{-# LANGUAGE MonadComprehensions #-} {-# LANGUAGE TypeFamilies #-} {-# OPTIONS_GHC -Wno-incomplete-uni-patterns #-} {- (c) The University of Glasgow 2006 (c) The GRASP/AQUA Project, Glasgow University, 1992-1998 The Desugarer: turning HsSyn into Core. -} module GHC.HsToCore ( -- * Desugaring operations deSugar, deSugarExpr ) where import GHC.Prelude import GHC.Driver.Session import GHC.Driver.Config import GHC.Driver.Config.Core.Lint ( endPassHscEnvIO ) import GHC.Driver.Config.HsToCore.Ticks import GHC.Driver.Config.HsToCore.Usage import GHC.Driver.Env import GHC.Driver.Backend import GHC.Driver.Plugins import GHC.Hs import GHC.HsToCore.Usage import GHC.HsToCore.Monad import GHC.HsToCore.Errors.Types import GHC.HsToCore.Expr import GHC.HsToCore.Binds import GHC.HsToCore.Foreign.Decl import GHC.HsToCore.Ticks import GHC.HsToCore.Breakpoints import GHC.HsToCore.Coverage import GHC.HsToCore.Docs import GHC.Tc.Types import GHC.Tc.Utils.Monad ( finalSafeMode, fixSafeInstances ) import GHC.Tc.Module ( runTcInteractive ) import GHC.Core.Type import GHC.Core.TyCo.Compare( eqType ) import GHC.Core.TyCon ( tyConDataCons ) import GHC.Core import GHC.Core.FVs ( exprsSomeFreeVarsList ) import GHC.Core.SimpleOpt ( simpleOptPgm, simpleOptExpr ) import GHC.Core.Utils import GHC.Core.Unfold.Make import GHC.Core.Coercion import GHC.Core.DataCon ( dataConWrapId ) import GHC.Core.Make import GHC.Core.Rules import GHC.Core.Opt.Pipeline.Types ( CoreToDo(..) ) import GHC.Core.Ppr import GHC.Builtin.Names import GHC.Builtin.Types.Prim import GHC.Builtin.Types import GHC.Data.FastString import GHC.Data.Maybe ( expectJust ) import GHC.Data.OrdList import GHC.Data.SizedSeq ( sizeSS ) import GHC.Utils.Error import GHC.Utils.Outputable import GHC.Utils.Panic.Plain import GHC.Utils.Misc import GHC.Utils.Monad import GHC.Utils.Logger import GHC.Types.Id import GHC.Types.Id.Info import GHC.Types.ForeignStubs import GHC.Types.Avail import GHC.Types.Basic import GHC.Types.Var.Set import GHC.Types.SrcLoc import GHC.Types.SourceFile import GHC.Types.TypeEnv import GHC.Types.Name import GHC.Types.Name.Set import GHC.Types.Name.Env import GHC.Types.Name.Ppr import GHC.Types.HpcInfo import GHC.Unit import GHC.Unit.Module.ModGuts import GHC.Unit.Module.ModIface import GHC.Unit.Module.Deps import Data.List (partition) import Data.IORef import Data.Traversable (for) {- ************************************************************************ * * * The main function: deSugar * * ************************************************************************ -} -- | Main entry point to the desugarer. deSugar :: HscEnv -> ModLocation -> TcGblEnv -> IO (Messages DsMessage, Maybe ModGuts) -- Can modify PCS by faulting in more declarations deSugar hsc_env mod_loc tcg_env@(TcGblEnv { tcg_mod = id_mod, tcg_semantic_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_merged = merged, 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_th_foreign_files = th_foreign_files_var, tcg_fords = fords, tcg_rules = rules, tcg_patsyns = patsyns, tcg_tcs = tcs, tcg_insts = insts, tcg_fam_insts = fam_insts, tcg_hpc = other_hpc_info, tcg_complete_matches = complete_matches, tcg_self_boot = self_boot }) = do { let dflags = hsc_dflags hsc_env logger = hsc_logger hsc_env print_unqual = mkPrintUnqualified (hsc_unit_env hsc_env) rdr_env ; withTiming logger (text "Desugar"<+>brackets (ppr mod)) (const ()) $ do { -- Desugar the program ; let export_set = availsToNameSet exports bcknd = backend dflags ; (binds_cvr, m_tickInfo) <- if not (isHsBootOrSig hsc_src) then addTicksToBinds (hsc_logger hsc_env) (initTicksConfig (hsc_dflags hsc_env)) mod mod_loc export_set (typeEnvTyCons type_env) binds else return (binds, Nothing) ; modBreaks <- for [ (i, s) | i <- hsc_interp hsc_env , (_, s) <- m_tickInfo , backendWantsBreakpointTicks (backend dflags) ] $ \(interp, specs) -> mkModBreaks interp mod specs ; ds_hpc_info <- case m_tickInfo of Just (orig_file2, ticks) | gopt Opt_Hpc $ hsc_dflags hsc_env -> do hashNo <- if gopt Opt_Hpc $ hsc_dflags hsc_env then writeMixEntries (hpcDir dflags) mod ticks orig_file2 else return 0 -- dummy hash when none are written pure $ HpcInfo (fromIntegral $ sizeSS ticks) hashNo _ -> pure $ emptyHpcInfo other_hpc_info ; (msgs, mb_res) <- initDs hsc_env tcg_env $ do { ds_ev_binds <- dsEvBinds ev_binds ; core_prs <- dsTopLHsBinds binds_cvr ; core_prs <- patchMagicDefns core_prs ; (spec_prs, spec_rules) <- dsImpSpecs imp_specs ; (ds_fords, foreign_prs) <- dsForeigns fords ; ds_rules <- mapMaybeM dsRule rules ; let hpc_init | gopt Opt_Hpc dflags = hpcInitCode (targetPlatform $ hsc_dflags hsc_env) mod ds_hpc_info | otherwise = mempty ; return ( ds_ev_binds , foreign_prs `appOL` core_prs `appOL` spec_prs , spec_rules ++ ds_rules , ds_fords `appendStubC` hpc_init) } ; case mb_res of { Nothing -> return (msgs, Nothing) ; Just (ds_ev_binds, all_prs, all_rules, ds_fords) -> 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 bcknd 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#! ; endPassHscEnvIO hsc_env print_unqual CoreDesugar final_pgm rules_for_imps ; let simpl_opts = initSimpleOpts dflags ; let (ds_binds, ds_rules_for_imps, occ_anald_binds) = simpleOptPgm simpl_opts mod final_pgm rules_for_imps -- The simpleOptPgm gets rid of type -- bindings plus any stupid dead code ; putDumpFileMaybe logger Opt_D_dump_occur_anal "Occurrence analysis" FormatCore (pprCoreBindings occ_anald_binds $$ pprRules ds_rules_for_imps ) ; endPassHscEnvIO hsc_env print_unqual CoreDesugarOpt ds_binds ds_rules_for_imps ; let used_names = mkUsedNames tcg_env pluginModules = map lpModule (loadedPlugins (hsc_plugins hsc_env)) home_unit = hsc_home_unit hsc_env ; let deps = mkDependencies home_unit (tcg_mod tcg_env) (tcg_imports tcg_env) (map mi_module pluginModules) ; used_th <- readIORef tc_splice_used ; dep_files <- readIORef dependent_files ; safe_mode <- finalSafeMode dflags tcg_env ; (needed_mods, needed_pkgs) <- readIORef (tcg_th_needed_deps tcg_env) ; let uc = initUsageConfig hsc_env ; let plugins = hsc_plugins hsc_env ; let fc = hsc_FC hsc_env ; let unit_env = hsc_unit_env hsc_env ; usages <- mkUsageInfo uc plugins fc unit_env mod (imp_mods imports) used_names dep_files merged needed_mods needed_pkgs -- id_mod /= mod when we are processing an hsig, but hsigs -- never desugared and compiled (there's no code!) -- Consequently, this should hold for any ModGuts that make -- past desugaring. See Note [Identity versus semantic module]. ; massert (id_mod == mod) ; foreign_files <- readIORef th_foreign_files_var ; docs <- extractDocs dflags tcg_env ; let mod_guts = ModGuts { mg_module = mod, mg_hsc_src = hsc_src, mg_loc = mkFileSrcSpan