% % (c) The University of Glasgow 2006 % (c) The GRASP/AQUA Project, Glasgow University, 1992-1998 % The Desugarer: turning HsSyn into Core. \begin{code}

    module Desugar ( deSugar, deSugarExpr ) where

    import DynFlags
    import StaticFlags
    import HscTypes
    import HsSyn
    import TcRnTypes
    import MkIface
    import Id
    import Name
    import CoreSyn
    import CoreSubst
    import PprCore
    import DsMonad
    import DsExpr
    import DsBinds
    import DsForeign
    import DsExpr		()	-- Forces DsExpr to be compiled; DsBinds only
    				-- depends on DsExpr.hi-boot.
    import Module
    import RdrName
    import NameSet
    import NameEnv
    import Rules
    import CoreMonad	( endPass, CoreToDo(..) )
    import ErrUtils
    import Outputable
    import SrcLoc
    import Coverage
    import Util
    import MonadUtils
    import OrdList
    import Data.List
    import Data.IORef

\end{code} %************************************************************************ %* * %* The main function: deSugar %* * %************************************************************************ \begin{code}

    -- | 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_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_ev_binds     = ev_binds,
    			    tcg_fords        = fords,
    			    tcg_rules        = rules,
    		    	    tcg_insts        = insts,
    		    	    tcg_fam_insts    = fam_insts,
    			    tcg_hpc          = other_hpc_info })

      = do	{ let dflags = hsc_dflags hsc_env
            ; showPass dflags "Desugar"

    	-- Desugar the program
            ; let export_set = availsToNameSet exports
    	; let auto_scc = mkAutoScc dflags mod export_set
            ; let target = hscTarget dflags
            ; let hpcInfo = emptyHpcInfo other_hpc_info
    	; (msgs, mb_res)
                  <- case target of
    	           HscNothing ->
                           return (emptyMessages,
                                   Just ([], nilOL, [], NoStubs, hpcInfo, emptyModBreaks))
                       _        -> do
                         (binds_cvr,ds_hpc_info, modBreaks)
    			 <- if (opt_Hpc
    				  || target == HscInterpreted)
    			       && (not (isHsBoot hsc_src))
                                  then addCoverageTicksToBinds dflags mod mod_loc
                                                               (typeEnvTyCons type_env) binds
                                  else return (binds, hpcInfo, emptyModBreaks)
                         initDs hsc_env mod rdr_env type_env $ do
    		       do { ds_ev_binds <- dsEvBinds ev_binds
    		          ; core_prs <- dsTopLHsBinds auto_scc binds_cvr
                              ; (spec_prs, spec_rules) <- dsImpSpecs imp_specs
    	                  ; (ds_fords, foreign_prs) <- dsForeigns fords
    		          ; rules <- mapMaybeM dsRule rules
    		          ; return ( ds_ev_binds
                                       , foreign_prs `appOL` core_prs `appOL` spec_prs
                                       , spec_rules ++ rules
                                       , ds_fords, ds_hpc_info, modBreaks) }

    	; case mb_res of {
    	   Nothing -> return (msgs, Nothing) ;
    	   Just (ds_ev_binds, all_prs, all_rules, ds_fords,ds_hpc_info, modBreaks) -> 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#!

    	-- Lint result if necessary, and print
            ; dumpIfSet_dyn dflags Opt_D_dump_ds "Desugared, before opt" $
                   (vcat [ pprCoreBindings final_pgm
                         , pprRules rules_for_imps ])

    	; (ds_binds, ds_rules_for_imps) <- simpleOptPgm dflags final_pgm rules_for_imps
    	      		 -- The simpleOptPgm gets rid of type
    			 -- bindings plus any stupid dead code

    	; endPass dflags CoreDesugar ds_binds ds_rules_for_imps

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

            ; let mod_guts = ModGuts {
    		mg_module    	= mod,
    		mg_boot	     	= isHsBoot hsc_src,
    		mg_exports   	= exports,
    		mg_deps	     	= deps,
    		mg_used_names   = used_names,
    		mg_dir_imps  	= imp_mods imports,
    	        mg_rdr_env   	= rdr_env,
    		mg_fix_env   	= fix_env,
    		mg_warns   	= warns,
    		mg_anns      	= anns,
    		mg_types     	= type_env,
    		mg_insts     	= insts,
    		mg_fam_insts 	= fam_insts,
    		mg_inst_env     = inst_env,
    		mg_fam_inst_env = fam_inst_env,
    	        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_info    = noVectInfo
                  }
            ; 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 :: [DsEvBind] -> [(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 (LetEvBind (NonRec b r) : bs) val_prs
      | isId b    = combineEvBinds bs ((b,r):val_prs)
      | otherwise = NonRec b r : combineEvBinds bs val_prs
    combineEvBinds (LetEvBind (Rec prs) : bs) val_prs
      = combineEvBinds bs (prs ++ val_prs)
    combineEvBinds (CaseEvBind x _ _ : _) _
      = pprPanic "topEvBindPairs" (ppr x)

\end{code} 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. \begin{code}

    mkAutoScc :: DynFlags -> Module -> NameSet -> AutoScc
    mkAutoScc dflags mod exports
      | not opt_SccProfilingOn 	-- No profiling
      = NoSccs
        -- Add auto-scc on all top-level things
      | dopt Opt_AutoSccsOnAllToplevs dflags
      = AddSccs mod (\id -> not $ isDerivedOccName $ getOccName id)
        -- See #1641.  This is pretty yucky, but I can't see a better way
        -- to identify compiler-generated Ids, and at least this should
        -- catch them all.
        -- Only on exported things
      | dopt Opt_AutoSccsOnExportedToplevs dflags
      = AddSccs mod (\id -> idName id `elemNameSet` exports)
      | otherwise
      = NoSccs

    deSugarExpr :: HscEnv
    	    -> Module -> GlobalRdrEnv -> TypeEnv
     	    -> LHsExpr Id
    	    -> IO (Messages, Maybe CoreExpr)
    -- Prints its own errors; returns Nothing if error occurred

    deSugarExpr hsc_env this_mod rdr_env type_env tc_expr = do
        let dflags = hsc_dflags hsc_env
        showPass dflags "Desugar"

        -- Do desugaring
        (msgs, mb_core_expr) <- initDs hsc_env this_mod rdr_env type_env $
                                       dsLExpr tc_expr

        case mb_core_expr of
          Nothing   -> return (msgs, Nothing)
          Just expr -> do

            -- Dump output
            dumpIfSet_dyn dflags Opt_D_dump_ds "Desugared" (pprCoreExpr expr)

            return (msgs, Just expr)

\end{code} %************************************************************************ %* * %* Add rules and export flags to binders %* * %************************************************************************ \begin{code}

    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 | target == HscInterpreted = isExternalName
    		| otherwise 		   = (`elemNameSet` exports)

\end{code} 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 %* * %************************************************************************ \begin{code}

    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 | RuleBndr (L _ var) <- vars]

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

    	; rhs' <- dsLExpr 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 becuase
    		-- 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
                                     name act fn_name final_bndrs args final_rhs
    	; return (Just rule)
    	} } }

\end{code} 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.