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

    The Desugarer: turning HsSyn into Core.
    -}

    {-# LANGUAGE CPP #-}

    module Desugar ( deSugar, deSugarExpr ) where

    import DynFlags
    import HscTypes
    import HsSyn
    import TcRnTypes
    import TcRnMonad ( finalSafeMode )
    import MkIface
    import Id
    import Name
    import Type
    import FamInstEnv
    import Coercion
    import InstEnv
    import Class
    import Avail
    import CoreSyn
    import CoreSubst
    import PprCore
    import DsMonad
    import DsExpr
    import DsBinds
    import DsForeign
    import Module
    import NameSet
    import NameEnv
    import Rules
    import TysPrim (eqReprPrimTyCon)
    import TysWiredIn (coercibleTyCon )
    import BasicTypes       ( Activation(.. ) )
    import CoreMonad        ( CoreToDo(..) )
    import CoreLint         ( endPassIO )
    import MkCore
    import FastString
    import ErrUtils
    import Outputable
    import SrcLoc
    import Coverage
    import Util
    import MonadUtils
    import OrdList
    import StaticPtrTable
    import Data.List
    import Data.IORef
    import Control.Monad( when )

    {-
    ************************************************************************
    *                                                                      *
    *              The main function: deSugar
    *                                                                      *
    ************************************************************************
    -}

    -- | Main entry point to the desugarer.
    deSugar :: HscEnv -> ModLocation -> TcGblEnv -> IO (Messages, Maybe ModGuts)
    -- Can modify PCS by faulting in more declarations

    deSugar hsc_env
            mod_loc
            tcg_env@(TcGblEnv { tcg_mod          = mod,
                                tcg_src          = hsc_src,
                                tcg_type_env     = type_env,
                                tcg_imports      = imports,
                                tcg_exports      = exports,
                                tcg_keep         = keep_var,
                                tcg_th_splice_used = tc_splice_used,
                                tcg_rdr_env      = rdr_env,
                                tcg_fix_env      = fix_env,
                                tcg_inst_env     = inst_env,
                                tcg_fam_inst_env = fam_inst_env,
                                tcg_warns        = warns,
                                tcg_anns         = anns,
                                tcg_binds        = binds,
                                tcg_imp_specs    = imp_specs,
                                tcg_dependent_files = dependent_files,
                                tcg_ev_binds     = ev_binds,
                                tcg_fords        = fords,
                                tcg_rules        = rules,
                                tcg_vects        = vects,
                                tcg_patsyns      = patsyns,
                                tcg_tcs          = tcs,
                                tcg_insts        = insts,
                                tcg_fam_insts    = fam_insts,
                                tcg_hpc          = other_hpc_info})

      = do { let dflags = hsc_dflags hsc_env
                 print_unqual = mkPrintUnqualified dflags rdr_env
            ; showPass dflags "Desugar"

            -- Desugar the program
            ; let export_set = availsToNameSet exports
                  target     = hscTarget dflags
                  hpcInfo    = emptyHpcInfo other_hpc_info

            ; (binds_cvr, ds_hpc_info, modBreaks)
                             <- if not (isHsBootOrSig hsc_src)
                                  then addTicksToBinds dflags mod mod_loc export_set
                                              (typeEnvTyCons type_env) binds
                                  else return (binds, hpcInfo, emptyModBreaks)

            ; (msgs, mb_res) <- initDs hsc_env mod rdr_env type_env fam_inst_env $
                           do { ds_ev_binds <- dsEvBinds ev_binds
                              ; core_prs <- dsTopLHsBinds binds_cvr
                              ; (spec_prs, spec_rules) <- dsImpSpecs imp_specs
                              ; (ds_fords, foreign_prs) <- dsForeigns fords
                              ; ds_rules <- mapMaybeM dsRule rules
                              ; ds_vects <- mapM dsVect vects
                              ; stBinds <- dsGetStaticBindsVar >>=
                                               liftIO . readIORef
                              ; let hpc_init
                                      | gopt Opt_Hpc dflags = hpcInitCode mod ds_hpc_info
                                      | otherwise = empty
                                    -- Stub to insert the static entries of the
                                    -- module into the static pointer table
                                    spt_init = sptInitCode mod stBinds
                              ; return ( ds_ev_binds
                                       , foreign_prs `appOL` core_prs `appOL` spec_prs
                                                     `appOL` toOL (map snd stBinds)
                                       , spec_rules ++ ds_rules, ds_vects
                                       , ds_fords `appendStubC` hpc_init
                                                  `appendStubC` spt_init) }