mod_loc, mg_exports = exports, mg_usages = usages, mg_deps = deps, mg_used_th = used_th, mg_rdr_env = rdr_env, mg_fix_env = fix_env, mg_warns = warns, mg_anns = anns, mg_tcs = tcs, mg_insts = fixSafeInstances safe_mode insts, mg_fam_insts = fam_insts, mg_inst_env = inst_env, mg_fam_inst_env = fam_inst_env, mg_boot_exports = bootExports self_boot, mg_patsyns = patsyns, mg_rules = ds_rules_for_imps, mg_binds = ds_binds, mg_foreign = ds_fords, mg_foreign_files = foreign_files, mg_hpc_info = ds_hpc_info, mg_modBreaks = modBreaks, mg_safe_haskell = safe_mode, mg_trust_pkg = imp_trust_own_pkg imports, mg_complete_matches = complete_matches, mg_docs = docs } ; return (msgs, Just mod_guts) }}}} mkFileSrcSpan :: ModLocation -> SrcSpan mkFileSrcSpan mod_loc = case ml_hs_file mod_loc of Just file_path -> mkGeneralSrcSpan (mkFastString file_path) Nothing -> interactiveSrcSpan -- Presumably 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 take 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 GhcTc -> IO (Messages DsMessage, Maybe CoreExpr) deSugarExpr hsc_env tc_expr = do let logger = hsc_logger hsc_env showPass logger "Desugar" -- Do desugaring (tc_msgs, mb_result) <- runTcInteractive hsc_env $ initDsTc $ dsLExpr tc_expr massert (isEmptyMessages tc_msgs) -- the type-checker isn't doing anything here -- mb_result is Nothing only when a failure happens in the type-checker, -- but mb_core_expr is Nothing when a failure happens in the desugarer let (ds_msgs, mb_core_expr) = expectJust "deSugarExpr" mb_result case mb_core_expr of Nothing -> return () Just expr -> putDumpFileMaybe logger Opt_D_dump_ds "Desugared" FormatCore (pprCoreExpr expr) -- callers (i.e. ioMsgMaybe) expect that no expression is returned if -- there are errors let final_res | errorsFound ds_msgs = Nothing | otherwise = mb_core_expr return (ds_msgs, final_res) {- ************************************************************************ * * * Add rules and export flags to binders * * ************************************************************************ -} addExportFlagsAndRules :: Backend -> NameSet -> NameSet -> [CoreRule] -> [(Id, t)] -> [(Id, t)] addExportFlagsAndRules bcknd exports keep_alive rules = mapFst add_one 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 | backendWantsGlobalBindings bcknd = 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 rewrite 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 rewrite rules * * ************************************************************************ -} dsRule :: LRuleDecl GhcTc -> DsM (Maybe CoreRule) dsRule (L loc (HsRule { rd_name = name , rd_act = rule_act , rd_tmvs = vars , rd_lhs = lhs , rd_rhs = rhs })) = putSrcSpanDs (locA 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 ; this_mod <- getModule ; (bndrs'', lhs'', rhs'') <- unfold_coerce bndrs' lhs' rhs' -- Substitute the dict bindings eagerly, -- and take the body apart into a (f args) form ; dflags <- getDynFlags ; case decomposeRuleLhs dflags bndrs'' lhs'' of { Left msg -> do { diagnosticDs 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 simpl_opts = initSimpleOpts dflags final_rhs = simpleOptExpr simpl_opts rhs'' -- De-crap it rule_name = unLoc name rule = mkRule this_mod False is_local rule_name rule_act fn_name final_bndrs args final_rhs ; dsWarnOrphanRule rule ; dsWarnRuleShadowing fn_id