DsBinds.hs 49.9 KB
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{-
(c) The University of Glasgow 2006
(c) The GRASP/AQUA Project, Glasgow University, 1992-1998

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Pattern-matching bindings (HsBinds and MonoBinds)
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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).
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-}
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{-# LANGUAGE CPP #-}
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module DsBinds ( dsTopLHsBinds, dsLHsBinds, decomposeRuleLhs, dsSpec,
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                 dsHsWrapper, dsTcEvBinds, dsTcEvBinds_s, dsEvBinds, dsMkUserRule
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  ) where
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#include "HsVersions.h"

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import {-# SOURCE #-}   DsExpr( dsLExpr )
import {-# SOURCE #-}   Match( matchWrapper )
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import DsMonad
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import DsGRHSs
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import DsUtils
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import HsSyn            -- lots of things
import CoreSyn          -- lots of things
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import Literal          ( Literal(MachStr) )
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import CoreSubst
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import OccurAnal        ( occurAnalyseExpr )
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import MkCore
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import CoreUtils
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import CoreArity ( etaExpand )
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import CoreUnfold
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import CoreFVs
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import Digraph
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import PrelNames
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import TysPrim ( mkProxyPrimTy )
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import TyCon
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import TcEvidence
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import TcType
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import Type
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import Coercion
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import TysWiredIn ( typeNatKind, typeSymbolKind )
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import Id
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import MkId(proxyHashId)
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import Class
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import Name
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import VarSet
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import Rules
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import VarEnv
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import Outputable
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import Module
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import SrcLoc
import Maybes
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import OrdList
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import Bag
import BasicTypes hiding ( TopLevel )
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import DynFlags
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import FastString
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import Util
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import MonadUtils
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import qualified GHC.LanguageExtensions as LangExt
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import Control.Monad
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{-**********************************************************************
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*                                                                      *
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           Desugaring a MonoBinds
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*                                                                      *
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**********************************************************************-}
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-- | Desugar top level binds, strict binds are treated like normal
-- binds since there is no good time to force before first usage.
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dsTopLHsBinds :: LHsBinds Id -> DsM (OrdList (Id,CoreExpr))
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dsTopLHsBinds binds = fmap (toOL . snd) (ds_lhs_binds binds)
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-- | 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') }
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------------------------
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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
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                -- Dictionary bindings are always VarBinds,
                -- so we only need do this here
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        ; let var' | inline_regardless = var `setIdUnfolding` mkCompulsoryUnfolding core_expr
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                   | otherwise         = var
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        ; let core_bind@(id,_) = makeCorePair dflags var' False 0 core_expr
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              force_var = if xopt LangExt.Strict dflags
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                          then [id]
                          else []
        ; return (force_var, [core_bind]) }

dsHsBind dflags
         (FunBind { fun_id = L _ fun, fun_matches = matches
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                  , fun_co_fn = co_fn, fun_tick = tick })
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 = do   { (args, body) <- matchWrapper
                           (FunRhs (noLoc $ idName fun) Prefix)
                           Nothing matches
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        ; let body' = mkOptTickBox tick body
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        ; rhs <- dsHsWrapper co_fn (mkLams args body')
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        ; let core_binds@(id,_) = makeCorePair dflags fun False 0 rhs
              force_var =
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                if xopt LangExt.Strict dflags
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                   && matchGroupArity matches == 0 -- no need to force lambdas
                then [id]
                else []
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        ; {- pprTrace "dsHsBind" (ppr fun <+> ppr (idInlinePragma fun)) $ -}
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           return (force_var, [core_binds]) }
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dsHsBind dflags
         (PatBind { pat_lhs = pat, pat_rhs = grhss, pat_rhs_ty = ty
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                  , pat_ticks = (rhs_tick, var_ticks) })
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  = do  { body_expr <- dsGuarded grhss ty
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        ; let body' = mkOptTickBox rhs_tick body_expr
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              pat'  = decideBangHood dflags pat
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        ; (force_var,sel_binds) <-
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            mkSelectorBinds var_ticks pat body'
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          -- We silently ignore inline pragmas; no makeCorePair
          -- Not so cool, but really doesn't matter
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        ; let force_var' = if isBangedLPat pat'
                           then [force_var]
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                           else []
        ; return (force_var', sel_binds) }
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        -- A common case: one exported variable, only non-strict binds
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        -- Non-recursive bindings come through this way
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        -- So do self-recursive bindings
        -- Bindings with complete signatures are AbsBindsSigs, below
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dsHsBind dflags
         (AbsBinds { abs_tvs = tyvars, abs_ev_vars = dicts
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                   , abs_exports = [export]
                   , abs_ev_binds = ev_binds, abs_binds = binds })
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  | ABE { abe_wrap = wrap, abe_poly = global
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        , abe_mono = local, abe_prags = prags } <- export
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  , not (xopt LangExt.Strict dflags)             -- Handle strict binds
  , not (anyBag (isBangedPatBind . unLoc) binds) --        in the next case
  = -- See Note [AbsBinds wrappers] in HsBinds
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    addDictsDs (toTcTypeBag (listToBag dicts)) $
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         -- addDictsDs: push type constraints deeper for pattern match check
    do { (_, bind_prs) <- ds_lhs_binds binds
       ; let core_bind = Rec bind_prs
       ; ds_binds <- dsTcEvBinds_s ev_binds
       ; rhs <- dsHsWrapper wrap $  -- Usually the identity
                mkLams tyvars $ mkLams dicts $
                mkCoreLets ds_binds $
                Let core_bind $
                Var local
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       ; (spec_binds, rules) <- dsSpecs rhs prags
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       ; let   global'  = addIdSpecialisations global rules
               main_bind = makeCorePair dflags global' (isDefaultMethod prags)
                                        (dictArity dicts) rhs
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       ; return ([], main_bind : fromOL spec_binds) }
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        -- 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) <- ds_lhs_binds binds
       ; let mk_bind (ABE { abe_wrap = wrap
                          , abe_poly = global
                          , abe_mono = local
                          , abe_prags = prags })
              = do { rhs <- dsHsWrapper wrap (Var local)
                   ; return (makeCorePair dflags global
                                          (isDefaultMethod prags)
                                          0 rhs) }
       ; main_binds <- mapM mk_bind exports

