content: {-
  (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 Eta.DeSugar.DsBinds ( dsTopLHsBinds, dsLHsBinds, decomposeRuleLhs, dsSpec,
                   dsHsWrapper, dsTcEvBinds, dsEvBinds
    ) where
  
  import {-# SOURCE #-}   Eta.DeSugar.DsExpr( dsLExpr )
  import {-# SOURCE #-}   Eta.DeSugar.Match( matchWrapper )
  
  import Eta.DeSugar.DsMonad
  import Eta.DeSugar.DsGRHSs
  import Eta.DeSugar.DsUtils
  import qualified Eta.LanguageExtensions as LangExt
  import Eta.HsSyn.HsSyn            -- lots of things
  import Eta.Core.CoreSyn          -- lots of things
  import Eta.BasicTypes.Literal          ( Literal(MachStr) )
  import Eta.Core.CoreSubst
  import Eta.SimplCore.OccurAnal        ( occurAnalyseExpr )
  import Eta.Core.MkCore
  import Eta.Core.CoreUtils
  import Eta.Core.CoreArity ( etaExpand )
  import Eta.Core.CoreUnfold
  import Eta.Core.CoreFVs
  import Eta.BasicTypes.UniqSupply
  import Eta.Utils.Digraph
  import Eta.Prelude.PrelNames
  import Eta.Prelude.TysPrim ( mkProxyPrimTy )
  import Eta.Types.TyCon      ( isTupleTyCon, tyConDataCons_maybe
                    , tyConName, isPromotedTyCon, isPromotedDataCon, tyConKind )
  import Eta.TypeCheck.TcEvidence
  import Eta.TypeCheck.TcType
  import Eta.Types.Type
  import Eta.Types.Kind (returnsConstraintKind)
  import qualified Eta.Types.Coercion as Coercion
  import Eta.Types.Coercion hiding (substCo)
  import Eta.Prelude.TysWiredIn ( eqBoxDataCon, coercibleDataCon, tupleCon )
  import Eta.BasicTypes.Id
  import Eta.BasicTypes.MkId(proxyHashId)
  import Eta.Types.Class
  import Eta.BasicTypes.DataCon  ( dataConWorkId )
  import Eta.BasicTypes.Name
  import Eta.BasicTypes.IdInfo   ( IdDetails(..) )
  import Eta.BasicTypes.Var
  import Eta.BasicTypes.VarSet
  import Eta.Specialise.Rules
  import Eta.BasicTypes.VarEnv
  import Eta.Utils.Outputable
  import Eta.BasicTypes.Module
  import Eta.BasicTypes.SrcLoc
  import Eta.Utils.Maybes
  import Eta.Utils.OrdList
  import Eta.Utils.Bag
  import Eta.BasicTypes.BasicTypes hiding ( TopLevel )
  import Eta.Main.DynFlags
  import Eta.Utils.FastString
  import Eta.Main.ErrUtils( MsgDoc )
  import Eta.Utils.ListSetOps( getNth )
  import Eta.Utils.Util
  import Control.Monad( when, liftM, foldM )
  import Eta.Utils.MonadUtils
  import Eta.Utils.Fingerprint(Fingerprint(..), fingerprintString)
  
  #include "HsVersions.h"
  
  {-
  ************************************************************************
  *                                                                      *
  \subsection[dsMonoBinds]{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 = fmap (toOL . snd) (ds_lhs_binds binds)
  
  -- | Desugar all other kind of bindings, Ids of strict binds are returned to
  -- later be forced in the binding gorup body, see Note [Desugar Strict binds]
  dsLHsBinds :: LHsBinds Id
             -> DsM ([Id], [(Id,CoreExpr)])
  dsLHsBinds binds = do { (force_vars, binds') <- ds_lhs_binds binds
                        ; return (force_vars, binds') }
  
