    -}

    ------------------------
    dsSpecs :: CoreExpr     -- Its rhs
            -> TcSpecPrags
            -> DsM ( OrdList (Id,CoreExpr)  -- Binding for specialised Ids
                   , [CoreRule] )           -- Rules for the Global Ids
    -- See Note [Handling SPECIALISE pragmas] in TcBinds
    dsSpecs _ IsDefaultMethod = return (nilOL, [])
    dsSpecs poly_rhs (SpecPrags sps)
      = do { pairs <- mapMaybeM (dsSpec (Just poly_rhs)) sps
           ; let (spec_binds_s, rules) = unzip pairs
           ; return (concatOL spec_binds_s, rules) }

    dsSpec :: Maybe CoreExpr        -- Just rhs => RULE is for a local binding
                                    -- Nothing => RULE is for an imported Id
                                    --            rhs is in the Id's unfolding
           -> Located TcSpecPrag
           -> DsM (Maybe (OrdList (Id,CoreExpr), CoreRule))
    dsSpec mb_poly_rhs (L loc (SpecPrag poly_id spec_co spec_inl))
      | isJust (isClassOpId_maybe poly_id)
      = putSrcSpanDs loc $
        do { warnDs NoReason (text "Ignoring useless SPECIALISE pragma for class method selector"
                              <+> quotes (ppr poly_id))
           ; return Nothing  }  -- There is no point in trying to specialise a class op
                                -- Moreover, classops don't (currently) have an inl_sat arity set
                                -- (it would be Just 0) and that in turn makes makeCorePair bleat

      | no_act_spec && isNeverActive rule_act
      = putSrcSpanDs loc $
        do { warnDs NoReason (text "Ignoring useless SPECIALISE pragma for NOINLINE function:"
                              <+> quotes (ppr poly_id))
           ; return Nothing  }  -- Function is NOINLINE, and the specialiation inherits that
                                -- See Note [Activation pragmas for SPECIALISE]

      | otherwise
      = putSrcSpanDs loc $
        do { uniq <- newUnique
           ; let poly_name = idName poly_id
                 spec_occ  = mkSpecOcc (getOccName poly_name)
                 spec_name = mkInternalName uniq spec_occ (getSrcSpan poly_name)
                 (spec_bndrs, spec_app) = collectHsWrapBinders spec_co
                   -- spec_co looks like
                   --         \spec_bndrs. [] spec_args
                   -- perhaps with the body of the lambda wrapped in some WpLets
                   -- E.g. /\a \(d:Eq a). let d2 = $df d in [] (Maybe a) d2

           ; core_app <- dsHsWrapper spec_app

           ; let ds_lhs  = core_app (Var poly_id)
                 spec_ty = mkLamTypes spec_bndrs (exprType ds_lhs)
           ; -- pprTrace "dsRule" (vcat [ text "Id:" <+> ppr poly_id
             --                         , text "spec_co:" <+> ppr spec_co
             --                         , text "ds_rhs:" <+> ppr ds_lhs ]) $
             case decomposeRuleLhs spec_bndrs ds_lhs of {
               Left msg -> do { warnDs NoReason msg; return Nothing } ;
               Right (rule_bndrs, _fn, args) -> do

           { dflags <- getDynFlags
           ; this_mod <- getModule
           ; let fn_unf    = realIdUnfolding poly_id
                 spec_unf  = specUnfolding spec_bndrs core_app arity_decrease fn_unf
                 spec_id   = mkLocalId spec_name spec_ty
                                `setInlinePragma` inl_prag
                                `setIdUnfolding`  spec_unf
                 arity_decrease = count isValArg args - count isId spec_bndrs

           ; rule <- dsMkUserRule this_mod is_local_id
                            (mkFastString ("SPEC " ++ showPpr dflags poly_name))
                            rule_act poly_name
                            rule_bndrs args
                            (mkVarApps (Var spec_id) spec_bndrs)

           ; let spec_rhs = mkLams spec_bndrs (core_app poly_rhs)

    -- Commented out: see Note [SPECIALISE on INLINE functions]
    --       ; when (isInlinePragma id_inl)
    --              (warnDs $ text "SPECIALISE pragma on INLINE function probably won't fire:"
    --                        <+> quotes (ppr poly_name))