            ; case mb_res of {
               Nothing -> return (msgs, Nothing) ;
               Just (ds_ev_binds, all_prs, all_rules, vects0, ds_fords) -> do

         do {       -- Add export flags to bindings
              keep_alive <- readIORef keep_var
            ; let (rules_for_locals, rules_for_imps) = partition isLocalRule all_rules
                  final_prs = addExportFlagsAndRules target export_set keep_alive
                                                     rules_for_locals (fromOL all_prs)

                  final_pgm = combineEvBinds ds_ev_binds final_prs
            -- Notice that we put the whole lot in a big Rec, even the foreign binds
            -- When compiling PrelFloat, which defines data Float = F# Float#
            -- we want F# to be in scope in the foreign marshalling code!
            -- You might think it doesn't matter, but the simplifier brings all top-level
            -- things into the in-scope set before simplifying; so we get no unfolding for F#!

    #ifdef DEBUG
              -- Debug only as pre-simple-optimisation program may be really big
            ; endPassIO hsc_env print_unqual CoreDesugar final_pgm rules_for_imps
    #endif
            ; (ds_binds, ds_rules_for_imps, ds_vects)
                <- simpleOptPgm dflags mod final_pgm rules_for_imps vects0
                             -- The simpleOptPgm gets rid of type
                             -- bindings plus any stupid dead code

            ; endPassIO hsc_env print_unqual CoreDesugarOpt ds_binds ds_rules_for_imps

            ; let used_names = mkUsedNames tcg_env
            ; deps <- mkDependencies tcg_env

            ; used_th <- readIORef tc_splice_used
            ; dep_files <- readIORef dependent_files
            ; safe_mode <- finalSafeMode dflags tcg_env

            ; let mod_guts = ModGuts {
                    mg_module       = mod,
                    mg_boot         = hsc_src == HsBootFile,
                    mg_exports      = exports,
                    mg_deps         = deps,
                    mg_used_names   = used_names,
                    mg_used_th      = used_th,
                    mg_dir_imps     = imp_mods imports,
                    mg_rdr_env      = rdr_env,
                    mg_fix_env      = fix_env,
                    mg_warns        = warns,
                    mg_anns         = anns,
                    mg_tcs          = tcs,
                    mg_insts        = insts,
                    mg_fam_insts    = fam_insts,
                    mg_inst_env     = inst_env,
                    mg_fam_inst_env = fam_inst_env,
                    mg_patsyns      = patsyns,
                    mg_rules        = ds_rules_for_imps,
                    mg_binds        = ds_binds,
                    mg_foreign      = ds_fords,
                    mg_hpc_info     = ds_hpc_info,
                    mg_modBreaks    = modBreaks,
                    mg_vect_decls   = ds_vects,
                    mg_vect_info    = noVectInfo,
                    mg_safe_haskell = safe_mode,
                    mg_trust_pkg    = imp_trust_own_pkg imports,
                    mg_dependent_files = dep_files
                  }
            ; return (msgs, Just mod_guts)
            }}}

    dsImpSpecs :: [LTcSpecPrag] -> DsM (OrdList (Id,CoreExpr), [CoreRule])
    dsImpSpecs imp_specs
     = do { spec_prs <- mapMaybeM (dsSpec Nothing) imp_specs
          ; let (spec_binds, spec_rules) = unzip spec_prs
          ; return (concatOL spec_binds, spec_rules) }