  ------------------------
  ds_lhs_binds :: LHsBinds Id -> DsM ([Id], [(Id,CoreExpr)])
  
  ds_lhs_binds binds
    = do { 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
          (FunBind { fun_id = L _ fun, fun_matches = matches
                    , fun_co_fn = co_fn, fun_tick = tick
                    , fun_infix = inf })
    = do { (args, body) <- matchWrapper (FunRhs (idName fun) inf) Nothing matches
          ; let body' = mkOptTickBox tick body
          ; rhs <- dsHsWrapper co_fn (mkLams args body')
          ; let core_binds@(id,_) = makeCorePair dflags fun False 0 rhs
                force_var =
                  if xopt LangExt.Strict dflags
                     && matchGroupArity matches == 0 -- no need to force lambdas
                  then [id]
                  else []
          ; {- pprTrace "dsHsBind" (ppr fun <+> ppr (idInlinePragma fun)) $ -}
             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, and recursive bindings
          -- that have been chopped up with type signatures
  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 (isBangedPatBind . unLoc) binds)
     = -- push type constraints deeper for pattern match check
      addDictsDs (toTcTypeBag (listToBag dicts)) $
      do  { (_, bind_prs) <- ds_lhs_binds binds
          ; let core_bind = Rec bind_prs
          ; ds_binds <- dsTcEvBinds ev_binds
          ; rhs <- dsHsWrapper wrap $  -- Usually the identity
                              mkLams tyvars $ mkLams dicts $
                              mkCoreLets ds_binds $
                              Let 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
  
          ; return ([], main_bind : fromOL spec_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]
    = -- push type constraints deeper for pattern match check
      addDictsDs (toTcTypeBag (listToBag dicts)) $
      do { (local_force_vars, bind_prs) <- ds_lhs_binds 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 ev_binds
          ; let poly_tup_rhs = mkLams tyvars $ mkLams dicts $
                               mkCoreLets ds_binds $
                               Let 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 })
                  = do { tup_id  <- newSysLocalDs tup_ty
                       ; rhs <- dsHsWrapper wrap $
                                   mkLams tyvars $ mkLams dicts $
                                   mkTupleSelector all_locals local tup_id $
                                   mkVarApps (Var poly_tup_id) (tyvars ++ dicts)
                       ; let 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 []})
  
  dsHsBind _ (PatSynBind{}) = panic "dsHsBind: PatSynBind"
  
  ------------------------
  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)
  
    | DFunId _ is_newtype <- idDetails gbl_id
    = (mk_dfun_w_stuff is_newtype, 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` mkInlineUnfolding (Just real_arity) rhs
           , etaExpand real_arity rhs)
  
         | otherwise
         = pprTrace "makeCorePair: arity missing" (ppr gbl_id) $
           (gbl_id `setIdUnfolding` mkInlineUnfolding Nothing rhs, rhs)
  
                  -- See Note [ClassOp/DFun selection] in TcInstDcls
                  -- See Note [Single-method classes]  in TcInstDcls
      mk_dfun_w_stuff is_newtype
         | is_newtype
         = gbl_id `setIdUnfolding`  mkInlineUnfolding (Just 0) rhs
                  `setInlinePragma` alwaysInlinePragma { inl_sat = Just 0 }
         | otherwise
         = gbl_id `setIdUnfolding`  mkDFunUnfolding dfun_bndrs dfun_constr dfun_args
                  `setInlinePragma` dfunInlinePragma
      (dfun_bndrs, dfun_body) = collectBinders (simpleOptExpr rhs)
      (dfun_con, dfun_args)   = collectArgs dfun_body
      dfun_constr | Var id <- dfun_con
                  , DataConWorkId con <- idDetails id
                  = con
                  | otherwise = pprPanic "makeCorePair: dfun" (ppr 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 polymoprhic
    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 [Implementing SPECIALISE pragmas]
  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  Example:
          f :: (Eq a, Ix b) => a -> b -> Bool
          {-# SPECIALISE f :: (Ix p, Ix q) => Int -> (p,q) -> Bool #-}
          f = <poly_rhs>
  
  From this the typechecker generates
  
      AbsBinds [ab] [d1,d2] [([ab], f, f_mono, prags)] binds
  
      SpecPrag (wrap_fn :: forall a b. (Eq a, Ix b) => XXX
                        -> forall p q. (Ix p, Ix q) => XXX[ Int/a, (p,q)/b ])
  
  Note that wrap_fn can transform *any* function with the right type prefix
      forall ab. (Eq a, Ix b) => XXX
  regardless of XXX.  It's sort of polymorphic in XXX.  This is
  useful: we use the same wrapper to transform each of the class ops, as
  well as the dict.
  