           ; return (Just (unitOL (spec_id, spec_rhs), rule))
                -- NB: do *not* use makeCorePair on (spec_id,spec_rhs), because
                --     makeCorePair overwrites the unfolding, which we have
                --     just created using specUnfolding
           } } }
      where
        is_local_id = isJust mb_poly_rhs
        poly_rhs | Just rhs <-  mb_poly_rhs
                 = rhs          -- Local Id; this is its rhs
                 | Just unfolding <- maybeUnfoldingTemplate (realIdUnfolding poly_id)
                 = unfolding    -- Imported Id; this is its unfolding
                                -- Use realIdUnfolding so we get the unfolding
                                -- even when it is a loop breaker.
                                -- We want to specialise recursive functions!
                 | otherwise = pprPanic "dsImpSpecs" (ppr poly_id)
                                -- The type checker has checked that it *has* an unfolding

        id_inl = idInlinePragma poly_id

        -- See Note [Activation pragmas for SPECIALISE]
        inl_prag | not (isDefaultInlinePragma spec_inl)    = spec_inl
                 | not is_local_id  -- See Note [Specialising imported functions]
                                     -- in OccurAnal
                 , isStrongLoopBreaker (idOccInfo poly_id) = neverInlinePragma
                 | otherwise                               = id_inl
         -- Get the INLINE pragma from SPECIALISE declaration, or,
         -- failing that, from the original Id

        spec_prag_act = inlinePragmaActivation spec_inl

        -- See Note [Activation pragmas for SPECIALISE]
        -- no_act_spec is True if the user didn't write an explicit
        -- phase specification in the SPECIALISE pragma
        no_act_spec = case inlinePragmaSpec spec_inl of
                        NoInline -> isNeverActive  spec_prag_act
                        _        -> isAlwaysActive spec_prag_act
        rule_act | no_act_spec = inlinePragmaActivation id_inl   -- Inherit
                 | otherwise   = spec_prag_act                   -- Specified by user

    dsMkUserRule :: Module -> Bool -> RuleName -> Activation
           -> Name -> [CoreBndr] -> [CoreExpr] -> CoreExpr -> DsM CoreRule
    dsMkUserRule this_mod is_local name act fn bndrs args rhs = do
        let rule = mkRule this_mod False is_local name act fn bndrs args rhs
        dflags <- getDynFlags
        when (isOrphan (ru_orphan rule) && wopt Opt_WarnOrphans dflags) $
            warnDs (Reason Opt_WarnOrphans) (ruleOrphWarn rule)
        return rule

    ruleOrphWarn :: CoreRule -> SDoc
    ruleOrphWarn rule = text "Orphan rule:" <+> ppr rule

    {- Note [SPECIALISE on INLINE functions]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    We used to warn that using SPECIALISE for a function marked INLINE
    would be a no-op; but it isn't!  Especially with worker/wrapper split
    we might have
       {-# INLINE f #-}
       f :: Ord a => Int -> a -> ...
       f d x y = case x of I# x' -> $wf d x' y

    We might want to specialise 'f' so that we in turn specialise '$wf'.
    We can't even /name/ '$wf' in the source code, so we can't specialise
    it even if we wanted to.  Trac #10721 is a case in point.

    Note [Activation pragmas for SPECIALISE]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    From a user SPECIALISE pragma for f, we generate
      a) A top-level binding    spec_fn = rhs
      b) A RULE                 f dOrd = spec_fn

    We need two pragma-like things:

    * spec_fn's inline pragma: inherited from f's inline pragma (ignoring
                               activation on SPEC), unless overriden by SPEC INLINE

    * Activation of RULE: from SPECIALISE pragma (if activation given)
                          otherwise from f's inline pragma

    This is not obvious (see Trac #5237)!

    Examples      Rule activation   Inline prag on spec'd fn
    ---------------------------------------------------------------------
    SPEC [n] f :: ty            [n]   Always, or NOINLINE [n]
                                      copy f's prag

    NOINLINE f
    SPEC [n] f :: ty            [n]   NOINLINE
                                      copy f's prag

    NOINLINE [k] f
    SPEC [n] f :: ty            [n]   NOINLINE [k]
                                      copy f's prag

    INLINE [k] f
    SPEC [n] f :: ty            [n]   INLINE [k]
                                      copy f's prag

    SPEC INLINE [n] f :: ty     [n]   INLINE [n]
                                      (ignore INLINE prag on f,
                                      same activation for rule and spec'd fn)

    NOINLINE [k] f
    SPEC f :: ty                [n]   INLINE [k]