rule ; return (Just rule) } } } dsWarnRuleShadowing :: Id -> CoreRule -> DsM () -- See Note [Rules and inlining/other rules] dsWarnRuleShadowing fn_id (Rule { ru_name = rule_name, ru_act = rule_act, ru_bndrs = bndrs, ru_args = args}) = do { check False fn_id -- We often have multiple rules for the same Id in a -- module. Maybe we should check that they don't overlap -- but currently we don't ; mapM_ (check True) arg_ids } where bndrs_set = mkVarSet bndrs arg_ids = filterOut (`elemVarSet` bndrs_set) $ exprsSomeFreeVarsList isId args check check_rules_too lhs_id | isLocalId lhs_id || canUnfold (idUnfolding lhs_id) -- If imported with no unfolding, no worries , idInlineActivation lhs_id `competesWith` rule_act = diagnosticDs (DsRuleMightInlineFirst rule_name lhs_id rule_act) | check_rules_too , bad_rule : _ <- get_bad_rules lhs_id = diagnosticDs (DsAnotherRuleMightFireFirst rule_name (ruleName bad_rule) lhs_id) | otherwise = return () get_bad_rules lhs_id = [ rule | rule <- idCoreRules lhs_id , ruleActivation rule `competesWith` rule_act ] dsWarnRuleShadowing _ _ = return () -- Not expecting built-in rules here -- 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, [k, t1, t2]) <- splitTyConApp_maybe (idType v) , tc `hasKey` coercibleTyConKey = do u <- newUnique let ty' = mkTyConApp eqReprPrimTyCon [k, k, t1, t2] v' = mkLocalCoVar (mkDerivedInternalName mkRepEqOcc u (getName v)) ty' box = Var (dataConWrapId coercibleDataCon) `mkTyApps` [k, t1, t2] `App` Coercion (mkCoVarCo v') (bndrs, wrap) <- go vs return (v':bndrs, mkCoreLet (NonRec v box) . 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 GHC.HsToCore.Expr.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 precisely 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 away, 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). See also Note [Getting the map/coerce RULE to work] in GHC.Core.SimpleOpt. Note [Rules and inlining/other rules] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ If you have f x = ... g x = ... {-# RULES "rule-for-f" forall x. f (g x) = ... #-} then there's a good chance that in a potential rule redex ...f (g e)... then 'f' or 'g' will inline before the rule can fire. Solution: add an INLINE [n] or NOINLINE [n] pragma to 'f' and 'g'. Note that this applies to all the free variables on the LHS, both the main function and things in its arguments. We also check if there are Ids on the LHS that have competing RULES. In the above example, suppose we had {-# RULES "rule-for-g" forally. g [y] = ... #-} Then "rule-for-f" and "rule-for-g" would compete. Better to add phase control, so "rule-for-f" has a chance to fire before "rule-for-g" becomes active; or perhaps after "rule-for-g" has become inactive. This is checked by 'competesWith' Class methods have a built-in RULE to select the method from the dictionary, so you can't change the phase on this. That makes id very dubious to match on class methods in RULE lhs's. See #10595. I'm not happy about this. For example in Control.Arrow we have {-# RULES "compose/arr" forall f g . (arr f) . (arr g) = arr (f . g) #-} and similar, which will elicit exactly these warnings, and risk never firing. But it's not clear what to do instead. We could make the class method rules inactive in phase 2, but that would delay when subsequent transformations could