  From these we generate:
  
      Rule:       forall p, q, (dp:Ix p), (dq:Ix q).
                      f Int (p,q) dInt ($dfInPair dp dq) = f_spec p q dp dq
  
      Spec bind:  f_spec = wrap_fn <poly_rhs>
  
  Note that
  
    * The LHS of the rule may mention dictionary *expressions* (eg
      $dfIxPair dp dq), and that is essential because the dp, dq are
      needed on the RHS.
  
    * The RHS of f_spec, <poly_rhs> has a *copy* of 'binds', so that it
      can fully specialise it.
  -}
  
  ------------------------
  dsSpecs :: CoreExpr     -- Its rhs
          -> TcSpecPrags
          -> DsM ( OrdList (Id,CoreExpr)  -- Binding for specialised Ids
                 , [CoreRule] )           -- Rules for the Global Ids
  -- See Note [Implementing SPECIALISE pragmas]
  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)
         ; (bndrs, ds_lhs) <- liftM collectBinders
                                    (dsHsWrapper spec_co (Var poly_id))
         ; let spec_ty = mkPiTypes bndrs (exprType ds_lhs)
         ; -- pprTrace "dsRule" (vcat [ ptext (sLit "Id:") <+> ppr poly_id
           --                         , ptext (sLit "spec_co:") <+> ppr spec_co
           --                         , ptext (sLit "ds_rhs:") <+> ppr ds_lhs ]) $
           case decomposeRuleLhs bndrs ds_lhs of {
             Left msg -> do { warnDs NoReason msg; return Nothing } ;
             Right (rule_bndrs, _fn, args) -> do
  
         { dflags <- getDynFlags
         ; let fn_unf    = realIdUnfolding poly_id
               unf_fvs   = stableUnfoldingVars fn_unf `orElse` emptyVarSet
               in_scope  = mkInScopeSet (unf_fvs `unionVarSet` exprsFreeVars args)
               spec_unf  = specUnfolding dflags (mkEmptySubst in_scope) bndrs args fn_unf
               spec_id   = mkLocalId spec_name spec_ty
                              `setInlinePragma` inl_prag
                              `setIdUnfolding`  spec_unf
               rule =  mkRule False {- Not auto -} is_local_id
                          (mkFastString ("SPEC " ++ showPpr dflags poly_name))
                          rule_act poly_name
                          rule_bndrs args
                          (mkVarApps (Var spec_id) bndrs)
  
         ; spec_rhs <- dsHsWrapper spec_co poly_rhs
  
         ; when (isInlinePragma id_inl && wopt Opt_WarnPointlessPragmas dflags)
                (warnDs (Reason Opt_WarnPointlessPragmas) (specOnInline 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
  
  
  specOnInline :: Name -> MsgDoc
  specOnInline f = ptext (sLit "SPECIALISE pragma on INLINE function probably won't fire:")
                   <+> quotes (ppr f)
  
  {-
  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 Nothing 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))
  
    | Just (fn_id, args) <- decompose fun2 args2
    , let extra_dict_bndrs = mk_extra_dict_bndrs fn_id args
    = -- pprTrace "decmposeRuleLhs" (vcat [ ptext (sLit "orig_bndrs:") <+> ppr orig_bndrs
      --                                  , ptext (sLit "orig_lhs:") <+> ppr orig_lhs
      --                                  , ptext (sLit "lhs1:")     <+> ppr lhs1
      --                                  , ptext (sLit "extra_dict_bndrs:") <+> ppr extra_dict_bndrs
      --                                  , ptext (sLit "fn_id:") <+> ppr fn_id
      --                                  , ptext (sLit "args:")   <+> ppr args]) $
      Right (orig_bndrs ++ extra_dict_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 dict binders: Note [Free dictionaries]
     mk_extra_dict_bndrs fn_id args
       = [ mkLocalId (localiseName (idName d)) (idType d)
         | d <- varSetElems (exprsFreeVars args `delVarSetList` (fn_id : orig_bndrs))
                -- fn_id: do not quantify over the function itself, which may
                -- itself be a dictionary (in pathological cases, Trac #10251)
         , isDictId d ]
  