    ************************************************************************
    *                                                                      *
    \subsection{Adding inline pragmas}
    *                                                                      *
    ************************************************************************
    -}

    decomposeRuleLhs :: [Var] -> CoreExpr -> Either SDoc ([Var], Id, [CoreExpr])
    -- (decomposeRuleLhs bndrs lhs) takes apart the LHS of a RULE,
    -- The 'bndrs' are the quantified binders of the rules, but decomposeRuleLhs
    -- may add some extra dictionary binders (see Note [Free dictionaries])
    --
    -- Returns an error message if the LHS isn't of the expected shape
    -- Note [Decomposing the left-hand side of a RULE]
    decomposeRuleLhs orig_bndrs orig_lhs
      | not (null unbound)    -- Check for things unbound on LHS
                              -- See Note [Unused spec binders]
      = Left (vcat (map dead_msg unbound))
      | Var funId <- fun2
      , Just con <- isDataConId_maybe funId
      = Left (constructor_msg con) -- See Note [No RULES on datacons]
      | Just (fn_id, args) <- decompose fun2 args2
      , let extra_bndrs = mk_extra_bndrs fn_id args
      = -- pprTrace "decmposeRuleLhs" (vcat [ text "orig_bndrs:" <+> ppr orig_bndrs
        --                                  , text "orig_lhs:" <+> ppr orig_lhs
        --                                  , text "lhs1:"     <+> ppr lhs1
        --                                  , text "extra_dict_bndrs:" <+> ppr extra_dict_bndrs
        --                                  , text "fn_id:" <+> ppr fn_id
        --                                  , text "args:"   <+> ppr args]) $
        Right (orig_bndrs ++ extra_bndrs, fn_id, args)

      | otherwise
      = Left bad_shape_msg
     where
       lhs1         = drop_dicts orig_lhs
       lhs2         = simpleOptExpr lhs1  -- See Note [Simplify rule LHS]
       (fun2,args2) = collectArgs lhs2

       lhs_fvs    = exprFreeVars lhs2
       unbound    = filterOut (`elemVarSet` lhs_fvs) orig_bndrs

       orig_bndr_set = mkVarSet orig_bndrs

            -- Add extra tyvar binders: Note [Free tyvars in rule LHS]
            -- and extra dict binders: Note [Free dictionaries in rule LHS]
       mk_extra_bndrs fn_id args
         = toposortTyVars unbound_tvs ++ unbound_dicts
         where
           unbound_tvs   = [ v | v <- unbound_vars, isTyVar v ]
           unbound_dicts = [ mkLocalId (localiseName (idName d)) (idType d)
                           | d <- unbound_vars, isDictId d ]
           unbound_vars  = [ v | v <- exprsFreeVarsList args
                               , not (v `elemVarSet` orig_bndr_set)
                               , not (v == fn_id) ]
             -- fn_id: do not quantify over the function itself, which may
             -- itself be a dictionary (in pathological cases, Trac #10251)

       decompose (Var fn_id) args
          | not (fn_id `elemVarSet` orig_bndr_set)
          = Just (fn_id, args)

       decompose _ _ = Nothing

       bad_shape_msg = hang (text "RULE left-hand side too complicated to desugar")
                          2 (vcat [ text "Optimised lhs:" <+> ppr lhs2
                                  , text "Orig lhs:" <+> ppr orig_lhs])
       dead_msg bndr = hang (sep [ text "Forall'd" <+> pp_bndr bndr
                                 , text "is not bound in RULE lhs"])
                          2 (vcat [ text "Orig bndrs:" <+> ppr orig_bndrs
                                  , text "Orig lhs:" <+> ppr orig_lhs
                                  , text "optimised lhs:" <+> ppr lhs2 ])
       pp_bndr bndr
        | isTyVar bndr                      = text "type variable" <+> quotes (ppr bndr)
        | Just pred <- evVarPred_maybe bndr = text "constraint" <+> quotes (ppr pred)
        | otherwise                         = text "variable" <+> quotes (ppr bndr)

       constructor_msg con = vcat
         [ text "A constructor," <+> ppr con <>
             text ", appears as outermost match in RULE lhs."
         , text "This rule will be ignored." ]

       drop_dicts :: CoreExpr -> CoreExpr
       drop_dicts e
           = wrap_lets needed bnds body
         where
           needed = orig_bndr_set `minusVarSet` exprFreeVars body
           (bnds, body) = split_lets (occurAnalyseExpr e)
               -- The occurAnalyseExpr drops dead bindings which is
               -- crucial to ensure that every binding is used later;
               -- which in turn makes wrap_lets work right

       split_lets :: CoreExpr -> ([(DictId,CoreExpr)], CoreExpr)
       split_lets (Let (NonRec d r) body)
         | isDictId d
         = ((d,r):bs, body')
         where (bs, body') = split_lets body

