X-Git-Url: http://git.megacz.com/?a=blobdiff_plain;f=compiler%2FdeSugar%2FDsBinds.lhs;h=cc7ecfd38ee5c424ca4a0da9e225447c66a39b1a;hb=81e41fe31fb27b6330762d93ce90eb98899beb24;hp=85487e6e816b16434b402a2b451cb82767e5e76a;hpb=cad6d4688bdc309b3e9953bf091535a8eeaa2515;p=ghc-hetmet.git diff --git a/compiler/deSugar/DsBinds.lhs b/compiler/deSugar/DsBinds.lhs index 85487e6..cc7ecfd 100644 --- a/compiler/deSugar/DsBinds.lhs +++ b/compiler/deSugar/DsBinds.lhs @@ -38,6 +38,7 @@ import TysPrim ( anyTypeOfKind ) import CostCentre import Module import Id +import Name ( localiseName ) import MkId ( seqId ) import Var ( Var, TyVar, tyVarKind ) import IdInfo ( vanillaIdInfo ) @@ -55,7 +56,6 @@ import Util ( count, lengthExceeds ) import MonadUtils import Control.Monad -import Data.List \end{code} %************************************************************************ @@ -115,92 +115,28 @@ dsHsBind _ rest ; sel_binds <- mkSelectorBinds pat body_expr ; return (sel_binds ++ rest) } -{- Note [Rules and inlining] - ~~~~~~~~~~~~~~~~~~~~~~~~~ - Common special case: no type or dictionary abstraction - This is a bit less trivial than you might suppose - The naive way woudl be to desguar to something like - f_lcl = ...f_lcl... -- The "binds" from AbsBinds - M.f = f_lcl -- Generated from "exports" - But we don't want that, because if M.f isn't exported, - it'll be inlined unconditionally at every call site (its rhs is - trivial). That would be ok unless it has RULES, which would - thereby be completely lost. Bad, bad, bad. - - Instead we want to generate - M.f = ...f_lcl... - f_lcl = M.f - Now all is cool. The RULES are attached to M.f (by SimplCore), - and f_lcl is rapidly inlined away. - - This does not happen in the same way to polymorphic binds, - because they desugar to - M.f = /\a. let f_lcl = ...f_lcl... in f_lcl - Although I'm a bit worried about whether full laziness might - float the f_lcl binding out and then inline M.f at its call site -} - dsHsBind auto_scc rest (AbsBinds [] [] exports binds) = do { core_prs <- ds_lhs_binds NoSccs binds ; let env = mkABEnv exports - ar_env = mkArityEnv binds do_one (lcl_id, rhs) | Just (_, gbl_id, _, spec_prags) <- lookupVarEnv env lcl_id - = WARN( not (null spec_prags), ppr gbl_id $$ ppr spec_prags ) -- Not overloaded - makeCorePair gbl_id (lookupArity ar_env lcl_id) - (addAutoScc auto_scc gbl_id rhs) + = do { let rhs' = addAutoScc auto_scc gbl_id rhs + ; (spec_binds, rules) <- dsSpecs gbl_id (Let (Rec core_prs) rhs') spec_prags + -- See Note [Specialising in no-dict case] + ; let gbl_id' = addIdSpecialisations gbl_id rules + main_bind = makeCorePair gbl_id' False 0 rhs' + ; return (main_bind : spec_binds) } - | otherwise = (lcl_id, rhs) + | otherwise = return [(lcl_id, rhs)] locals' = [(lcl_id, Var gbl_id) | (_, gbl_id, lcl_id, _) <- exports] -- Note [Rules and inlining] - ; return (map do_one core_prs ++ locals' ++ rest) } + ; export_binds <- mapM do_one core_prs + ; return (concat export_binds ++ locals' ++ rest) } -- No Rec needed here (contrast the other AbsBinds cases) -- because we can rely on the enclosing dsBind to wrap in Rec -{- Note [Abstracting over tyvars only] - ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ - When abstracting over type variable only (not dictionaries), we don't really need to - built a tuple and select from it, as we do in the general case. Instead we can take - - AbsBinds [a,b] [ ([a,b], fg, fl, _), - ([b], gg, gl, _) ] - { fl = e1 - gl = e2 - h = e3 } - - and desugar it to - - fg = /\ab. let B in e1 - gg = /\b. let a = () in let B in S(e2) - h = /\ab. let B in e3 - - where B is the *non-recursive* binding - fl = fg a b - gl = gg b - h = h a b -- See (b); note shadowing! - - Notice (a) g has a different number of type variables to f, so we must - use the mkArbitraryType thing to fill in the gaps. - We use a type-let to do that. - - (b) The local variable h isn't in the exports, and rather than - clone a fresh copy we simply replace h by (h a b), where - the two h's have different types! Shadowing happens here, - which looks confusing but works fine. - - (c) The result is *still* quadratic-sized if there are a lot of - small bindings. So if there are more than some small - number (10), we filter the binding set B by the free - variables of the particular RHS. Tiresome. - - Why got to this trouble? It's a common case, and it removes the - quadratic-sized tuple desugaring. Less clutter, hopefullly faster - compilation, especially in a case where there are a *lot* of - bindings. --} - - dsHsBind auto_scc rest (AbsBinds tyvars [] exports binds) | opt_DsMultiTyVar -- This (static) debug flag just lets us -- switch on and off this optimisation to @@ -217,9 +153,7 @@ dsHsBind auto_scc rest (AbsBinds tyvars [] exports binds) where fvs = exprSomeFreeVars (`elemVarSet` bndrs) rhs - ar_env = mkArityEnv binds env = mkABEnv exports - mk_lg_bind lcl_id gbl_id tyvars = NonRec (setIdInfo lcl_id vanillaIdInfo) -- Nuke the IdInfo so that no old unfoldings @@ -229,21 +163,22 @@ dsHsBind auto_scc rest (AbsBinds tyvars [] exports binds) do_one lg_binds (lcl_id, rhs) | Just (id_tvs, gbl_id, _, spec_prags) <- lookupVarEnv env lcl_id - = WARN( not (null spec_prags), ppr gbl_id $$ ppr spec_prags ) -- Not overloaded - (let rhs' = addAutoScc auto_scc gbl_id $ - mkLams id_tvs $ - mkLets [ NonRec tv (Type (lookupVarEnv_NF arby_env tv)) - | tv <- tyvars, not (tv `elem` id_tvs)] $ - add_lets lg_binds rhs - in return (mk_lg_bind lcl_id gbl_id id_tvs, - makeCorePair gbl_id (lookupArity ar_env lcl_id) rhs')) + = do { let rhs' = addAutoScc auto_scc gbl_id $ + mkLams id_tvs $ + mkLets [ NonRec tv (Type (lookupVarEnv_NF arby_env tv)) + | tv <- tyvars, not (tv `elem` id_tvs)] $ + add_lets lg_binds rhs + ; (spec_binds, rules) <- dsSpecs gbl_id rhs' spec_prags + ; let gbl_id' = addIdSpecialisations gbl_id rules + main_bind = makeCorePair gbl_id' False 0 rhs' + ; return (mk_lg_bind lcl_id gbl_id' id_tvs, main_bind : spec_binds) } | otherwise = do { non_exp_gbl_id <- newUniqueId lcl_id (mkForAllTys tyvars (idType lcl_id)) ; return (mk_lg_bind lcl_id non_exp_gbl_id tyvars, - (non_exp_gbl_id, mkLams tyvars (add_lets lg_binds rhs))) } + [(non_exp_gbl_id, mkLams tyvars (add_lets lg_binds rhs))]) } ; (_, core_prs') <- fixDs (\ ~(lg_binds, _) -> mapAndUnzipM (do_one lg_binds) core_prs) - ; return (core_prs' ++ rest) } + ; return (concat core_prs' ++ rest) } -- Another common case: one exported variable -- Non-recursive bindings come through this way @@ -254,25 +189,23 @@ dsHsBind auto_scc rest = ASSERT( all (`elem` tyvars) all_tyvars ) do { core_prs <- ds_lhs_binds NoSccs binds - ; let -- Always treat the binds as recursive, because the typechecker - -- makes rather mixed-up dictionary bindings + ; let -- Always treat the binds as recursive, because the + -- typechecker makes rather mixed-up dictionary bindings core_bind = Rec core_prs - inl_arity = lookupArity (mkArityEnv binds) local + rhs = addAutoScc auto_scc global $ + mkLams tyvars $ mkLams dicts $ Let core_bind (Var local) - ; (spec_binds, rules) <- dsSpecs all_tyvars dicts tyvars global - local inl_arity core_bind prags + ; (spec_binds, rules) <- dsSpecs global rhs prags ; let global' = addIdSpecialisations global rules - rhs = addAutoScc auto_scc