     decompose (Var fn_id) args
        | not (fn_id `elemVarSet` orig_bndr_set)
        = Just (fn_id, args)
  
     decompose _ _ = Nothing
  
     bad_shape_msg = hang (ptext (sLit "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 [ ptext (sLit "Forall'd") <+> pp_bndr bndr
                               , ptext (sLit "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                      = ptext (sLit "type variable") <+> quotes (ppr bndr)
      | Just pred <- evVarPred_maybe bndr = ptext (sLit "constraint") <+> quotes (ppr pred)
      | otherwise                         = ptext (sLit "variable") <+> quotes (ppr bndr)
  
     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 e
       | Let (NonRec d r) body <- e
       , isDictId d
       , (bs, body') <- split_lets body
       = ((d,r):bs, body')
       | otherwise
       = ([], e)
  
     wrap_lets :: VarSet -> [(DictId,CoreExpr)] -> CoreExpr -> CoreExpr
     wrap_lets _ [] body = body
     wrap_lets needed ((d, r) : bs) body
       | rhs_fvs `intersectsVarSet` needed = Let (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 [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 ther 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 :: MonadBstractIOST (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 [Free dictionaries]
  ~~~~~~~~~~~~~~~~~~~~~~~~
  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.
  
  
  ************************************************************************
  *                                                                      *
                  Desugaring evidence
  *                                                                      *
  ************************************************************************
  
  -}
  
  dsHsWrapper :: HsWrapper -> CoreExpr -> DsM CoreExpr
  dsHsWrapper WpHole            e = return e
  dsHsWrapper (WpTyApp ty)      e = return $ App e (Type ty)
  dsHsWrapper (WpLet ev_binds)  e = do bs <- dsTcEvBinds ev_binds
                                       return (mkCoreLets bs e)
  dsHsWrapper (WpCompose c1 c2) e = do { e1 <- dsHsWrapper c2 e
                                       ; dsHsWrapper c1 e1 }
  dsHsWrapper (WpFun c1 c2 t1 _) e = do { x <- newSysLocalDs t1
                                        ; e1 <- dsHsWrapper c1 (Var x)
                                        ; e2 <- dsHsWrapper c2 (e `mkCoreAppDs` e1)
                                        ; return (Lam x e2) }
  dsHsWrapper (WpCast co)       e = ASSERT(tcCoercionRole co == Representational)
                                    dsTcCoercion co (mkCast e)
  dsHsWrapper (WpEvLam ev)      e = return $ Lam ev e
  dsHsWrapper (WpTyLam tv)      e = return $ Lam tv e
  dsHsWrapper (WpEvApp    tm)   e = liftM (App e) (dsEvTerm tm)
  
  --------------------------------------
  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 v r)) = liftM (NonRec v) (dsEvTerm r)
      ds_scc (CyclicSCC bs)            = liftM Rec (mapM ds_pair bs)
  
      ds_pair (EvBind v r) = liftM ((,) v) (dsEvTerm r)
  
  sccEvBinds :: Bag EvBind -> [SCC EvBind]
  sccEvBinds bs = stronglyConnCompFromEdgedVertices edges
    where
      edges :: [(EvBind, EvVar, [EvVar])]
      edges = foldrBag ((:) . mk_node) [] bs
  
      mk_node :: EvBind -> (EvBind, EvVar, [EvVar])
      mk_node b@(EvBind var term) = (b, var, varSetElems (evVarsOfTerm term))
  
  
  ---------------------------------------
  dsEvTerm :: EvTerm -> DsM CoreExpr
  dsEvTerm (EvId v) = return (Var v)
  
  dsEvTerm (EvCast tm co)
    = do { tm' <- dsEvTerm tm
         ; dsTcCoercion co $ mkCast tm' }
                          -- 'v' is always a lifted evidence variable so it is
                          -- unnecessary to call varToCoreExpr v here.
  
