X-Git-Url: http://git.megacz.com/?p=ghc-hetmet.git;a=blobdiff_plain;f=compiler%2Ftypecheck%2FTcSimplify.lhs;h=342114b9fbb04e3ab9316409ecab54c8bec2d0fb;hp=19557c6dd7e21e2268a05219aa18b0ab250a80b4;hb=84923cc7de2a93c22a2f72daf9ac863959efae13;hpb=5a8a219c131b56c75dbb3110da3ab5ae3715ff66 diff --git a/compiler/typecheck/TcSimplify.lhs b/compiler/typecheck/TcSimplify.lhs index 19557c6..342114b 100644 --- a/compiler/typecheck/TcSimplify.lhs +++ b/compiler/typecheck/TcSimplify.lhs @@ -1,9 +1,9 @@ % +% (c) The University of Glasgow 2006 % (c) The GRASP/AQUA Project, Glasgow University, 1992-1998 % -\section[TcSimplify]{TcSimplify} - +TcSimplify \begin{code} module TcSimplify ( @@ -12,68 +12,47 @@ module TcSimplify ( tcSimplifyRuleLhs, tcSimplifyIPs, tcSimplifySuperClasses, tcSimplifyTop, tcSimplifyInteractive, - tcSimplifyBracket, + tcSimplifyBracket, tcSimplifyCheckPat, tcSimplifyDeriv, tcSimplifyDefault, - bindInstsOfLocalFuns + bindInstsOfLocalFuns, bindIrreds, ) where #include "HsVersions.h" import {-# SOURCE #-} TcUnify( unifyType ) -import TypeRep ( Type(..) ) -import HsSyn ( HsBind(..), HsExpr(..), LHsExpr, emptyLHsBinds ) -import TcHsSyn ( mkHsApp, mkHsTyApp, mkHsDictApp ) +import HsSyn import TcRnMonad -import Inst ( lookupInst, LookupInstResult(..), - tyVarsOfInst, fdPredsOfInsts, newDicts, - isDict, isClassDict, isLinearInst, linearInstType, - isMethodFor, isMethod, - instToId, tyVarsOfInsts, cloneDict, - ipNamesOfInsts, ipNamesOfInst, dictPred, - fdPredsOfInst, - newDictsAtLoc, tcInstClassOp, - getDictClassTys, isTyVarDict, instLoc, - zonkInst, tidyInsts, tidyMoreInsts, - pprInsts, pprDictsInFull, pprInstInFull, tcGetInstEnvs, - isInheritableInst, pprDictsTheta - ) -import TcEnv ( tcGetGlobalTyVars, tcLookupId, findGlobals, pprBinders, - lclEnvElts, tcMetaTy ) -import InstEnv ( lookupInstEnv, classInstances, pprInstances ) -import TcMType ( zonkTcTyVarsAndFV, tcInstTyVars, zonkTcPredType, - checkAmbiguity, checkInstTermination ) -import TcType ( TcTyVar, TcTyVarSet, ThetaType, TcPredType, tidyPred, - mkClassPred, isOverloadedTy, mkTyConApp, isSkolemTyVar, - mkTyVarTy, tcGetTyVar, isTyVarClassPred, mkTyVarTys, - tyVarsOfPred, tcEqType, pprPred, mkPredTy, tcIsTyVarTy ) -import TcIface ( checkWiredInTyCon ) -import Id ( idType, mkUserLocal ) -import Var ( TyVar ) -import TyCon ( TyCon ) -import Name ( Name, getOccName, getSrcLoc ) -import NameSet ( NameSet, mkNameSet, elemNameSet ) -import Class ( classBigSig, classKey ) -import FunDeps ( oclose, grow, improve, pprEquation ) -import PrelInfo ( isNumericClass, isStandardClass ) -import PrelNames ( splitName, fstName, sndName, integerTyConName, - showClassKey, eqClassKey, ordClassKey ) -import Type ( zipTopTvSubst, substTheta, substTy ) -import TysWiredIn ( pairTyCon, doubleTy, doubleTyCon ) -import ErrUtils ( Message ) -import BasicTypes ( TopLevelFlag, isNotTopLevel ) +import Inst +import TcEnv +import InstEnv +import TcGadt +import TcType +import TcMType +import TcIface +import Var +import Name +import NameSet +import Class +import FunDeps +import PrelInfo +import PrelNames +import Type +import TysWiredIn +import ErrUtils +import BasicTypes import VarSet -import VarEnv ( TidyEnv ) +import VarEnv import FiniteMap import Bag import Outputable -import ListSetOps ( equivClasses ) -import Util ( zipEqual, isSingleton ) -import List ( partition ) -import SrcLoc ( Located(..) ) -import DynFlags ( DynFlag(..) ) -import StaticFlags +import ListSetOps +import Util +import SrcLoc +import DynFlags + +import Data.List \end{code} @@ -87,6 +66,24 @@ import StaticFlags Notes on functional dependencies (a bug) -------------------------------------- +Consider this: + + class C a b | a -> b + class D a b | a -> b + + instance D a b => C a b -- Undecidable + -- (Not sure if it's crucial to this eg) + f :: C a b => a -> Bool + f _ = True + + g :: C a b => a -> Bool + g = f + +Here f typechecks, but g does not!! Reason: before doing improvement, +we reduce the (C a b1) constraint from