        -- handle "unlifted lets" too, needed for "map/coerce"
       split_lets (Case r d _ [(DEFAULT, _, body)])
         | isCoVar d
         = ((d,r):bs, body')
         where (bs, body') = split_lets body

       split_lets e = ([], e)

       wrap_lets :: VarSet -> [(DictId,CoreExpr)] -> CoreExpr -> CoreExpr
       wrap_lets _ [] body = body
       wrap_lets needed ((d, r) : bs) body
         | rhs_fvs `intersectsVarSet` needed = mkCoreLet (NonRec d r) (wrap_lets needed' bs body)
         | otherwise                         = wrap_lets needed bs body
         where
           rhs_fvs = exprFreeVars r
           needed' = (needed `minusVarSet` rhs_fvs) `extendVarSet` d

    {-
    Note [Decomposing the left-hand side of a RULE]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    There are several things going on here.
    * drop_dicts: see Note [Drop dictionary bindings on rule LHS]
    * simpleOptExpr: see Note [Simplify rule LHS]
    * extra_dict_bndrs: see Note [Free dictionaries]

    Note [Free tyvars on rule LHS]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Consider
      data T a = C

      foo :: T a -> Int
      foo C = 1

      {-# RULES "myrule"  foo C = 1 #-}

    After type checking the LHS becomes (foo alpha (C alpha)), where alpha
    is an unbound meta-tyvar.  The zonker in TcHsSyn is careful not to
    turn the free alpha into Any (as it usually does).  Instead it turns it
    into a TyVar 'a'.  See TcHsSyn Note [Zonking the LHS of a RULE].

    Now we must quantify over that 'a'.  It's /really/ inconvenient to do that
    in the zonker, because the HsExpr data type is very large.  But it's /easy/
    to do it here in the desugarer.

    Moreover, we have to do something rather similar for dictionaries;
    see Note [Free dictionaries on rule LHS].   So that's why we look for
    type variables free on the LHS, and quantify over them.

    Note [Free dictionaries on rule LHS]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    When the LHS of a specialisation rule, (/\as\ds. f es) has a free dict,
    which is presumably in scope at the function definition site, we can quantify
    over it too.  *Any* dict with that type will do.

    So for example when you have
            f :: Eq a => a -> a
            f = <rhs>
            ... SPECIALISE f :: Int -> Int ...

    Then we get the SpecPrag
            SpecPrag (f Int dInt)

    And from that we want the rule

            RULE forall dInt. f Int dInt = f_spec
            f_spec = let f = <rhs> in f Int dInt

    But be careful!  That dInt might be GHC.Base.$fOrdInt, which is an External
    Name, and you can't bind them in a lambda or forall without getting things
    confused.   Likewise it might have an InlineRule or something, which would be
    utterly bogus. So we really make a fresh Id, with the same unique and type
    as the old one, but with an Internal name and no IdInfo.

    Note [Drop dictionary bindings on rule LHS]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    drop_dicts drops dictionary bindings on the LHS where possible.
       E.g.  let d:Eq [Int] = $fEqList $fEqInt in f d
         --> f d
       Reasoning here is that there is only one d:Eq [Int], and so we can
       quantify over it. That makes 'd' free in the LHS, but that is later
       picked up by extra_dict_bndrs (Note [Dead spec binders]).

       NB 1: We can only drop the binding if the RHS doesn't bind
             one of the orig_bndrs, which we assume occur on RHS.
             Example
                f :: (Eq a) => b -> a -> a
                {-# SPECIALISE f :: Eq a => b -> [a] -> [a] #-}
             Here we want to end up with
                RULE forall d:Eq a.  f ($dfEqList d) = f_spec d
             Of course, the ($dfEqlist d) in the pattern makes it less likely
             to match, but there is no other way to get d:Eq a

       NB 2: We do drop_dicts *before* simplOptEpxr, so that we expect all
             the evidence bindings to be wrapped around the outside of the
             LHS.  (After simplOptExpr they'll usually have been inlined.)
             dsHsWrapper does dependency analysis, so that civilised ones
             will be simple NonRec bindings.  We don't handle recursive
             dictionaries!