  dsEvTerm (EvDFunApp df tys tms) = do { tms' <- mapM dsEvTerm tms
                                       ; return (Var df `mkTyApps` tys `mkApps` tms') }
  
  dsEvTerm (EvCoercion (TcCoVarCo v)) = return (Var v)  -- See Note [Simple coercions]
  dsEvTerm (EvCoercion co)            = dsTcCoercion co mkEqBox
  
  dsEvTerm (EvTupleSel tm n)
     = do { tup <- dsEvTerm tm
          ; let scrut_ty  = exprType tup
                (tc, tys) = splitTyConApp scrut_ty
                Just [dc] = tyConDataCons_maybe tc
                xs = mkTemplateLocals tys
                the_x = getNth xs n
          ; ASSERT( isTupleTyCon tc )
            return $
            Case tup (mkWildValBinder scrut_ty) (idType the_x) [(DataAlt dc, xs, Var the_x)] }
  
  dsEvTerm (EvTupleMk tms)
    = do { tms' <- mapM dsEvTerm tms
         ; let tys = map exprType tms'
         ; return $ Var (dataConWorkId dc) `mkTyApps` tys `mkApps` tms' }
    where
      dc = tupleCon ConstraintTuple (length tms)
  
  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' }
    where
  
  dsEvTerm (EvDelayedError ty msg) = return $ Var errorId `mkTyApps` [ty] `mkApps` [litMsg]
    where
      errorId = rUNTIME_ERROR_ID
      litMsg  = Lit (MachStr (fastStringToByteString msg))
  
  dsEvTerm (EvLit l) =
    case l of
      EvNum n -> mkIntegerExpr n
      EvStr s -> mkStringExprFS s
  
  dsEvTerm (EvTypeable ev) = dsEvTypeable ev
  
  dsEvTerm (EvCallStack cs) = dsEvCallStack cs
  
  dsEvTypeable :: EvTypeable -> DsM CoreExpr
  dsEvTypeable ev =
    do tyCl      <- dsLookupTyCon typeableClassName
       typeRepTc <- dsLookupTyCon typeRepTyConName
       let tyRepType = mkTyConApp typeRepTc []
  
       (ty, rep) <-
          case ev of
  
            EvTypeableTyCon tc ks ->
              do ctr       <- dsLookupGlobalId mkPolyTyConAppName
                 mkTyCon   <- dsLookupGlobalId mkTyConName
                 dflags    <- getDynFlags
                 let mkRep cRep kReps tReps =
                       mkApps (Var ctr) [ cRep, mkListExpr tyRepType kReps
                                              , mkListExpr tyRepType tReps ]
  
                 let kindRep k =
                       case splitTyConApp_maybe k of
                         Nothing -> panic "dsEvTypeable: not a kind constructor"
                         Just (kc,ks) ->
                           do kcRep <- tyConRep dflags mkTyCon kc
                              reps  <- mapM kindRep ks
                              return (mkRep kcRep [] reps)
  
                 tcRep     <- tyConRep dflags mkTyCon tc
  
                 kReps     <- mapM kindRep ks
  
                 return ( mkTyConApp tc ks
                        , mkRep tcRep kReps []
                        )
  
            EvTypeableTyApp t1 t2 ->
              do e1  <- getRep tyCl t1
                 e2  <- getRep tyCl t2
                 ctr <- dsLookupGlobalId mkAppTyName
  
                 return ( mkAppTy (snd t1) (snd t2)
                        , mkApps (Var ctr) [ e1, e2 ]
                        )
  
            EvTypeableTyLit ty ->
              do str <- case (isNumLitTy ty, isStrLitTy ty) of
                          (Just n, _) -> return (show n)
                          (_, Just n) -> return (show n)
                          _ -> panic "dsEvTypeable: malformed TyLit evidence"
                 ctr <- dsLookupGlobalId typeLitTypeRepName
                 tag <- mkStringExpr str
                 return (ty, mkApps (Var ctr) [ tag ])
  
       -- TyRep -> Typeable t
       -- see also: Note [Memoising typeOf]
       repName <- newSysLocalDs tyRepType
       let proxyT = mkProxyPrimTy (typeKind ty) ty
           method = bindNonRec repName rep