the call of f to (D a b1). + +Here is a more complicated example: + | > class Foo a b | a->b | > | > class Bar a b | a->b @@ -258,9 +255,9 @@ any other type variables. - -------------------------------------- - Notes on ambiguity - -------------------------------------- +------------------------------------- + Note [Ambiguity] +------------------------------------- It's very hard to be certain when a type is ambiguous. Consider @@ -400,8 +397,8 @@ When m is later unified with [], we can solve both constraints. Notes on implicit parameters -------------------------------------- -Question 1: can we "inherit" implicit parameters -~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +Note [Inheriting implicit parameters] +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Consider this: f x = (x::Int) + ?y @@ -426,6 +423,21 @@ BOTTOM LINE: when *inferring types* you *must* quantify over implicit parameters. See the predicate isFreeWhenInferring. +Note [Implicit parameters and ambiguity] +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +What type should we infer for this? + f x = (show ?y, x::Int) +Since we must quantify over the ?y, the most plausible type is + f :: (Show a, ?y::a) => Int -> (String, Int) +But notice that the type of the RHS is (String,Int), with no type +varibables mentioned at all! The type of f looks ambiguous. But +it isn't, because at a call site we might have + let ?y = 5::Int in f 7 +and all is well. In effect, implicit parameters are, well, parameters, +so we can take their type variables into account as part of the +"tau-tvs" stuff. This is done in the function 'FunDeps.grow'. + + Question 2: type signatures ~~~~~~~~~~~~~~~~~~~~~~~~~~~ BUT WATCH OUT: When you supply a type signature, we can't force you @@ -644,82 +656,98 @@ tcSimplifyInfer :: SDoc -> TcTyVarSet -- fv(T); type vars -> [Inst] -- Wanted - -> TcM ([TcTyVar], -- Tyvars to quantify (zonked) - TcDictBinds, -- Bindings - [TcId]) -- Dict Ids that must be bound here (zonked) + -> TcM ([TcTyVar], -- Tyvars to quantify (zonked and quantified) + [Inst], -- Dict Ids that must be bound here (zonked) + TcDictBinds) -- Bindings -- Any free (escaping) Insts are tossed into the environment \end{code} \begin{code} -tcSimplifyInfer doc tau_tvs wanted_lie - = inferLoop doc (varSetElems tau_tvs) - wanted_lie `thenM` \ (qtvs, frees, binds, irreds) -> - - extendLIEs frees `thenM_` - returnM (qtvs, binds, map instToId irreds) - -inferLoop doc tau_tvs wanteds - = -- Step 1 - zonkTcTyVarsAndFV tau_tvs `thenM` \ tau_tvs' -> - mappM zonkInst wanteds `thenM` \ wanteds' -> - tcGetGlobalTyVars `thenM` \ gbl_tvs -> - let - preds = fdPredsOfInsts wanteds' - qtvs = grow preds tau_tvs' `minusVarSet` oclose preds gbl_tvs - - try_me inst - | isFreeWhenInferring qtvs inst = Free - | isClassDict inst = DontReduceUnlessConstant -- Dicts - | otherwise = ReduceMe NoSCs -- Lits and Methods - in - traceTc (text "infloop" <+> vcat [ppr tau_tvs', ppr wanteds', ppr preds, - ppr (grow preds tau_tvs'), ppr qtvs]) `thenM_` - -- Step 2 - reduceContext doc try_me [] wanteds' `thenM` \ (no_improvement, frees, binds, irreds) -> - - -- Step 3 - if no_improvement then - returnM (varSetElems qtvs, frees, binds, irreds) - else - -- If improvement did some unification, we go round again. There - -- are two subtleties: - -- a) We start again with irreds, not wanteds - -- Using an instance decl might have introduced a fresh type variable - -- which might have been unified, so we'd get an infinite loop - -- if we started again with wanteds! See example [LOOP] - -- - -- b) It's also essential to re-process frees, because unification - -- might mean that a type variable that looked free isn't now. - -- - -- Hence the (irreds ++ frees) - - -- However, NOTICE that when we are done, we might have some bindings, but - -- the final qtvs might be empty. See [NO TYVARS] below. - - inferLoop doc tau_tvs (irreds ++ frees) `thenM` \ (qtvs1, frees1, binds1, irreds1) -> - returnM (qtvs1, frees1, binds `unionBags` binds1, irreds1) +tcSimplifyInfer doc tau_tvs wanted + = do { tau_tvs' <- zonkTcTyVarsAndFV (varSetElems tau_tvs) + ; wanted' <- mappM zonkInst wanted -- Zonk before deciding quantified tyvars + ; gbl_tvs <- tcGetGlobalTyVars + ; let preds = fdPredsOfInsts wanted' + qtvs = grow preds tau_tvs' `minusVarSet` oclose preds gbl_tvs + (free, bound) = partition (isFreeWhenInferring qtvs) wanted' + ; traceTc (text "infer" <+> (ppr preds $$ ppr (grow preds tau_tvs') $$ ppr gbl_tvs $$ ppr (oclose preds gbl_tvs) $$ ppr free $$ ppr bound)) + ; extendLIEs free + + -- To make types simple, reduce as much as possible + ; let try_me inst = ReduceMe AddSCs + ; (irreds, binds) <- checkLoop (mkRedEnv doc try_me []) bound + + ; qtvs' <- zonkQuantifiedTyVars (varSetElems qtvs) + + -- We can't abstract over implications + ; let (dicts, implics) = partition isDict irreds + ; loc <- getInstLoc (ImplicOrigin doc) + ; implic_bind <- bindIrreds loc qtvs' dicts implics + + ; return (qtvs', dicts, binds `unionBags` implic_bind) } + -- NB: when we are done, we might have some bindings, but + -- the final qtvs might be empty. See Note [NO TYVARS] below. \end{code} -Example [LOOP] +\begin{code} +----------------------------------------------------------- +-- tcSimplifyInferCheck is used when we know the constraints we are to simplify +-- against, but we don't know the type variables over which we are going to quantify. +-- This happens when we have a type signature for a mutually recursive group +tcSimplifyInferCheck + :: InstLoc + -> TcTyVarSet -- fv(T) + -> [Inst] -- Given + -> [Inst] -- Wanted + -> TcM ([TyVar], -- Fully zonked, and quantified + TcDictBinds) -- Bindings - class If b t e r | b t e -> r - instance If T t e t - instance If F t e e - class Lte a b c | a b -> c where lte :: a -> b -> c - instance Lte Z b T - instance (Lte a b l,If l b a c) => Max a b c +tcSimplifyInferCheck loc tau_tvs givens wanteds + = do { (irreds, binds) <- innerCheckLoop loc givens wanteds -Wanted: Max Z (S x) y + -- Figure out which type variables to quantify over + -- You might think it should just be the signature tyvars, + -- but in bizarre cases you can get extra ones + -- f :: forall a. Num a => a -> a + -- f x = fst (g (x, head [])) + 1 + -- g a b = (b,a) + -- Here we infer g :: forall a b. a -> b -> (b,a) + -- We don't want g to be monomorphic in b just because + -- f isn't quantified over b. + ; let all_tvs = varSetElems (tau_tvs `unionVarSet` tyVarsOfInsts givens) + ; all_tvs <- zonkTcTyVarsAndFV all_tvs + ; gbl_tvs <- tcGetGlobalTyVars + ; let qtvs = varSetElems (all_tvs `minusVarSet` gbl_tvs) + -- We could close gbl_tvs, but its not necessary for + -- soundness, and it'll only affect which tyvars, not which + -- dictionaries, we quantify over -Then we'll reduce using the Max instance to: - (Lte Z (S x) l, If l (S x) Z y) -and improve by binding l->T, after which we can do some reduction -on both the Lte and If constraints. What we *can't* do is start again -with (Max Z (S x) y)! + ; qtvs' <- zonkQuantifiedTyVars qtvs -[NO TYVARS] + -- Now we are back to normal (c.f. tcSimplCheck) + ; implic_bind <- bindIrreds loc qtvs' givens irreds + ; return (qtvs', binds `unionBags` implic_bind) } +\end{code} + +Note [Squashing methods] +~~~~~~~~~~~~~~~~~~~~~~~~~ +Be careful if you want to float methods more: + truncate :: forall a. RealFrac a => forall b. Integral b => a -> b +From an application (truncate f i) we get + t1 = truncate at f + t2 = t1 at i +If we have also have a second occurrence of truncate, we get + t3 = truncate at f + t4 = t3 at i +When simplifying with i,f free, we might still notice that +t1=t3; but alas, the binding for t2 (which mentions t1) +may continue to float out! + + +Note [NO TYVARS] +~~~~~~~~~~~~~~~~~ class Y a b | a -> b where y :: a -> X b @@ -744,18 +772,20 @@ The net effect of [NO TYVARS] \begin{code} isFreeWhenInferring :: TyVarSet -> Inst -> Bool isFreeWhenInferring qtvs inst - = isFreeWrtTyVars qtvs inst -- Constrains no quantified vars - && isInheritableInst inst -- And no implicit parameter involved - -- (see "Notes on implicit parameters") + = isFreeWrtTyVars qtvs inst -- Constrains no quantified vars + && isInheritableInst inst -- and no implicit parameter involved + -- see Note [Inheriting implicit parameters] +{- No longer used (with implication constraints) isFreeWhenChecking :: TyVarSet -- Quantified tyvars -> NameSet -- Quantified implicit parameters -> Inst -> Bool isFreeWhenChecking qtvs ips inst = isFreeWrtTyVars qtvs inst && isFreeWrtIPs ips inst +-} -isFreeWrtTyVars qtvs inst = not (tyVarsOfInst inst `intersectsVarSet` qtvs) +isFreeWrtTyVars qtvs inst = tyVarsOfInst inst `disjointVarSet` qtvs isFreeWrtIPs ips inst = not (any (`elemNameSet` ips) (ipNamesOfInst inst)) \end{code} @@ -770,109 +800,205 @@ isFreeWrtIPs ips inst = not (any (`elemNameSet` ips) (ipNamesOfInst inst)) we are going to quantify over. For example, a class or instance declaration. \begin{code} -tcSimplifyCheck - :: SDoc - -> [TcTyVar] -- Quantify over these - -> [Inst] -- Given - -> [Inst] -- Wanted - -> TcM TcDictBinds -- Bindings - +----------------------------------------------------------- -- tcSimplifyCheck is used when checking expression type signatures, -- class decls, instance decls etc. +tcSimplifyCheck :: InstLoc + -> [TcTyVar] -- Quantify over these + -> [Inst] -- Given + -> [Inst] -- Wanted + -> TcM TcDictBinds -- Bindings +tcSimplifyCheck loc qtvs givens wanteds + = ASSERT( all isTcTyVar qtvs && all isSkolemTyVar qtvs ) + do { (irreds, binds) <- innerCheckLoop loc givens wanteds + ; implic_bind <- bindIrreds loc qtvs givens irreds + ; return (binds `unionBags` implic_bind) } + +----------------------------------------------------------- +-- tcSimplifyCheckPat is used for existential pattern match +tcSimplifyCheckPat :: InstLoc + -> [CoVar] -> Refinement + -> [TcTyVar] -- Quantify over these + -> [Inst] -- Given + -> [Inst] -- Wanted + -> TcM TcDictBinds -- Bindings +tcSimplifyCheckPat loc co_vars reft qtvs givens wanteds + = ASSERT( all isTcTyVar qtvs && all isSkolemTyVar qtvs ) + do { (irreds, binds) <- innerCheckLoop loc givens wanteds + ; implic_bind <- bindIrredsR loc qtvs co_vars reft + givens irreds + ; return (binds `unionBags` implic_bind) } + +----------------------------------------------------------- +bindIrreds :: InstLoc -> [TcTyVar] + -> [Inst] -> [Inst] + -> TcM TcDictBinds +bindIrreds loc qtvs givens irreds + = bindIrredsR loc qtvs [] emptyRefinement givens irreds + +bindIrredsR :: InstLoc -> [TcTyVar] -> [CoVar] + -> Refinement -> [Inst] -> [Inst] + -> TcM TcDictBinds +-- Make a binding that binds 'irreds', by generating an implication +-- constraint for them, *and* throwing the constraint into the LIE +bindIrredsR loc qtvs co_vars reft givens irreds + | null irreds + = return emptyBag + | otherwise + = do { let givens' = filter isDict givens + -- The givens can include methods + -- See Note [Pruning the givens in an implication constraint] + + -- If there are no 'givens', then it's safe to + -- partition the 'wanteds' by their qtvs, thereby trimming irreds + -- See Note [Freeness and implications] + ; irreds' <- if null givens' + then do + { let qtv_set = mkVarSet qtvs + (frees, real_irreds) = partition (isFreeWrtTyVars qtv_set) irreds + ; extendLIEs frees + ; return real_irreds } + else return irreds + + ; let all_tvs = qtvs ++ co_vars -- Abstract over all these + ; (implics, bind) <- makeImplicationBind loc all_tvs reft givens' irreds' + -- This call does the real work + ; extendLIEs implics + ; return bind } + + +makeImplicationBind :: InstLoc -> [TcTyVar] -> Refinement + -> [Inst] -> [Inst] + -> TcM ([Inst], TcDictBinds) +-- Make a binding that binds 'irreds', by generating an implication +-- constraint for them, *and* throwing the constraint into the LIE +-- The binding looks like +-- (ir1, .., irn) = f qtvs givens +-- where f is (evidence for) the new implication constraint -- --- NB: tcSimplifyCheck