        NB3: In the common case of a non-overloaded, but perhaps-polymorphic
             specialisation, we don't need to bind *any* dictionaries for use
             in the RHS. For example (Trac #8331)
                 {-# SPECIALIZE INLINE useAbstractMonad :: ReaderST s Int #-}
                 useAbstractMonad :: MonadAbstractIOST m => m Int
             Here, deriving (MonadAbstractIOST (ReaderST s)) is a lot of code
             but the RHS uses no dictionaries, so we want to end up with
                 RULE forall s (d :: MonadAbstractIOST (ReaderT s)).
                    useAbstractMonad (ReaderT s) d = $suseAbstractMonad s

       Trac #8848 is a good example of where there are some intersting
       dictionary bindings to discard.

    The drop_dicts algorithm is based on these observations:

      * Given (let d = rhs in e) where d is a DictId,
        matching 'e' will bind e's free variables.

      * So we want to keep the binding if one of the needed variables (for
        which we need a binding) is in fv(rhs) but not already in fv(e).

      * The "needed variables" are simply the orig_bndrs.  Consider
           f :: (Eq a, Show b) => a -> b -> String
           ... SPECIALISE f :: (Show b) => Int -> b -> String ...
        Then orig_bndrs includes the *quantified* dictionaries of the type
        namely (dsb::Show b), but not the one for Eq Int

    So we work inside out, applying the above criterion at each step.

    Note [Simplify rule LHS]
    ~~~~~~~~~~~~~~~~~~~~~~~~
    simplOptExpr occurrence-analyses and simplifies the LHS:

       (a) Inline any remaining dictionary bindings (which hopefully
           occur just once)

       (b) Substitute trivial lets, so that they don't get in the way.
           Note that we substitute the function too; we might
           have this as a LHS:  let f71 = M.f Int in f71

       (c) Do eta reduction.  To see why, consider the fold/build rule,
           which without simplification looked like:
              fold k z (build (/\a. g a))  ==>  ...
           This doesn't match unless you do eta reduction on the build argument.
           Similarly for a LHS like
             augment g (build h)
           we do not want to get
             augment (\a. g a) (build h)
           otherwise we don't match when given an argument like
              augment (\a. h a a) (build h)

    Note [Matching seqId]
    ~~~~~~~~~~~~~~~~~~~
    The desugarer turns (seq e r) into (case e of _ -> r), via a special-case hack
    and this code turns it back into an application of seq!
    See Note [Rules for seq] in MkId for the details.

    Note [Unused spec binders]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~
    Consider
            f :: a -> a
            ... SPECIALISE f :: Eq a => a -> a ...
    It's true that this *is* a more specialised type, but the rule
    we get is something like this:
            f_spec d = f
            RULE: f = f_spec d
    Note that the rule is bogus, because it mentions a 'd' that is
    not bound on the LHS!  But it's a silly specialisation anyway, because
    the constraint is unused.  We could bind 'd' to (error "unused")
    but it seems better to reject the program because it's almost certainly
    a mistake.  That's what the isDeadBinder call detects.

    Note [No RULES on datacons]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~

    Previously, `RULES` like

        "JustNothing" forall x . Just x = Nothing

    were allowed. Simon Peyton Jones says this seems to have been a
    mistake, that such rules have never been supported intentionally,
    and that he doesn't know if they can break in horrible ways.
    Furthermore, Ben Gamari and Reid Barton are considering trying to
    detect the presence of "static data" that the simplifier doesn't
    need to traverse at all. Such rules do not play well with that.
    So for now, we ban them altogether as requested by #13290. See also #7398.