                  $ mkLams [mkWildValBinder proxyT] (Var repName)
  
       -- package up the method as `Typeable` dictionary
       return $ mkCast method $ mkSymCo $ getTypeableCo tyCl ty
  
    where
    -- co: method -> Typeable k t
    getTypeableCo tc t =
      case instNewTyCon_maybe tc [typeKind t, t] of
        Just (_,co) -> co
        _           -> panic "Class `Typeable` is not a `newtype`."
  
    -- Typeable t -> TyRep
    getRep tc (ev,t) =
      do typeableExpr <- dsEvTerm ev
         let co     = getTypeableCo tc t
             method = mkCast typeableExpr co
             proxy  = mkTyApps (Var proxyHashId) [typeKind t, t]
         return (mkApps method [proxy])
  
    -- This part could be cached
    tyConRep _ mkTyCon tc =
      do pkgStr  <- mkStringExprFS pkg_fs
         modStr  <- mkStringExprFS modl_fs
         nameStr <- mkStringExprFS name_fs
         return (mkApps (Var mkTyCon) [ int64 high, int64 low
                                      , pkgStr, modStr, nameStr
                                      ])
      where
      tycon_name                = tyConName tc
      modl                      = nameModule tycon_name
      pkg                       = moduleUnitId modl
  
      modl_fs                   = moduleNameFS (moduleName modl)
      pkg_fs                    = unitIdFS pkg
      name_fs                   = occNameFS (nameOccName tycon_name)
      hash_name_fs
        | isPromotedTyCon tc    = appendFS (mkFastString "$k") name_fs
        | isPromotedDataCon tc  = appendFS (mkFastString "$c") name_fs
        | isTupleTyCon tc &&
          returnsConstraintKind (tyConKind tc)
                                = appendFS (mkFastString "$p") name_fs
        | otherwise             = name_fs
  
      hashThis = unwords $ map unpackFS [pkg_fs, modl_fs, hash_name_fs]
      Fingerprint high low = fingerprintString hashThis
  
      int64 = mkWord64LitWord64
  
  {- 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.
  -}
  
  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 (mkCast tmExpr ip_co))
    case cs of
      EvCsPushCall name loc tm -> mkPush (occNameFS $ getOccName name) loc tm
      EvCsEmpty -> return emptyCS
  
  ---------------------------------------
  dsTcCoercion :: TcCoercion -> (Coercion -> CoreExpr) -> DsM CoreExpr
  -- This is the crucial function that moves
  -- from TcCoercions to Coercions; see Note [TcCoercions] in Coercion
  -- e.g.  dsTcCoercion (trans g1 g2) k
  --       = case g1 of EqBox g1# ->
  --         case g2 of EqBox g2# ->
  --         k (trans g1# g2#)
  -- thing_inside will get a coercion at the role requested
  dsTcCoercion co thing_inside
    = do { us <- newUniqueSupply
         ; let eqvs_covs :: [(EqVar,CoVar)]
               eqvs_covs = zipWith mk_co_var (varSetElems (coVarsOfTcCo co))
                                             (uniqsFromSupply us)
  
               subst = mkCvSubst emptyInScopeSet [(eqv, mkCoVarCo cov) | (eqv, cov) <- eqvs_covs]
               result_expr = thing_inside (ds_tc_coercion subst co)
               result_ty   = exprType result_expr
  
         ; return (foldr (wrap_in_case result_ty) result_expr eqvs_covs) }
    where
      mk_co_var :: Id -> Unique -> (Id, Id)
      mk_co_var eqv uniq = (eqv, mkUserLocal occ uniq ty loc)
         where
           eq_nm = idName eqv
           occ = nameOccName eq_nm
           loc = nameSrcSpan eq_nm
           ty  = mkCoercionType (getEqPredRole (evVarPred eqv)) ty1 ty2
           (ty1, ty2) = getEqPredTys (evVarPred eqv)
  
      wrap_in_case result_ty (eqv, cov) body
        = case getEqPredRole (evVarPred eqv) of
           Nominal          -> Case (Var eqv) eqv result_ty [(DataAlt eqBoxDataCon, [cov], body)]