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

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

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

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-- AbsBindsSig is a combination of AbsBinds and FunBind
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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)) $
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             -- addDictsDs: push type constraints deeper for pattern match check
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    do { (args, body) <- matchWrapper
                           (FunRhs (noLoc $ idName global) Prefix)
                           Nothing matches
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       ; let body' = mkOptTickBox tick body
       ; fun_rhs <- dsHsWrapper co_fn $
                    mkLams args body'
       ; let force_vars
               | xopt LangExt.Strict dflags
               , matchGroupArity matches == 0 -- no need to force lambdas
               = [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)

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

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------------------------
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makeCorePair :: DynFlags -> Id -> Bool -> Arity -> CoreExpr -> (Id, CoreExpr)
makeCorePair dflags gbl_id is_default_method dict_arity rhs
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  | is_default_method                 -- Default methods are *always* inlined
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  = (gbl_id `setIdUnfolding` mkCompulsoryUnfolding rhs, rhs)

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  | otherwise
  = case inlinePragmaSpec inline_prag of
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          EmptyInlineSpec -> (gbl_id, rhs)
          NoInline        -> (gbl_id, rhs)
          Inlinable       -> (gbl_id `setIdUnfolding` inlinable_unf, rhs)
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          Inline          -> inline_pair
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  where
    inline_prag   = idInlinePragma gbl_id
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    inlinable_unf = mkInlinableUnfolding dflags rhs
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    inline_pair
       | Just arity <- inlinePragmaSat inline_prag
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        -- Add an Unfolding for an INLINE (but not for NOINLINE)
        -- And eta-expand the RHS; see Note [Eta-expanding INLINE things]
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       , let real_arity = dict_arity + arity
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        -- NB: The arity in the InlineRule takes account of the dictionaries
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       = ( 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)
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dictArity :: [Var] -> Arity
-- Don't count coercion variables in arity
dictArity dicts = count isId dicts
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{-
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Note [Desugaring AbsBinds]
~~~~~~~~~~~~~~~~~~~~~~~~~~
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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 }

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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
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        f_lcl = ...f_lcl...     -- The "binds" from AbsBinds
        M.f = f_lcl             -- Generated from "exports"
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But we don't want that, because if M.f isn't exported,
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it'll be inlined unconditionally at every call site (its rhs is
trivial).  That would be ok unless it has RULES, which would
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thereby be completely lost.  Bad, bad, bad.

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

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

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The top-level AbsBinds for $cround has no tyvars or dicts (because the
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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

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

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        fg = /\ab. let B in e1
        gg = /\b. let a = () in let B in S(e2)
        h  = /\ab. let B in e3
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where B is the *non-recursive* binding
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        fl = fg a b
        gl = gg b
        h  = h a b    -- See (b); note shadowing!
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Notice (a) g has a different number of type variables to f, so we must
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             use the mkArbitraryType thing to fill in the gaps.
             We use a type-let to do that.
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         (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.
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         (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.
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Why got to this trouble?  It's a common case, and it removes the
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quadratic-sized tuple desugaring.  Less clutter, hopefully faster
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compilation, especially in a case where there are a *lot* of
bindings.


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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
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happen as a result of method sharing), there's a danger that we never
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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
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has the arity with which it is declared in the source code.  In this
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example it has arity 2 (one for the Eq and one for x). Doing this
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should mean that (foo d) is a PAP and we don't share it.
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Note [Nested arities]
~~~~~~~~~~~~~~~~~~~~~
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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!
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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.