    combineEvBinds :: [CoreBind] -> [(Id,CoreExpr)] -> [CoreBind]
    -- Top-level bindings can include coercion bindings, but not via superclasses
    -- See Note [Top-level evidence]
    combineEvBinds [] val_prs
      = [Rec val_prs]
    combineEvBinds (NonRec b r : bs) val_prs
      | isId b    = combineEvBinds bs ((b,r):val_prs)
      | otherwise = NonRec b r : combineEvBinds bs val_prs
    combineEvBinds (Rec prs : bs) val_prs
      = combineEvBinds bs (prs ++ val_prs)

    {-
    Note [Top-level evidence]
    ~~~~~~~~~~~~~~~~~~~~~~~~~
    Top-level evidence bindings may be mutually recursive with the top-level value
    bindings, so we must put those in a Rec.  But we can't put them *all* in a Rec
    because the occurrence analyser doesn't teke account of type/coercion variables
    when computing dependencies.

    So we pull out the type/coercion variables (which are in dependency order),
    and Rec the rest.
    -}

    deSugarExpr :: HscEnv -> LHsExpr Id -> IO (Messages, Maybe CoreExpr)

    deSugarExpr hsc_env tc_expr
      = do { let dflags       = hsc_dflags hsc_env
                 icntxt       = hsc_IC hsc_env
                 rdr_env      = ic_rn_gbl_env icntxt
                 type_env     = mkTypeEnvWithImplicits (ic_tythings icntxt)
                 fam_insts    = snd (ic_instances icntxt)
                 fam_inst_env = extendFamInstEnvList emptyFamInstEnv fam_insts
                 -- This stuff is a half baked version of TcRnDriver.setInteractiveContext

           ; showPass dflags "Desugar"

             -- Do desugaring
           ; (msgs, mb_core_expr) <- initDs hsc_env (icInteractiveModule icntxt) rdr_env
                                            type_env fam_inst_env $
                                     dsLExpr tc_expr

           ; case mb_core_expr of
                Nothing   -> return ()
                Just expr -> dumpIfSet_dyn dflags Opt_D_dump_ds "Desugared" (pprCoreExpr expr)

           ; return (msgs, mb_core_expr) }

    {-
    ************************************************************************
    *                                                                      *
    *              Add rules and export flags to binders
    *                                                                      *
    ************************************************************************
    -}

    addExportFlagsAndRules
        :: HscTarget -> NameSet -> NameSet -> [CoreRule]
        -> [(Id, t)] -> [(Id, t)]
    addExportFlagsAndRules target exports keep_alive rules prs
      = mapFst add_one prs
      where
        add_one bndr = add_rules name (add_export name bndr)
           where
             name = idName bndr

        ---------- Rules --------
            -- See Note [Attach rules to local ids]
            -- NB: the binder might have some existing rules,
            -- arising from specialisation pragmas
        add_rules name bndr
            | Just rules <- lookupNameEnv rule_base name
            = bndr `addIdSpecialisations` rules
            | otherwise
            = bndr
        rule_base = extendRuleBaseList emptyRuleBase rules

        ---------- Export flag --------
        -- See Note [Adding export flags]
        add_export name bndr
            | dont_discard name = setIdExported bndr
            | otherwise         = bndr

        dont_discard :: Name -> Bool
        dont_discard name = is_exported name
                         || name `elemNameSet` keep_alive

            -- In interactive mode, we don't want to discard any top-level
            -- entities at all (eg. do not inline them away during
            -- simplification), and retain them all in the TypeEnv so they are
            -- available from the command line.
            --
            -- isExternalName separates the user-defined top-level names from those
            -- introduced by the type checker.
        is_exported :: Name -> Bool
        is_exported | targetRetainsAllBindings target = isExternalName
                    | otherwise                       = (`elemNameSet` exports)

    {-
    Note [Adding export flags]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~
    Set the no-discard flag if either
            a) the Id is exported
            b) it's mentioned in the RHS of an orphan rule
            c) it's in the keep-alive set

    It means that the binding won't be discarded EVEN if the binding
    ends up being trivial (v = w) -- the simplifier would usually just
    substitute w for v throughout, but we don't apply the substitution to
    the rules (maybe we should?), so this substitution would make the rule
    bogus.