fire. -} {- ************************************************************************ * * * Magic definitions * * ************************************************************************ Note [Patching magic definitions] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ We sometimes need to have access to defined Ids in pure contexts. Usually, we simply "wire in" these entities, as we do for types in GHC.Builtin.Types and for Ids in GHC.Types.Id.Make. See Note [Wired-in Ids] in GHC.Types.Id.Make. However, it is sometimes *much* easier to define entities in Haskell, even if we need pure access; note that wiring-in an Id requires all entities used in its definition *also* to be wired in, transitively and recursively. This can be a huge pain. The little trick documented here allows us to have the best of both worlds. Motivating example: unsafeCoerce#. See [Wiring in unsafeCoerce#] for the details. The trick is to * Define the known-key Id in a library module, with a stub definition, unsafeCoerce# :: ..a suitable type signature.. unsafeCoerce# = error "urk" * Magically over-write its RHS here in the desugarer, in patchMagicDefns. This update can be done with full access to the DsM monad, and hence, dsLookupGlobal. We thus do not have to wire in all the entities used internally, a potentially big win. This step should not change the Name or type of the Id. Because an Id stores its unfolding directly (as opposed to in the second component of a (Id, CoreExpr) pair), the patchMagicDefns function returns a new Id to use. Here are the moving parts: - patchMagicDefns checks whether we're in a module with magic definitions; if so, patch the magic definitions. If not, skip. - patchMagicDefn just looks up in an environment to find a magic defn and patches it in. - magicDefns holds the magic definitions. - magicDefnsEnv allows for quick access to magicDefns. - magicDefnModules, built also from magicDefns, contains the modules that need careful attention. Note [Wiring in unsafeCoerce#] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ We want (Haskell) unsafeCoerce# :: forall (r1 :: RuntimeRep) (r2 :: RuntimeRep) (a :: TYPE r1) (b :: TYPE r2). a -> b unsafeCoerce# x = case unsafeEqualityProof @r1 @r2 of UnsafeRefl -> case unsafeEqualityProof @a @b of UnsafeRefl -> x or (Core) unsafeCoerce# :: forall (r1 :: RuntimeRep) (r2 :: RuntimeRep) (a :: TYPE r1) (b :: TYPE r2). a -> b unsafeCoerce# = \ @r1 @r2 @a @b (x :: a). case unsafeEqualityProof @RuntimeRep @r1 @r2 of UnsafeRefl (co1 :: r1 ~# r2) -> case unsafeEqualityProof @(TYPE r2) @(a |> TYPE co1) @b of UnsafeRefl (co2 :: (a |> TYPE co1) ~# b) -> (x |> (GRefl :: a ~# (a |> TYPE co1)) ; co2) It looks like we can write this in Haskell directly, but we can't: the representation polymorphism checks defeat us. Note that `x` is a representation-polymorphic variable. So we must wire it in with a compulsory unfolding, like other representation-polymorphic primops. The challenge is that UnsafeEquality is a GADT, and wiring in a GADT is *hard*: it has a worker separate from its wrapper, with all manner of complications. (Simon and Richard tried to do this. We nearly wept.) The solution is documented in Note [Patching magic definitions]. We now simply look up the UnsafeEquality GADT in the environment, leaving us only to wire in unsafeCoerce# directly. Wrinkle: see