           Representational -> Case (Var eqv) eqv result_ty [(DataAlt coercibleDataCon, [cov], body)]
           Phantom          -> panic "wrap_in_case/phantom"
  
  ds_tc_coercion :: CvSubst -> TcCoercion -> Coercion
  -- If the incoming TcCoercion if of type (a ~ b)   (resp.  Coercible a b)
  --                 the result is of type (a ~# b)  (reps.  a ~# b)
  -- The VarEnv maps EqVars of type (a ~ b) to Coercions of type (a ~# b) (resp. and so on)
  -- No need for InScope set etc because the
  ds_tc_coercion subst tc_co
    = go tc_co
    where
      go (TcRefl r ty)            = Refl r (Coercion.substTy subst ty)
      go (TcTyConAppCo r tc cos)  = mkTyConAppCo r tc (map go cos)
      go (TcAppCo co1 co2)        = mkAppCo (go co1) (go co2)
      go (TcForAllCo tv co)       = mkForAllCo tv' (ds_tc_coercion subst' co)
                                where
                                  (subst', tv') = Coercion.substTyVarBndr subst tv
      go (TcAxiomInstCo ax ind cos)
                                  = AxiomInstCo ax ind (map go cos)
      go (TcPhantomCo ty1 ty2)    = UnivCo (fsLit "ds_tc_coercion") Phantom ty1 ty2
      go (TcSymCo co)             = mkSymCo (go co)
      go (TcTransCo co1 co2)      = mkTransCo (go co1) (go co2)
      go (TcNthCo n co)           = mkNthCo n (go co)
      go (TcLRCo lr co)           = mkLRCo lr (go co)
      go (TcSubCo co)             = mkSubCo (go co)
      go (TcLetCo bs co)          = ds_tc_coercion (ds_co_binds bs) co
      go (TcCastCo co1 co2)       = mkCoCast (go co1) (go co2)
      go (TcCoVarCo v)            = ds_ev_id subst v
      go (TcAxiomRuleCo co ts cs) = AxiomRuleCo co (map (Coercion.substTy subst) ts) (map go cs)
      go (TcCoercion co)          = co
  
      ds_co_binds :: TcEvBinds -> CvSubst
      ds_co_binds (EvBinds bs)      = foldl ds_scc subst (sccEvBinds bs)
      ds_co_binds eb@(TcEvBinds {}) = pprPanic "ds_co_binds" (ppr eb)
  
      ds_scc :: CvSubst -> SCC EvBind -> CvSubst
      ds_scc subst (AcyclicSCC (EvBind v ev_term))
        = extendCvSubstAndInScope subst v (ds_co_term subst ev_term)
      ds_scc _ (CyclicSCC other) = pprPanic "ds_scc:cyclic" (ppr other $$ ppr tc_co)
  
      ds_co_term :: CvSubst -> EvTerm -> Coercion
      ds_co_term subst (EvCoercion tc_co) = ds_tc_coercion subst tc_co
      ds_co_term subst (EvId v)           = ds_ev_id subst v
      ds_co_term subst (EvCast tm co)     = mkCoCast (ds_co_term subst tm) (ds_tc_coercion subst co)
      ds_co_term _ other = pprPanic "ds_co_term" (ppr other $$ ppr tc_co)
  
      ds_ev_id :: CvSubst -> EqVar -> Coercion
      ds_ev_id subst v
       | Just co <- Coercion.lookupCoVar subst v = co
       | otherwise  = pprPanic "ds_tc_coercion" (ppr v $$ ppr tc_co)
  
  {-
  Note [Simple coercions]
  ~~~~~~~~~~~~~~~~~~~~~~~
  We have a special case for coercions that are simple variables.
  Suppose   cv :: a ~ b   is in scope
  Lacking the special case, if we see
          f a b cv
  we'd desguar to
          f a b (case cv of EqBox (cv# :: a ~# b) -> EqBox cv#)
  which is a bit stupid.  The special case does the obvious thing.
  
  This turns out to be important when desugaring the LHS of a RULE
  (see Trac #7837).  Suppose we have
      normalise        :: (a ~ Scalar a) => a -> a
      normalise_Double :: Double -> Double
      {-# RULES "normalise" normalise = normalise_Double #-}
  
  Then the RULE we want looks like
       forall a, (cv:a~Scalar a).
         normalise a cv = normalise_Double
  But without the special case we generate the redundant box/unbox,
  which simpleOpt (currently) doesn't remove. So the rule never matches.
  
  Maybe simpleOpt should be smarter.  But it seems like a good plan
  to simply never generate the redundant box/unbox in the first place.
  -}
  