    ************************************************************************
    *                                                                      *
                    Desugaring evidence
    *                                                                      *
    ************************************************************************

    -}

    dsHsWrapper :: HsWrapper -> DsM (CoreExpr -> CoreExpr)
    dsHsWrapper WpHole            = return $ \e -> e
    dsHsWrapper (WpTyApp ty)      = return $ \e -> App e (Type ty)
    dsHsWrapper (WpEvLam ev)      = return $ Lam ev
    dsHsWrapper (WpTyLam tv)      = return $ Lam tv
    dsHsWrapper (WpLet ev_binds)  = do { bs <- dsTcEvBinds ev_binds
                                       ; return (mkCoreLets bs) }
    dsHsWrapper (WpCompose c1 c2) = do { w1 <- dsHsWrapper c1
                                       ; w2 <- dsHsWrapper c2
                                       ; return (w1 . w2) }
     -- See comments on WpFun in TcEvidence for an explanation of what
     -- the specification of this clause is
    dsHsWrapper (WpFun c1 c2 t1 doc)
                                  = do { x  <- newSysLocalDsNoLP t1
                                       ; w1 <- dsHsWrapper c1
                                       ; w2 <- dsHsWrapper c2
                                       ; let app f a = mkCoreAppDs (text "dsHsWrapper") f a
                                             arg     = w1 (Var x)
                                       ; (_, ok) <- askNoErrsDs $ dsNoLevPolyExpr arg doc
                                       ; if ok
                                         then return (\e -> (Lam x (w2 (app e arg))))
                                         else return id }  -- this return is irrelevant
    dsHsWrapper (WpCast co)       = ASSERT(coercionRole co == Representational)
                                    return $ \e -> mkCastDs e co
    dsHsWrapper (WpEvApp tm)      = do { core_tm <- dsEvTerm tm
                                       ; return (\e -> App e core_tm) }

    --------------------------------------
    dsTcEvBinds_s :: [TcEvBinds] -> DsM [CoreBind]
    dsTcEvBinds_s []       = return []
    dsTcEvBinds_s (b:rest) = ASSERT( null rest )  -- Zonker ensures null
                             dsTcEvBinds b

    dsTcEvBinds :: TcEvBinds -> DsM [CoreBind]
    dsTcEvBinds (TcEvBinds {}) = panic "dsEvBinds"    -- Zonker has got rid of this
    dsTcEvBinds (EvBinds bs)   = dsEvBinds bs

    dsEvBinds :: Bag EvBind -> DsM [CoreBind]
    dsEvBinds bs = mapM ds_scc (sccEvBinds bs)
      where
        ds_scc (AcyclicSCC (EvBind { eb_lhs = v, eb_rhs = r}))
                              = liftM (NonRec v) (dsEvTerm r)
        ds_scc (CyclicSCC bs) = liftM Rec (mapM dsEvBind bs)

    dsEvBind :: EvBind -> DsM (Id, CoreExpr)
    dsEvBind (EvBind { eb_lhs = v, eb_rhs = r}) = liftM ((,) v) (dsEvTerm r)

    {-**********************************************************************
    *                                                                      *
               Desugaring EvTerms
    *                                                                      *
    **********************************************************************-}

    dsEvTerm :: EvTerm -> DsM CoreExpr
    dsEvTerm (EvId v)           = return (Var v)
    dsEvTerm (EvCallStack cs)   = dsEvCallStack cs
    dsEvTerm (EvTypeable ty ev) = dsEvTypeable ty ev
    dsEvTerm (EvLit (EvNum n))  = mkNaturalExpr n
    dsEvTerm (EvLit (EvStr s))  = mkStringExprFS s

    dsEvTerm (EvCast tm co)
      = do { tm' <- dsEvTerm tm
           ; return $ mkCastDs tm' co }

    dsEvTerm (EvDFunApp df tys tms)
      = do { tms' <- mapM dsEvTerm tms
           ; return $ Var df `mkTyApps` tys `mkApps` tms' }
      -- The use of mkApps here is OK vis-a-vis levity polymorphism because
      -- the terms are always evidence variables with types of kind Constraint

    dsEvTerm (EvCoercion co) = return (Coercion co)
    dsEvTerm (EvSuperClass d n)
      = do { d' <- dsEvTerm d
           ; let (cls, tys) = getClassPredTys (exprType d')
                 sc_sel_id  = classSCSelId cls n    -- Zero-indexed
           ; return $ Var sc_sel_id `mkTyApps` tys `App` d' }

    dsEvTerm (EvSelector sel_id tys tms)
      = do { tms' <- mapM dsEvTerm tms
           ; return $ Var sel_id `mkTyApps` tys `mkApps` tms' }

    dsEvTerm (EvDelayedError ty msg) = return $ dsEvDelayedError ty msg

    dsEvDelayedError :: Type -> FastString -> CoreExpr
    dsEvDelayedError ty msg
      = Var errorId `mkTyApps` [getRuntimeRep ty, ty] `mkApps` [litMsg]
      where
        errorId = tYPE_ERROR_ID
        litMsg  = Lit (MachStr (fastStringToByteString msg))