Note [Always expose compulsory unfoldings] in GHC.Iface.Tidy -} -- Postcondition: the returned Ids are in one-to-one correspondence as the -- input Ids; each returned Id has the same type as the passed-in Id. -- See Note [Patching magic definitions] patchMagicDefns :: OrdList (Id,CoreExpr) -> DsM (OrdList (Id,CoreExpr)) patchMagicDefns pairs -- optimization: check whether we're in a magic module before looking -- at all the ids = do { this_mod <- getModule ; if this_mod `elemModuleSet` magicDefnModules then traverse patchMagicDefn pairs else return pairs } patchMagicDefn :: (Id, CoreExpr) -> DsM (Id, CoreExpr) patchMagicDefn orig_pair@(orig_id, orig_rhs) | Just mk_magic_pair <- lookupNameEnv magicDefnsEnv (getName orig_id) = do { magic_pair@(magic_id, _) <- mk_magic_pair orig_id orig_rhs -- Patching should not change the Name or the type of the Id ; massert (getUnique magic_id == getUnique orig_id) ; massert (varType magic_id `eqType` varType orig_id) ; return magic_pair } | otherwise = return orig_pair magicDefns :: [(Name, Id -> CoreExpr -- old Id and RHS -> DsM (Id, CoreExpr) -- new Id and RHS )] magicDefns = [ (unsafeCoercePrimName, mkUnsafeCoercePrimPair) ] magicDefnsEnv :: NameEnv (Id -> CoreExpr -> DsM (Id, CoreExpr)) magicDefnsEnv = mkNameEnv magicDefns magicDefnModules :: ModuleSet magicDefnModules = mkModuleSet $ map (nameModule . getName . fst) magicDefns mkUnsafeCoercePrimPair :: Id -> CoreExpr -> DsM (Id, CoreExpr) -- See Note [Wiring in unsafeCoerce#] for the defn we are creating here mkUnsafeCoercePrimPair _old_id old_expr = do { unsafe_equality_proof_id <- dsLookupGlobalId unsafeEqualityProofName ; unsafe_equality_tc <- dsLookupTyCon unsafeEqualityTyConName ; let [unsafe_refl_data_con] = tyConDataCons unsafe_equality_tc rhs = mkLams [ runtimeRep1TyVar, runtimeRep2TyVar , openAlphaTyVar, openBetaTyVar , x ] $ mkSingleAltCase scrut1 (mkWildValBinder ManyTy scrut1_ty) (DataAlt unsafe_refl_data_con) [rr_cv] $ mkSingleAltCase scrut2 (mkWildValBinder ManyTy scrut2_ty) (DataAlt unsafe_refl_data_con) [ab_cv] $ Var x `mkCast` x_co [x, rr_cv, ab_cv] = mkTemplateLocals [ openAlphaTy -- x :: a , rr_cv_ty -- rr_cv :: r1 ~# r2 , ab_cv_ty -- ab_cv :: (alpha |> alpha_co ~# beta) ] -- Returns (scrutinee, scrutinee type, type of covar in AltCon) unsafe_equality k a b = ( mkTyApps (Var unsafe_equality_proof_id) [k,b,a] , mkTyConApp unsafe_equality_tc [k,b,a] , mkHeteroPrimEqPred k k a b ) -- NB: UnsafeRefl :: (b ~# a) -> UnsafeEquality a b, so we have to -- carefully swap the arguments above (scrut1, scrut1_ty, rr_cv_ty) = unsafe_equality runtimeRepTy runtimeRep1Ty runtimeRep2Ty (scrut2, scrut2_ty, ab_cv_ty) = unsafe_equality (mkTYPEapp runtimeRep2Ty) (openAlphaTy `mkCastTy` alpha_co) openBetaTy -- alpha_co :: TYPE r1 ~# TYPE r2 -- alpha_co = TYPE rr_cv alpha_co = mkTyConAppCo Nominal tYPETyCon [mkCoVarCo rr_cv] -- x_co :: alpha ~R# beta x_co = mkGReflCo Representational openAlphaTy (MCo alpha_co) `mkTransCo` mkSubCo (mkCoVarCo ab_cv) info = noCafIdInfo `setInlinePragInfo` alwaysInlinePragma `setUnfoldingInfo` mkCompulsoryUnfolding rhs `setArityInfo` arity ty = mkSpecForAllTys [ runtimeRep1TyVar, runtimeRep2TyVar , openAlphaTyVar, openBetaTyVar ] $ mkVisFunTyMany openAlphaTy openBetaTy arity = 1 id = mkExportedVanillaId unsafeCoercePrimName ty `setIdInfo` info ; return (id, old_expr) }