global $ - mkLams tyvars $ mkLams dicts $ Let core_bind (Var local) - main_bind = makeCorePair global' (inl_arity + dictArity dicts) rhs + main_bind = makeCorePair global' (isDefaultMethod prags) + (dictArity dicts) rhs ; return (main_bind : spec_binds ++ rest) } dsHsBind auto_scc rest (AbsBinds all_tyvars dicts exports binds) = do { core_prs <- ds_lhs_binds NoSccs binds ; let env = mkABEnv exports - ar_env = mkArityEnv binds do_one (lcl_id,rhs) | Just (_, gbl_id, _, _prags) <- lookupVarEnv env lcl_id = (lcl_id, addAutoScc auto_scc gbl_id rhs) | otherwise = (lcl_id,rhs) @@ -280,30 +213,30 @@ dsHsBind auto_scc rest (AbsBinds all_tyvars dicts exports binds) -- Rec because of mixed-up dictionary bindings core_bind = Rec (map do_one core_prs) - tup_expr = mkBigCoreVarTup locals - tup_ty = exprType tup_expr - poly_tup_expr = mkLams all_tyvars $ mkLams dicts $ - Let core_bind tup_expr - locals = [local | (_, _, local, _) <- exports] - local_tys = map idType locals + tup_expr = mkBigCoreVarTup locals + tup_ty = exprType tup_expr + poly_tup_rhs = mkLams all_tyvars $ mkLams dicts $ + Let core_bind tup_expr + locals = [local | (_, _, local, _) <- exports] + local_tys = map idType locals - ; poly_tup_id <- newSysLocalDs (exprType poly_tup_expr) + ; poly_tup_id <- newSysLocalDs (exprType poly_tup_rhs) - ; let mk_bind ((tyvars, global, local, spec_prags), n) -- locals!!n == local + ; let mk_bind ((tyvars, global, _, spec_prags), n) -- locals!!n == local = -- Need to make fresh locals to bind in the selector, -- because some of the tyvars will be bound to 'Any' do { let ty_args = map mk_ty_arg all_tyvars substitute = substTyWith all_tyvars ty_args ; locals' <- newSysLocalsDs (map substitute local_tys) ; tup_id <- newSysLocalDs (substitute tup_ty) - ; (spec_binds, rules) <- dsSpecs all_tyvars dicts tyvars global local - (lookupArity ar_env local) core_bind - spec_prags - ; let global' = addIdSpecialisations global rules - rhs = mkLams tyvars $ mkLams dicts $ + ; let rhs = mkLams tyvars $ mkLams dicts $ mkTupleSelector locals' (locals' !! n) tup_id $ mkVarApps (mkTyApps (Var poly_tup_id) ty_args) dicts + ; (spec_binds, rules) <- dsSpecs global + (Let (NonRec poly_tup_id poly_tup_rhs) rhs) + spec_prags + ; let global' = addIdSpecialisations global rules ; return ((global', rhs) : spec_binds) } where mk_ty_arg all_tyvar @@ -313,55 +246,128 @@ dsHsBind auto_scc rest (AbsBinds all_tyvars dicts exports binds) ; export_binds_s <- mapM mk_bind (exports `zip` [0..]) -- Don't scc (auto-)annotate the tuple itself. - ; return ((poly_tup_id, poly_tup_expr) : + ; return ((poly_tup_id, poly_tup_rhs) : (concat export_binds_s ++ rest)) } ------------------------ -makeCorePair :: Id-> Arity -> CoreExpr -> (Id, CoreExpr) -makeCorePair gbl_id arity rhs - | isInlinePragma (idInlinePragma gbl_id) +makeCorePair :: Id -> Bool -> Arity -> CoreExpr -> (Id, CoreExpr) +makeCorePair gbl_id is_default_method dict_arity rhs + | is_default_method -- Default methods are *always* inlined + = (gbl_id `setIdUnfolding` mkCompulsoryUnfolding rhs, rhs) + + | not (isInlinePragma inline_prag) + = (gbl_id, rhs) + + | Just arity <- inlinePragmaSat inline_prag -- Add an Unfolding for an INLINE (but not for NOINLINE) -- And eta-expand the RHS; see Note [Eta-expanding INLINE things] - = (gbl_id `setIdUnfolding` mkInlineRule InlSat rhs arity, + = (gbl_id `setIdUnfolding` mkInlineRule rhs (Just (dict_arity + arity)), + -- NB: The arity in the InlineRule takes account of the dictionaries etaExpand arity rhs) + | otherwise - = (gbl_id, rhs) + = (gbl_id `setIdUnfolding` mkInlineRule rhs Nothing, rhs) + where + inline_prag = idInlinePragma gbl_id + +dictArity :: [Var] -> Arity +-- Don't count coercion variables in arity +dictArity dicts = count isId dicts + ------------------------ -type AbsBindEnv = VarEnv ([TyVar], Id, Id, [LSpecPrag]) +type AbsBindEnv = VarEnv ([TyVar], Id, Id, TcSpecPrags) -- Maps the "lcl_id" for an AbsBind to -- its "gbl_id" and associated pragmas, if any -mkABEnv :: [([TyVar], Id, Id, [LSpecPrag])] -> AbsBindEnv +mkABEnv :: [([TyVar], Id, Id, TcSpecPrags)] -> AbsBindEnv -- Takes the exports of a AbsBinds, and returns a mapping -- lcl_id -> (tyvars, gbl_id, lcl_id, prags) mkABEnv exports = mkVarEnv [ (lcl_id, export) | export@(_, _, lcl_id, _) <- exports] +\end{code} -mkArityEnv :: LHsBinds Id -> IdEnv Arity - -- Maps a local to the arity of its definition -mkArityEnv binds = foldrBag (plusVarEnv . lhsBindArity) emptyVarEnv binds +Note [Rules and inlining] +~~~~~~~~~~~~~~~~~~~~~~~~~ +Common special case: no type or dictionary abstraction +This is a bit less trivial than you might suppose +The naive way woudl be to desguar to something like + f_lcl = ...f_lcl... -- The "binds" from AbsBinds + M.f = f_lcl -- Generated from "exports" +But we don't want that, because if M.f isn't exported, +it'll be inlined unconditionally at every call site (its rhs is +trivial). That would be ok unless it has RULES, which would +thereby be completely lost. Bad, bad, bad. + +Instead we want to generate + M.f = ...f_lcl... + f_lcl = M.f +Now all is cool. The RULES are attached to M.f (by SimplCore), +and f_lcl is rapidly inlined away. + +This does not happen in the same way to polymorphic binds, +because they desugar to + M.f = /\a. let f_lcl = ...f_lcl... in f_lcl +Although I'm a bit worried about whether full laziness might +float the f_lcl binding out and then inline M.f at its call site -} + +Note [Specialising in no-dict case] +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +Even if there are no tyvars or dicts, we may have specialisation pragmas. +Class methods can generate + AbsBinds [] [] [( ... spec-prag] + { AbsBinds [tvs] [dicts] ...blah } +So the overloading is in the nested AbsBinds. A good example is in GHC.Float: + + class (Real a, Fractional a) => RealFrac a where + round :: (Integral b) => a -> b + + instance RealFrac Float where + {-# SPECIALIZE round :: Float -> Int #-} + +The top-level AbsBinds for $cround has no tyvars or dicts (because the +instance does not). But the method is locally overloaded! + +Note [Abstracting over tyvars only] +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +When abstracting over type variable only (not dictionaries), we don't really need to +built a tuple and select from it, as we do in the general case. Instead we can take -lhsBindArity :: LHsBind Id -> IdEnv Arity -lhsBindArity (L _ (FunBind { fun_id = id, fun_matches = ms })) - = unitVarEnv (unLoc id) (matchGroupArity ms) -lhsBindArity (L _ (AbsBinds { abs_exports = exports - , abs_dicts = dicts - , abs_binds = binds })) - = mkVarEnv [ (gbl, lookupArity ar_env lcl + n_val_dicts) - | (_, gbl, lcl, _) <- exports] - where -- See Note [Nested arities] - ar_env = mkArityEnv binds - n_val_dicts = dictArity dicts + AbsBinds [a,b] [ ([a,b], fg, fl, _), + ([b], gg, gl, _) ] + { fl = e1 + gl = e2 + h = e3 } -lhsBindArity _ = emptyVarEnv -- PatBind/VarBind +and desugar it to -dictArity :: [Var] -> Arity --- Don't count coercion variables in arity -dictArity dicts = count isId dicts + fg = /\ab. let B in e1 + gg = /\b. let a = () in let B in S(e2) + h = /\ab. let B in e3 + +where B is the *non-recursive* binding + fl = fg a b + gl = gg b + h = h a b -- See (b); note