    {-**********************************************************************
    *                                                                      *
               Desugaring Typeable dictionaries
    *                                                                      *
    **********************************************************************-}

    dsEvTypeable :: Type -> EvTypeable -> DsM CoreExpr
    -- Return a CoreExpr :: Typeable ty
    -- This code is tightly coupled to the representation
    -- of TypeRep, in base library Data.Typeable.Internals
    dsEvTypeable ty ev
      = do { tyCl <- dsLookupTyCon typeableClassName    -- Typeable
           ; let kind = typeKind ty
                 Just typeable_data_con
                     = tyConSingleDataCon_maybe tyCl    -- "Data constructor"
                                                        -- for Typeable

           ; rep_expr <- ds_ev_typeable ty ev           -- :: TypeRep a

           -- Package up the method as `Typeable` dictionary
           ; return $ mkConApp typeable_data_con [Type kind, Type ty, rep_expr] }

    type TypeRepExpr = CoreExpr

    -- | Returns a @CoreExpr :: TypeRep ty@
    ds_ev_typeable :: Type -> EvTypeable -> DsM CoreExpr
    ds_ev_typeable ty (EvTypeableTyCon tc kind_ev)
      = do { mkTrCon <- dsLookupGlobalId mkTrConName
                        -- mkTrCon :: forall k (a :: k). TyCon -> TypeRep k -> TypeRep a
           ; someTypeRepTyCon <- dsLookupTyCon someTypeRepTyConName
           ; someTypeRepDataCon <- dsLookupDataCon someTypeRepDataConName
                        -- SomeTypeRep :: forall k (a :: k). TypeRep a -> SomeTypeRep

           ; tc_rep <- tyConRep tc                      -- :: TyCon
           ; let ks = tyConAppArgs ty
                 -- Construct a SomeTypeRep
                 toSomeTypeRep :: Type -> EvTerm -> DsM CoreExpr
                 toSomeTypeRep t ev = do
                     rep <- getRep ev t
                     return $ mkCoreConApps someTypeRepDataCon [Type (typeKind t), Type t, rep]
           ; kind_arg_reps <- sequence $ zipWith toSomeTypeRep ks kind_ev   -- :: TypeRep t
           ; let -- :: [SomeTypeRep]
                 kind_args = mkListExpr (mkTyConTy someTypeRepTyCon) kind_arg_reps

             -- Note that we use the kind of the type, not the TyCon from which it
             -- is constructed since the latter may be kind polymorphic whereas the
             -- former we know is not (we checked in the solver).
           ; return $ mkApps (Var mkTrCon) [ Type (typeKind ty)
                                           , Type ty
                                           , tc_rep
                                           , kind_args ]
           }

    ds_ev_typeable ty (EvTypeableTyApp ev1 ev2)
      | Just (t1,t2) <- splitAppTy_maybe ty
      = do { e1  <- getRep ev1 t1
           ; e2  <- getRep ev2 t2
           ; mkTrApp <- dsLookupGlobalId mkTrAppName
                        -- mkTrApp :: forall k1 k2 (a :: k1 -> k2) (b :: k1).
                        --            TypeRep a -> TypeRep b -> TypeRep (a b)
           ; let (k1, k2) = splitFunTy (typeKind t1)
           ; return $ mkApps (mkTyApps (Var mkTrApp) [ k1, k2, t1, t2 ])
                             [ e1, e2 ] }

    ds_ev_typeable ty (EvTypeableTrFun ev1 ev2)
      | Just (t1,t2) <- splitFunTy_maybe ty
      = do { e1 <- getRep ev1 t1
           ; e2 <- getRep ev2 t2
           ; mkTrFun <- dsLookupGlobalId mkTrFunName
                        -- mkTrFun :: forall r1 r2 (a :: TYPE r1) (b :: TYPE r2).
                        --            TypeRep a -> TypeRep b -> TypeRep (a -> b)
           ; let r1 = getRuntimeRep t1
                 r2 = getRuntimeRep t2
           ; return $ mkApps (mkTyApps (Var mkTrFun) [r1, r2, t1, t2])
                             [ e1, e2 ]
           }

    ds_ev_typeable ty (EvTypeableTyLit ev)
      = do { fun  <- dsLookupGlobalId tr_fun
           ; dict <- dsEvTerm ev       -- Of type KnownNat/KnownSym
           ; let proxy = mkTyApps (Var proxyHashId) [ty_kind, ty]
           ; return (mkApps (mkTyApps (Var fun) [ty]) [ dict, proxy ]) }
      where
        ty_kind = typeKind ty