    You might wonder why exported Ids aren't already marked as such;
    it's just because the type checker is rather busy already and
    I didn't want to pass in yet another mapping.

    Note [Attach rules to local ids]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Find the rules for locally-defined Ids; then we can attach them
    to the binders in the top-level bindings

    Reason
      - It makes the rules easier to look up
      - It means that transformation rules and specialisations for
        locally defined Ids are handled uniformly
      - It keeps alive things that are referred to only from a rule
        (the occurrence analyser knows about rules attached to Ids)
      - It makes sure that, when we apply a rule, the free vars
        of the RHS are more likely to be in scope
      - The imported rules are carried in the in-scope set
        which is extended on each iteration by the new wave of
        local binders; any rules which aren't on the binding will
        thereby get dropped

    ************************************************************************
    *                                                                      *
    *              Desugaring transformation rules
    *                                                                      *
    ************************************************************************
    -}

    dsRule :: LRuleDecl Id -> DsM (Maybe CoreRule)
    dsRule (L loc (HsRule name act vars lhs _tv_lhs rhs _fv_rhs))
      = putSrcSpanDs loc $
        do  { let bndrs' = [var | L _ (RuleBndr (L _ var)) <- vars]

            ; lhs' <- unsetGOptM Opt_EnableRewriteRules $
                      unsetWOptM Opt_WarnIdentities $
                      dsLExpr lhs   -- Note [Desugaring RULE left hand sides]

            ; rhs' <- dsLExpr rhs
            ; dflags <- getDynFlags

            ; (bndrs'', lhs'', rhs'') <- unfold_coerce bndrs' lhs' rhs'

            -- Substitute the dict bindings eagerly,
            -- and take the body apart into a (f args) form
            ; case decomposeRuleLhs bndrs'' lhs'' of {
                    Left msg -> do { warnDs msg; return Nothing } ;
                    Right (final_bndrs, fn_id, args) -> do

            { let is_local = isLocalId fn_id
                    -- NB: isLocalId is False of implicit Ids.  This is good because
                    -- we don't want to attach rules to the bindings of implicit Ids,
                    -- because they don't show up in the bindings until just before code gen
                  fn_name   = idName fn_id
                  final_rhs = simpleOptExpr rhs''    -- De-crap it
                  rule      = mkRule False {- Not auto -} is_local
                                     (unLoc name) act fn_name final_bndrs args
                                     final_rhs

                  inline_shadows_rule   -- Function can be inlined before rule fires
                    | wopt Opt_WarnInlineRuleShadowing dflags
                    , isLocalId fn_id || hasSomeUnfolding (idUnfolding fn_id)
                           -- If imported with no unfolding, no worries
                    = case (idInlineActivation fn_id, act) of
                        (NeverActive, _)    -> False
                        (AlwaysActive, _)   -> True
                        (ActiveBefore {}, _) -> True
                        (ActiveAfter {}, NeverActive)     -> True
                        (ActiveAfter n, ActiveAfter r)    -> r < n  -- Rule active strictly first
                        (ActiveAfter {}, AlwaysActive)    -> False
                        (ActiveAfter {}, ActiveBefore {}) -> False
                    | otherwise = False

            ; when inline_shadows_rule $
              warnDs (vcat [ hang (ptext (sLit "Rule")
                                   <+> doubleQuotes (ftext $ unLoc name)
                                   <+> ptext (sLit "may never fire"))
                                2 (ptext (sLit "because") <+> quotes (ppr fn_id)
                                   <+> ptext (sLit "might inline first"))
                           , ptext (sLit "Probable fix: add an INLINE[n] or NOINLINE[n] pragma on")
                             <+> quotes (ppr fn_id) ])