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The simplest thing is to return it in the polymorphic
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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.

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-}
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------------------------
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dsSpecs :: CoreExpr     -- Its rhs
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        -> TcSpecPrags
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        -> DsM ( OrdList (Id,CoreExpr)  -- Binding for specialised Ids
               , [CoreRule] )           -- Rules for the Global Ids
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-- See Note [Handling SPECIALISE pragmas] in TcBinds
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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) }

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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
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       -> Located TcSpecPrag
       -> DsM (Maybe (OrdList (Id,CoreExpr), CoreRule))
dsSpec mb_poly_rhs (L loc (SpecPrag poly_id spec_co spec_inl))
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  | isJust (isClassOpId_maybe poly_id)
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  = putSrcSpanDs loc $
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    do { warnDs NoReason (text "Ignoring useless SPECIALISE pragma for class method selector"
                          <+> quotes (ppr poly_id))
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       ; return Nothing  }  -- There is no point in trying to specialise a class op
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                            -- Moreover, classops don't (currently) have an inl_sat arity set
                            -- (it would be Just 0) and that in turn makes makeCorePair bleat
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  | no_act_spec && isNeverActive rule_act
  = putSrcSpanDs loc $
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    do { warnDs NoReason (text "Ignoring useless SPECIALISE pragma for NOINLINE function:"
                          <+> quotes (ppr poly_id))
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       ; return Nothing  }  -- Function is NOINLINE, and the specialiation inherits that
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                            -- See Note [Activation pragmas for SPECIALISE]
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  | otherwise
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  = putSrcSpanDs loc $
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    do { uniq <- newUnique
       ; let poly_name = idName poly_id
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             spec_occ  = mkSpecOcc (getOccName poly_name)
             spec_name = mkInternalName uniq spec_occ (getSrcSpan poly_name)
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       ; (bndrs, ds_lhs) <- liftM collectBinders
                                  (dsHsWrapper spec_co (Var poly_id))
       ; let spec_ty = mkPiTypes bndrs (exprType ds_lhs)
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       ; -- pprTrace "dsRule" (vcat [ text "Id:" <+> ppr poly_id
         --                         , text "spec_co:" <+> ppr spec_co
         --                         , text "ds_rhs:" <+> ppr ds_lhs ]) $
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         case decomposeRuleLhs bndrs ds_lhs of {
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           Left msg -> do { warnDs NoReason msg; return Nothing } ;
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           Right (rule_bndrs, _fn, args) -> do
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       { dflags <- getDynFlags
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       ; this_mod <- getModule
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       ; 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
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             spec_id   = mkLocalId spec_name spec_ty
                            `setInlinePragma` inl_prag
                            `setIdUnfolding`  spec_unf
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       ; rule <- dsMkUserRule this_mod is_local_id
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                        (mkFastString ("SPEC " ++ showPpr dflags poly_name))
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                        rule_act poly_name
                        rule_bndrs args
                        (mkVarApps (Var spec_id) bndrs)
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       ; spec_rhs <- dsHsWrapper spec_co poly_rhs
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-- Commented out: see Note [SPECIALISE on INLINE functions]
--       ; when (isInlinePragma id_inl)
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--              (warnDs $ text "SPECIALISE pragma on INLINE function probably won't fire:"
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--                        <+> quotes (ppr poly_name))
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       ; 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
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       } } }
  where
    is_local_id = isJust mb_poly_rhs
    poly_rhs | Just rhs <-  mb_poly_rhs
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             = rhs          -- Local Id; this is its rhs
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             | Just unfolding <- maybeUnfoldingTemplate (realIdUnfolding poly_id)
             = unfolding    -- Imported Id; this is its unfolding
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                            -- Use realIdUnfolding so we get the unfolding
                            -- even when it is a loop breaker.
                            -- We want to specialise recursive functions!
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             | otherwise = pprPanic "dsImpSpecs" (ppr poly_id)
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                            -- The type checker has checked that it *has* an unfolding
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    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]
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                                 -- in OccurAnal
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             , 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


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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) $
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        warnDs (Reason Opt_WarnOrphans) (ruleOrphWarn rule)
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    return rule

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

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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:

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* spec_fn's inline pragma: inherited from f's inline pragma (ignoring
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                           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
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SPEC [n] f :: ty            [n]   INLINE [k]
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                                  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]