        -- tr_fun is the Name of
        --       typeNatTypeRep    :: KnownNat    a => Proxy# a -> TypeRep a
        -- of    typeSymbolTypeRep :: KnownSymbol a => Proxy# a -> TypeRep a
        tr_fun | ty_kind `eqType` typeNatKind    = typeNatTypeRepName
               | ty_kind `eqType` typeSymbolKind = typeSymbolTypeRepName
               | otherwise = panic "dsEvTypeable: unknown type lit kind"

    ds_ev_typeable ty ev
      = pprPanic "dsEvTypeable" (ppr ty $$ ppr ev)

    getRep :: EvTerm          -- ^ EvTerm for @Typeable ty@
           -> Type            -- ^ The type @ty@
           -> DsM TypeRepExpr -- ^ Return @CoreExpr :: TypeRep ty@
                              -- namely @typeRep# dict@
    -- Remember that
    --   typeRep# :: forall k (a::k). Typeable k a -> TypeRep a
    getRep ev ty
      = do { typeable_expr <- dsEvTerm ev
           ; typeRepId     <- dsLookupGlobalId typeRepIdName
           ; let ty_args = [typeKind ty, ty]
           ; return (mkApps (mkTyApps (Var typeRepId) ty_args) [ typeable_expr ]) }

    tyConRep :: TyCon -> DsM CoreExpr
    -- Returns CoreExpr :: TyCon
    tyConRep tc
      | Just tc_rep_nm <- tyConRepName_maybe tc
      = do { tc_rep_id <- dsLookupGlobalId tc_rep_nm
           ; return (Var tc_rep_id) }
      | otherwise
      = pprPanic "tyConRep" (ppr tc)

    {- Note [Memoising typeOf]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~
    See #3245, #9203

    IMPORTANT: we don't want to recalculate the TypeRep once per call with
    the proxy argument.  This is what went wrong in #3245 and #9203. So we
    help GHC by manually keeping the 'rep' *outside* the lambda.
    -}

    {-**********************************************************************
    *                                                                      *
               Desugaring EvCallStack evidence
    *                                                                      *
    **********************************************************************-}

    dsEvCallStack :: EvCallStack -> DsM CoreExpr
    -- See Note [Overview of implicit CallStacks] in TcEvidence.hs
    dsEvCallStack cs = do
      df            <- getDynFlags
      m             <- getModule
      srcLocDataCon <- dsLookupDataCon srcLocDataConName
      let mkSrcLoc l =
            liftM (mkCoreConApps srcLocDataCon)
                  (sequence [ mkStringExprFS (unitIdFS $ moduleUnitId m)
                            , mkStringExprFS (moduleNameFS $ moduleName m)
                            , mkStringExprFS (srcSpanFile l)
                            , return $ mkIntExprInt df (srcSpanStartLine l)
                            , return $ mkIntExprInt df (srcSpanStartCol l)
                            , return $ mkIntExprInt df (srcSpanEndLine l)
                            , return $ mkIntExprInt df (srcSpanEndCol l)
                            ])

      emptyCS <- Var <$> dsLookupGlobalId emptyCallStackName

      pushCSVar <- dsLookupGlobalId pushCallStackName
      let pushCS name loc rest =
            mkCoreApps (Var pushCSVar) [mkCoreTup [name, loc], rest]

      let mkPush name loc tm = do
            nameExpr <- mkStringExprFS name
            locExpr <- mkSrcLoc loc
            case tm of
              EvCallStack EvCsEmpty -> return (pushCS nameExpr locExpr emptyCS)
              _ -> do tmExpr  <- dsEvTerm tm
                      -- at this point tmExpr :: IP sym CallStack
                      -- but we need the actual CallStack to pass to pushCS,
                      -- so we use unwrapIP to strip the dictionary wrapper
                      -- See Note [Overview of implicit CallStacks]
                      let ip_co = unwrapIP (exprType tmExpr)
                      return (pushCS nameExpr locExpr (mkCastDs tmExpr ip_co))
      case cs of
        EvCsPushCall name loc tm -> mkPush (occNameFS $ getOccName name) loc tm
        EvCsEmpty -> return emptyCS