shadowing! + +Notice (a) g has a different number of type variables to f, so we must + use the mkArbitraryType thing to fill in the gaps. + We use a type-let to do that. + + (b) The local variable h isn't in the exports, and rather than + clone a fresh copy we simply replace h by (h a b), where + the two h's have different types! Shadowing happens here, + which looks confusing but works fine. + + (c) The result is *still* quadratic-sized if there are a lot of + small bindings. So if there are more than some small + number (10), we filter the binding set B by the free + variables of the particular RHS. Tiresome. + +Why got to this trouble? It's a common case, and it removes the +quadratic-sized tuple desugaring. Less clutter, hopefullly faster +compilation, especially in a case where there are a *lot* of +bindings. -lookupArity :: IdEnv Arity -> Id -> Arity -lookupArity ar_env id = lookupVarEnv ar_env id `orElse` 0 -\end{code} Note [Eta-expanding INLINE things] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ @@ -376,9 +382,9 @@ get to do the inlining, which is a Terribly Bad thing given that the user said "inline"! To avoid this we pre-emptively eta-expand the definition, so that foo -has arity 2 (one for the Eq and one for x); and that in turn should -mean that (foo d) is a PAP and we don't share it. - +has the arity with which it is declared in the source code. In this +example it has arity 2 (one for the Eq and one for x). Doing this +should mean that (foo d) is a PAP and we don't share it. Note [Nested arities] ~~~~~~~~~~~~~~~~~~~~~ @@ -397,44 +403,58 @@ gotten from the binding for fromT_1. It might be better to have just one level of AbsBinds, but that requires more thought! +Note [Implementing SPECIALISE pragmas] +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +Example: + f :: (Eq a, Ix b) => a -> b -> Bool + {-# SPECIALISE f :: (Ix p, Ix q) => Int -> (p,q) -> Bool #-} + f = + +From this the typechecker generates + + AbsBinds [ab] [d1,d2] [([ab], f, f_mono, prags)] binds + + SpecPrag (wrap_fn :: forall a b. (Eq a, Ix b) => XXX + -> forall p q. (Ix p, Ix q) => XXX[ Int/a, (p,q)/b ]) + +Note that wrap_fn can transform *any* function with the right type prefix + forall ab. (Eq a, Ix b) => XXX +regardless of XXX. It's sort of polymorphic in XXX. This is +useful: we use the same wrapper to transform each of the class ops, as +well as the dict. + +From these we generate: + + Rule: forall p, q, (dp:Ix p), (dq:Ix q). + f Int (p,q) dInt ($dfInPair dp dq) = f_spec p q dp dq + + Spec bind: f_spec = wrap_fn + +Note that + + * The LHS of the rule may mention dictionary *expressions* (eg + $dfIxPair dp dq), and that is essential because the dp, dq are + needed on the RHS. + + * The RHS of f_spec, has a *copy* of 'binds', so that it + can fully specialise it. \begin{code} ------------------------ -dsSpecs :: [TyVar] -> [DictId] -> [TyVar] - -> Id -> Id -> Arity -- Global, local, arity of local - -> CoreBind -> [LSpecPrag] +dsSpecs :: Id -- The polymorphic Id + -> CoreExpr -- Its rhs + -> TcSpecPrags -> DsM ( [(Id,CoreExpr)] -- Binding for specialised Ids , [CoreRule] ) -- Rules for the Global Ids --- Example: --- f :: (Eq a, Ix b) => a -> b -> b --- {-# SPECIALISE f :: Ix b => Int -> b -> b #-} --- --- AbsBinds [ab] [d1,d2] [([ab], f, f_mono, prags)] binds --- --- SpecPrag (/\b.\(d:Ix b). f Int b dInt d) --- (forall b. Ix b => Int -> b -> b) --- --- Rule: forall b,(d:Ix b). f Int b dInt d = f_spec b d --- --- Spec bind: f_spec = Let f = /\ab \(d1:Eq a)(d2:Ix b). let binds in f_mono --- /\b.\(d:Ix b). in f Int b dInt d --- The idea is that f occurs just once, so it'll be --- inlined and specialised --- --- Given SpecPrag (/\as.