            ; return (Just rule)
            } } }

    -- See Note [Desugaring coerce as cast]
    unfold_coerce :: [Id] -> CoreExpr -> CoreExpr -> DsM ([Var], CoreExpr, CoreExpr)
    unfold_coerce bndrs lhs rhs = do
        (bndrs', wrap) <- go bndrs
        return (bndrs', wrap lhs, wrap rhs)
      where
        go :: [Id] -> DsM ([Id], CoreExpr -> CoreExpr)
        go []     = return ([], id)
        go (v:vs)
            | Just (tc, args) <- splitTyConApp_maybe (idType v)
            , tc == coercibleTyCon = do
                let ty' = mkTyConApp eqReprPrimTyCon args
                v' <- mkDerivedLocalM mkRepEqOcc v ty'

                (bndrs, wrap) <- go vs
                return (v':bndrs, mkCoreLet (NonRec v (mkEqBox (mkCoVarCo v'))) . wrap)
            | otherwise = do
                (bndrs,wrap) <- go vs
                return (v:bndrs, wrap)

    {-
    Note [Desugaring RULE left hand sides]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    For the LHS of a RULE we do *not* want to desugar
        [x]   to    build (\cn. x `c` n)
    We want to leave explicit lists simply as chains
    of cons's. We can achieve that slightly indirectly by
    switching off EnableRewriteRules.  See DsExpr.dsExplicitList.

    That keeps the desugaring of list comprehensions simple too.

    Nor do we want to warn of conversion identities on the LHS;
    the rule is precisly to optimise them:
      {-# RULES "fromRational/id" fromRational = id :: Rational -> Rational #-}

    Note [Desugaring coerce as cast]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    We want the user to express a rule saying roughly “mapping a coercion over a
    list can be replaced by a coercion”. But the cast operator of Core (▷) cannot
    be written in Haskell. So we use `coerce` for that (#2110). The user writes
        map coerce = coerce
    as a RULE, and this optimizes any kind of mapped' casts aways, including `map
    MkNewtype`.

    For that we replace any forall'ed `c :: Coercible a b` value in a RULE by
    corresponding `co :: a ~#R b` and wrap the LHS and the RHS in
    `let c = MkCoercible co in ...`. This is later simplified to the desired form
    by simpleOptExpr (for the LHS) resp. the simplifiers (for the RHS).

    ************************************************************************
    *                                                                      *
    *              Desugaring vectorisation declarations
    *                                                                      *
    ************************************************************************
    -}

    dsVect :: LVectDecl Id -> DsM CoreVect
    dsVect (L loc (HsVect _ (L _ v) rhs))
      = putSrcSpanDs loc $
        do { rhs' <- dsLExpr rhs
           ; return $ Vect v rhs'
           }
    dsVect (L _loc (HsNoVect _ (L _ v)))
      = return $ NoVect v
    dsVect (L _loc (HsVectTypeOut isScalar tycon rhs_tycon))
      = return $ VectType isScalar tycon' rhs_tycon
      where
        tycon' | Just ty <- coreView $ mkTyConTy tycon
               , (tycon', []) <- splitTyConApp ty      = tycon'
               | otherwise                             = tycon
    dsVect vd@(L _ (HsVectTypeIn _ _ _ _))
      = pprPanic "Desugar.dsVect: unexpected 'HsVectTypeIn'" (ppr vd)
    dsVect (L _loc (HsVectClassOut cls))
      = return $ VectClass (classTyCon cls)
    dsVect vc@(L _ (HsVectClassIn _ _))
      = pprPanic "Desugar.dsVect: unexpected 'HsVectClassIn'" (ppr vc)
    dsVect (L _loc (HsVectInstOut inst))
      = return $ VectInst (instanceDFunId inst)
    dsVect vi@(L _ (HsVectInstIn _))
      = pprPanic "Desugar.dsVect: unexpected 'HsVectInstIn'" (ppr vi)