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************************************************************************
*                                                                      *
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\subsection{Adding inline pragmas}
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*                                                                      *
************************************************************************
-}
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decomposeRuleLhs :: [Var] -> CoreExpr -> Either SDoc ([Var], Id, [CoreExpr])
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-- (decomposeRuleLhs bndrs lhs) takes apart the LHS of a RULE,
-- The 'bndrs' are the quantified binders of the rules, but decomposeRuleLhs
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-- may add some extra dictionary binders (see Note [Free dictionaries])
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--
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-- Returns Nothing if the LHS isn't of the expected shape
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-- 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))

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  | Just (fn_id, args) <- decompose fun2 args2
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  , let extra_bndrs = mk_extra_bndrs fn_id args
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  = -- 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]) $
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    Right (orig_bndrs ++ extra_bndrs, fn_id, args)
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  | otherwise
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  = Left bad_shape_msg
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 where
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   lhs1         = drop_dicts orig_lhs
   lhs2         = simpleOptExpr lhs1  -- See Note [Simplify rule LHS]
   (fun2,args2) = collectArgs lhs2

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   lhs_fvs    = exprFreeVars lhs2
   unbound    = filterOut (`elemVarSet` lhs_fvs) orig_bndrs
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   orig_bndr_set = mkVarSet orig_bndrs
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        -- 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)
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   decompose (Var fn_id) args
      | not (fn_id `elemVarSet` orig_bndr_set)
      = Just (fn_id, args)

   decompose _ _ = Nothing
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   bad_shape_msg = hang (text "RULE left-hand side too complicated to desugar")
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                      2 (vcat [ text "Optimised lhs:" <+> ppr lhs2
                              , text "Orig lhs:" <+> ppr orig_lhs])
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   dead_msg bndr = hang (sep [ text "Forall'd" <+> pp_bndr bndr
                             , text "is not bound in RULE lhs"])
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                      2 (vcat [ text "Orig bndrs:" <+> ppr orig_bndrs
                              , text "Orig lhs:" <+> ppr orig_lhs
                              , text "optimised lhs:" <+> ppr lhs2 ])
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   pp_bndr bndr
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    | isTyVar bndr                      = text "type variable" <+> quotes (ppr bndr)
    | Just pred <- evVarPred_maybe bndr = text "constraint" <+> quotes (ppr pred)
    | otherwise                         = text "variable" <+> quotes (ppr bndr)
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   drop_dicts :: CoreExpr -> CoreExpr
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   drop_dicts e
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       = wrap_lets needed bnds body
     where
       needed = orig_bndr_set `minusVarSet` exprFreeVars body
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       (bnds, body) = split_lets (occurAnalyseExpr e)
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           -- The occurAnalyseExpr drops dead bindings which is
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           -- crucial to ensure that every binding is used later;
           -- which in turn makes wrap_lets work right
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   split_lets :: CoreExpr -> ([(DictId,CoreExpr)], CoreExpr)
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   split_lets (Let (NonRec d r) body)
     | isDictId d
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     = ((d,r):bs, body')
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     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)
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   wrap_lets :: VarSet -> [(DictId,CoreExpr)] -> CoreExpr -> CoreExpr
   wrap_lets _ [] body = body
   wrap_lets needed ((d, r) : bs) body
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     | rhs_fvs `intersectsVarSet` needed = mkCoreLet (NonRec d r) (wrap_lets needed' bs body)
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     | otherwise                         = wrap_lets needed bs body
     where
       rhs_fvs = exprFreeVars r
       needed' = (needed `minusVarSet` rhs_fvs) `extendVarSet` d
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{-
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Note [Decomposing the left-hand side of a RULE]
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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There are several things going on here.
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* drop_dicts: see Note [Drop dictionary bindings on rule LHS]
* simpleOptExpr: see Note [Simplify rule LHS]
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* extra_dict_bndrs: see Note [Free dictionaries]
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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 skolem '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.

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Note [Drop dictionary bindings on rule LHS]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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drop_dicts drops dictionary bindings on the LHS where possible.
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   E.g.  let d:Eq [Int] = $fEqList $fEqInt in f d
     --> f d
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   Reasoning here is that there is only one d:Eq [Int], and so we can
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   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
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         one of the orig_bndrs, which we assume occur on RHS.
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         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
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         to match, but there is no other way to get d:Eq a
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   NB 2: We do drop_dicts *before* simplOptEpxr, so that we expect all
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         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!

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    NB3: In the common case of a non-overloaded, but perhaps-polymorphic
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         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
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             RULE forall s (d :: MonadAbstractIOST (ReaderT s)).
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                useAbstractMonad (ReaderT s) d = $suseAbstractMonad s

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

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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 alr