\ds. f es) t, we have --- the defn f_spec as ds = let-nonrec f = /\fas\fds. let f_mono = in f_mono --- in f es --- and the RULE forall as, ds. f es = f_spec as ds --- --- It is *possible* that 'es' does not mention all of the dictionaries 'ds' --- (a bit silly, because then the - -dsSpecs all_tvs dicts tvs poly_id mono_id inl_arity mono_bind prags - = do { pairs <- mapMaybeM spec_one prags - ; let (spec_binds_s, rules) = unzip pairs - ; return (concat spec_binds_s, rules) } +-- See Note [Implementing SPECIALISE pragmas] +dsSpecs poly_id poly_rhs prags + = case prags of + IsDefaultMethod -> return ([], []) + SpecPrags sps -> do { pairs <- mapMaybeM spec_one sps + ; let (spec_binds_s, rules) = unzip pairs + ; return (concat spec_binds_s, rules) } where - spec_one :: LSpecPrag -> DsM (Maybe ([(Id,CoreExpr)], CoreRule)) + spec_one :: Located TcSpecPrag -> DsM (Maybe ([(Id,CoreExpr)], CoreRule)) spec_one (L loc (SpecPrag spec_co spec_inl)) = putSrcSpanDs loc $ do { let poly_name = idName poly_id @@ -452,10 +472,9 @@ dsSpecs all_tvs dicts tvs poly_id mono_id inl_arity mono_bind prags bs | not (null bs) -> do { warnDs (dead_msg bs); return Nothing } | otherwise -> do - { (spec_unf, unf_pairs) <- specUnfolding wrap_fn (idUnfolding poly_id) + { (spec_unf, unf_pairs) <- specUnfolding wrap_fn (realIdUnfolding poly_id) - ; let f_body = fix_up (Let mono_bind (Var mono_id)) - spec_ty = exprType ds_spec_expr + ; let spec_ty = exprType ds_spec_expr spec_id = mkLocalId spec_name spec_ty `setInlinePragma` inl_prag `setIdUnfolding` spec_unf @@ -464,32 +483,22 @@ dsSpecs all_tvs dicts tvs poly_id mono_id inl_arity mono_bind prags -- Get the INLINE pragma from SPECIALISE declaration, or, -- failing that, from the original Id - spec_id_arity = inl_arity + count isDictId bndrs - - extra_dict_bndrs = [ localiseId d -- See Note [Constant rule dicts] - | d <- varSetElems (exprFreeVars ds_spec_expr) - , isDictId d] - -- Note [Const rule dicts] + extra_dict_bndrs = [ mkLocalId (localiseName (idName d)) (idType d) + -- See Note [Constant rule dicts] + | d <- varSetElems (exprFreeVars ds_spec_expr) + , isDictId d] rule = mkLocalRule (mkFastString ("SPEC " ++ showSDoc (ppr poly_name))) AlwaysActive poly_name (extra_dict_bndrs ++ bndrs) args (mkVarApps (Var spec_id) bndrs) - spec_rhs = wrap_fn (mkLams (tvs ++ dicts) f_body) - spec_pair = makeCorePair spec_id spec_id_arity spec_rhs + spec_rhs = wrap_fn poly_rhs + spec_pair = makeCorePair spec_id False (dictArity bndrs) spec_rhs ; return (Just (spec_pair : unf_pairs, rule)) } } } } - -- Bind to Any any of all_ptvs that aren't - -- relevant for this particular function - fix_up body | null void_tvs = body - | otherwise = mkTyApps (mkLams void_tvs body) $ - map dsMkArbitraryType void_tvs - - void_tvs = all_tvs \\ tvs - dead_msg bs = vcat [ sep [ptext (sLit "Useless constraint") <> plural bs <+> ptext (sLit "in specialied type:"), nest 2 (pprTheta (map get_pred bs))] @@ -544,7 +553,7 @@ the constraint is unused. We could bind 'd' to (error "unused") but it seems better to reject the program because it's almost certainly a mistake. That's what the isDeadBinder call detects. -Note [Const rule dicts] +Note [Constant rule dicts] ~~~~~~~~~~~~~~~~~~~~~~~ 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 @@ -565,8 +574,9 @@ And from that we want the rule 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. Hence the use of 'localiseId' to make it Internal. - +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. %************************************************************************ %* *