does not consult the --- global type variables in the environment; so you don't --- need to worry about setting them before calling tcSimplifyCheck -tcSimplifyCheck doc qtvs givens wanted_lie - = ASSERT( all isSkolemTyVar qtvs ) - do { (qtvs', frees, binds) <- tcSimplCheck doc get_qtvs AddSCs givens wanted_lie - ; extendLIEs frees - ; return binds } +-- This binding must line up the 'rhs' in reduceImplication +makeImplicationBind loc all_tvs reft + givens -- Guaranteed all Dicts + irreds + | null irreds -- If there are no irreds, we are done + = return ([], emptyBag) + | otherwise -- Otherwise we must generate a binding + = do { uniq <- newUnique + ; span <- getSrcSpanM + ; let name = mkInternalName uniq (mkVarOcc "ic") (srcSpanStart span) + implic_inst = ImplicInst { tci_name = name, tci_reft = reft, + tci_tyvars = all_tvs, + tci_given = givens, + tci_wanted = irreds, tci_loc = loc } + + ; let n_irreds = length irreds + irred_ids = map instToId irreds + tup_ty = mkTupleTy Boxed n_irreds (map idType irred_ids) + pat = TuplePat (map nlVarPat irred_ids) Boxed tup_ty + rhs = L span (mkHsWrap co (HsVar (instToId implic_inst))) + co = mkWpApps (map instToId givens) <.> mkWpTyApps (mkTyVarTys all_tvs) + bind | n_irreds==1 = VarBind (head irred_ids) rhs + | otherwise = PatBind { pat_lhs = L span pat, + pat_rhs = unguardedGRHSs rhs, + pat_rhs_ty = tup_ty, + bind_fvs = placeHolderNames } + ; -- pprTrace "Make implic inst" (ppr implic_inst) $ + return ([implic_inst], unitBag (L span bind)) } + +----------------------------------------------------------- +topCheckLoop :: SDoc + -> [Inst] -- Wanted + -> TcM ([Inst], TcDictBinds) + +topCheckLoop doc wanteds + = checkLoop (mkRedEnv doc try_me []) wanteds where --- get_qtvs = zonkTcTyVarsAndFV qtvs - get_qtvs = return (mkVarSet qtvs) -- All skolems - + try_me inst = ReduceMe AddSCs --- tcSimplifyInferCheck is used when we know the constraints we are to simplify --- against, but we don't know the type variables over which we are going to quantify. --- This happens when we have a type signature for a mutually recursive group -tcSimplifyInferCheck - :: SDoc - -> TcTyVarSet -- fv(T) - -> [Inst] -- Given - -> [Inst] -- Wanted - -> TcM ([TcTyVar], -- Variables over which to quantify - TcDictBinds) -- Bindings +----------------------------------------------------------- +innerCheckLoop :: InstLoc + -> [Inst] -- Given + -> [Inst] -- Wanted + -> TcM ([Inst], TcDictBinds) -tcSimplifyInferCheck doc tau_tvs givens wanted_lie - = do { (qtvs', frees, binds) <- tcSimplCheck doc get_qtvs AddSCs givens wanted_lie - ; extendLIEs frees - ; return (qtvs', binds) } +innerCheckLoop inst_loc givens wanteds + = checkLoop env wanteds where - -- Figure out which type variables to quantify over - -- You might think it should just be the signature tyvars, - -- but in bizarre cases you can get extra ones - -- f :: forall a. Num a => a -> a - -- f x = fst (g (x, head [])) + 1 - -- g a b = (b,a) - -- Here we infer g :: forall a b. a -> b -> (b,a) - -- We don't want g to be monomorphic in b just because - -- f isn't quantified over b. - all_tvs = varSetElems (tau_tvs `unionVarSet` tyVarsOfInsts givens) - - get_qtvs = zonkTcTyVarsAndFV all_tvs `thenM` \ all_tvs' -> - tcGetGlobalTyVars `thenM` \ gbl_tvs -> - let - qtvs = all_tvs' `minusVarSet` gbl_tvs - -- We could close gbl_tvs, but its not necessary for - -- soundness, and it'll only affect which tyvars, not which - -- dictionaries, we quantify over - in - returnM qtvs + env = mkRedEnv (pprInstLoc inst_loc) try_me givens + + try_me inst | isMethodOrLit inst = ReduceMe AddSCs + | otherwise = Stop + -- When checking against a given signature + -- we MUST be very gentle: Note [Check gently] \end{code} -Here is the workhorse function for all three wrappers. +Note [Check gently] +~~~~~~~~~~~~~~~~~~~~ +We have to very careful about not simplifying too vigorously +Example: + data T a where + MkT :: a -> T [a] + + f :: Show b => T b -> b + f (MkT x) = show [x] + +Inside the pattern match, which binds (a:*, x:a), we know that + b ~ [a] +Hence we have a dictionary for Show [a] available; and indeed we +need it. We are going to build an implication contraint + forall a. (b~[a]) => Show [a] +Later, we will solve this constraint using the knowledge (Show b) + +But we MUST NOT reduce (Show [a]) to (Show a), else the whole +thing becomes insoluble. So we simplify gently (get rid of literals +and methods only, plus common up equal things), deferring the real +work until top level, when we solve the implication constraint +with topCheckLooop. + \begin{code} -tcSimplCheck doc get_qtvs want_scs givens wanted_lie - = do { (qtvs, frees, binds, irreds) <- check_loop givens wanted_lie +----------------------------------------------------------- +checkLoop :: RedEnv + -> [Inst] -- Wanted + -> TcM ([Inst], TcDictBinds) +-- Precondition: givens are completely rigid + +checkLoop env wanteds + = do { -- Givens are skolems, so no need to zonk them + wanteds' <- mappM zonkInst wanteds + + ; (improved, binds, irreds) <- reduceContext env wanteds' + + ; if not improved then + return (irreds, binds) + else do + + -- If improvement did some unification, we go round again. + -- We start again with irreds, not wanteds + -- Using an instance decl might have introduced a fresh type variable + -- which might have been unified, so we'd get an infinite loop + -- if we started again with wanteds! See Note [LOOP] + { (irreds1, binds1) <- checkLoop env irreds + ; return (irreds1, binds `unionBags` binds1) } } +\end{code} - -- Complain about any irreducible ones - ; if not (null irreds) - then do { givens' <- mappM zonkInst given_dicts_and_ips - ; groupErrs (addNoInstanceErrs (Just doc) givens') irreds } - else return () +Note [LOOP] +~~~~~~~~~~~ + class If b t e r | b t e -> r + instance If T t e t + instance If F t e e + class Lte a b c | a b -> c where lte :: a -> b -> c + instance Lte Z b T + instance (Lte a b l,If l b a c) => Max a b c + +Wanted: Max Z (S x) y + +Then we'll reduce using the Max instance to: + (Lte Z (S x) l, If l (S x) Z y) +and improve by binding l->T, after which we can do some reduction +on both the Lte and If constraints. What we *can't* do is start again +with (Max Z (S x) y)! - ; returnM (qtvs, frees, binds) } - where - given_dicts_and_ips = filter (not . isMethod) givens - -- For error reporting, filter out methods, which are - -- only added to the given set as an optimisation - - ip_set = mkNameSet (ipNamesOfInsts givens) - - check_loop givens wanteds - = -- Step 1 - mappM zonkInst givens `thenM` \ givens' -> - mappM zonkInst wanteds `thenM` \ wanteds' -> - get_qtvs `thenM` \ qtvs' -> - - -- Step 2 - let - -- When checking against a given signature we always reduce - -- until we find a match against something given, or can't reduce - try_me inst | isFreeWhenChecking qtvs' ip_set inst = Free - | otherwise = ReduceMe want_scs - in - reduceContext doc try_me givens' wanteds' `thenM` \ (no_improvement, frees, binds, irreds) -> - - -- Step 3 - if no_improvement then - returnM (varSetElems qtvs', frees, binds, irreds) - else - check_loop givens' (irreds ++ frees) `thenM` \ (qtvs', frees1, binds1, irreds1) -> - returnM (qtvs', frees1, binds `unionBags` binds1, irreds1) -\end{code} %************************************************************************ @@ -920,14 +1046,20 @@ Two more nasty cases are in tcrun033 \begin{code} -tcSimplifySuperClasses qtvs givens sc_wanteds - = ASSERT( all isSkolemTyVar qtvs ) - do { (_, frees, binds1) <- tcSimplCheck doc get_qtvs NoSCs givens sc_wanteds - ; binds2 <- tc_simplify_top doc False NoSCs frees - ; return (binds1 `unionBags` binds2) } +tcSimplifySuperClasses + :: InstLoc + -> [Inst] -- Given + -> [Inst] -- Wanted + -> TcM TcDictBinds +tcSimplifySuperClasses loc givens sc_wanteds + = do { (irreds, binds1) <- checkLoop env sc_wanteds + ; let (tidy_env, tidy_irreds) = tidyInsts irreds + ; reportNoInstances tidy_env (Just (loc, givens)) tidy_irreds + ; return binds1 } where - get_qtvs = return (mkVarSet qtvs) - doc = ptext SLIT("instance declaration superclass context") + env = mkRedEnv (pprInstLoc loc) try_me givens + try_me inst = ReduceMe NoSCs + -- Like topCheckLoop, but with NoSCs \end{code} @@ -1039,7 +1171,7 @@ tcSimplifyRestricted -- Used for restricted binding groups -> [Name] -- Things bound in this group -> TcTyVarSet -- Free in the type of the RHSs -> [Inst] -- Free in the RHSs - -> TcM ([TcTyVar], -- Tyvars to quantify (zonked) + -> TcM ([TyVar], -- Tyvars to quantify (zonked and quantified) TcDictBinds) -- Bindings -- tcSimpifyRestricted returns no constraints to -- quantify over; by definition there are none. @@ -1047,30 +1179,32 @@ tcSimplifyRestricted -- Used for restricted binding groups tcSimplifyRestricted doc top_lvl bndrs tau_tvs wanteds -- Zonk everything in sight - = mappM zonkInst wanteds `thenM` \ wanteds' -> - zonkTcTyVarsAndFV (varSetElems tau_tvs) `thenM` \ tau_tvs' -> - tcGetGlobalTyVars `thenM` \ gbl_tvs' -> + = do { wanteds' <- mappM zonkInst wanteds - -- 'reduceMe': Reduce as far as we can. Don't stop at + -- 'ReduceMe': Reduce as far as we can. Don't stop at -- dicts; the idea is to get rid of as many type -- variables as possible, and we don't want to stop -- at (say) Monad (ST s), because that reduces -- immediately, with no constraint on s. -- -- BUT do no improvement! See Plan D above - reduceContextWithoutImprovement - doc reduceMe wanteds' `thenM` \ (_frees, _binds, constrained_dicts) -> + -- HOWEVER, some unification may take place, if we instantiate + -- a method Inst with an equality constraint + ; let env = mkNoImproveRedEnv doc (\i -> ReduceMe AddSCs) + ; (_imp, _binds, constrained_dicts) <- reduceContext env wanteds' -- Next, figure out the tyvars we will quantify over - let - constrained_tvs = tyVarsOfInsts constrained_dicts - qtvs = (tau_tvs' `minusVarSet` oclose (fdPredsOfInsts constrained_dicts) gbl_tvs') - `minusVarSet` constrained_tvs - in - traceTc (text "tcSimplifyRestricted" <+> vcat [ - pprInsts wanteds, pprInsts _frees, pprInsts constrained_dicts, + ; tau_tvs' <- zonkTcTyVarsAndFV (varSetElems tau_tvs) + ; gbl_tvs' <- tcGetGlobalTyVars + ; constrained_dicts' <- mappM zonkInst constrained_dicts + + ; let constrained_tvs' = tyVarsOfInsts constrained_dicts' + qtvs = (tau_tvs' `minusVarSet` oclose (fdPredsOfInsts constrained_dicts) gbl_tvs') + `minusVarSet` constrained_tvs' + ; traceTc (text "tcSimplifyRestricted" <+> vcat [ + pprInsts wanteds, pprInsts constrained_dicts', ppr _binds, - ppr constrained_tvs, ppr tau_tvs', ppr qtvs ]) `thenM_` + ppr constrained_tvs', ppr tau_tvs', ppr qtvs ]) -- The first step may have squashed more methods than -- necessary, so try again, this time more gently, knowing the exact @@ -1088,27 +1222,23 @@ tcSimplifyRestricted doc top_lvl bndrs tau_tvs wanteds -- -- At top level, we *do* squash methods becuase we want to -- expose implicit parameters to the test that follows - let - is_nested_group = isNotTopLevel top_lvl - try_me inst | isFreeWrtTyVars qtvs inst, - (is_nested_group || isDict inst) = Free - | otherwise = ReduceMe AddSCs - in - reduceContextWithoutImprovement - doc try_me wanteds' `thenM` \ (frees, binds, irreds) -> - ASSERT( null irreds ) + ; let is_nested_group = isNotTopLevel top_lvl + try_me inst | isFreeWrtTyVars qtvs inst, + (is_nested_group || isDict inst) = Stop + | otherwise = ReduceMe AddSCs + env = mkNoImproveRedEnv doc try_me + ; (_imp, binds, irreds) <- reduceContext env wanteds' -- See "Notes on implicit parameters, Question 4: top level" - if is_nested_group then - extendLIEs frees `thenM_` - returnM (varSetElems qtvs, binds) - else - let - (non_ips, bad_ips) = partition isClassDict frees - in - addTopIPErrs bndrs bad_ips `thenM_` - extendLIEs non_ips `thenM_` - returnM (varSetElems qtvs, binds) + ; ASSERT( all (isFreeWrtTyVars qtvs) irreds ) -- None should be captured + if is_nested_group then + extendLIEs irreds + else do { let (bad_ips, non_ips) = partition isIPDict irreds + ; addTopIPErrs bndrs bad_ips + ; extendLIEs non_ips } + + ; qtvs' <- zonkQuantifiedTyVars (varSetElems qtvs) + ; return (qtvs', binds) } \end{code} @@ -1185,10 +1315,9 @@ tcSimplifyRuleLhs wanteds | otherwise = do { w' <- zonkInst w -- So that (3::Int) does not generate a call -- to fromInteger; this looks fragile to me - ; lookup_result <- lookupInst w' + ; lookup_result <- lookupSimpleInst w' ; case lookup_result of GenInst ws' rhs -> go dicts (addBind binds w rhs) (ws' ++ ws) - SimpleInst rhs -> go dicts (addBind binds w rhs) ws NoInstance -> pprPanic "tcSimplifyRuleLhs" (ppr w) } \end{code} @@ -1203,8 +1332,8 @@ this bracket again at its usage site. \begin{code} tcSimplifyBracket :: [Inst] -> TcM () tcSimplifyBracket wanteds - = simpleReduceLoop doc reduceMe wanteds `thenM_` - returnM () + = do { topCheckLoop doc wanteds + ; return () } where doc = text "tcSimplifyBracket" \end{code} @@ -1234,30 +1363,34 @@ force the binding for ?x to be of type Int. tcSimplifyIPs :: [Inst] -- The implicit parameters bound here -> [Inst] -- Wanted -> TcM TcDictBinds + -- We need a loop so that we do improvement, and then + -- (next time round) generate a binding to connect the two + -- let ?x = e in ?x + -- Here the two ?x's have different types, and improvement + -- makes them the same. + tcSimplifyIPs given_ips wanteds - = simpl_loop given_ips wanteds `thenM` \ (frees, binds) -> - extendLIEs frees `thenM_` - returnM binds + = do { wanteds' <- mappM zonkInst wanteds + ; given_ips' <- mappM zonkInst given_ips + -- Unusually for checking, we *must* zonk the given_ips + + ; let env = mkRedEnv doc try_me given_ips' + ; (improved, binds, irreds) <- reduceContext env wanteds' + + ; if not improved then + ASSERT( all is_free irreds ) + do { extendLIEs irreds + ; return binds } + else + tcSimplifyIPs given_ips wanteds } where - doc = text "tcSimplifyIPs" <+> ppr given_ips - ip_set = mkNameSet (ipNamesOfInsts given_ips) + doc = text "tcSimplifyIPs" <+> ppr given_ips + ip_set = mkNameSet (ipNamesOfInsts given_ips) + is_free inst = isFreeWrtIPs ip_set inst -- Simplify any methods that mention the implicit parameter - try_me inst | isFreeWrtIPs ip_set inst = Free - | otherwise = ReduceMe NoSCs - - simpl_loop givens wanteds - = mappM zonkInst givens `thenM` \ givens' -> - mappM zonkInst wanteds `thenM` \ wanteds' -> - - reduceContext doc try_me givens' wanteds' `thenM` \ (no_improvement, frees, binds, irreds) -> - - if no_improvement then - ASSERT( null irreds ) - returnM (frees, binds) - else - simpl_loop givens' (irreds ++ frees) `thenM` \ (frees1, binds1) -> - returnM (frees1, binds `unionBags` binds1) + try_me inst | is_free inst = Stop + | otherwise = ReduceMe NoSCs \end{code} @@ -1302,12 +1435,12 @@ bindInstsOfLocalFuns wanteds local_ids returnM emptyLHsBinds | otherwise - = simpleReduceLoop doc try_me for_me `thenM` \ (frees, binds, irreds) -> - ASSERT( null irreds ) - extendLIEs not_for_me `thenM_` - extendLIEs frees `thenM_` - returnM binds + = do { (irreds, binds) <- checkLoop env for_me + ; extendLIEs not_for_me + ; extendLIEs irreds + ; return binds } where + env = mkRedEnv doc try_me [] doc = text "bindInsts" <+> ppr local_ids overloaded_ids = filter is_overloaded local_ids is_overloaded id = isOverloadedTy (idType id) @@ -1317,7 +1450,7 @@ bindInstsOfLocalFuns wanteds local_ids -- so it's worth building a set, so that -- lookup (in isMethodFor) is faster try_me inst | isMethod inst = ReduceMe NoSCs - | otherwise = Free + | otherwise = Stop \end{code} @@ -1330,20 +1463,54 @@ bindInstsOfLocalFuns wanteds local_ids The main control over context reduction is here \begin{code} +data RedEnv + = RedEnv { red_doc :: SDoc -- The context + , red_try_me :: Inst -> WhatToDo + , red_improve :: Bool -- True <=> do improvement + , red_givens :: [Inst] -- All guaranteed rigid + -- Always dicts + -- but see Note [Rigidity] + , red_stack :: (Int, [Inst]) -- Recursion stack (for err msg) + -- See Note [RedStack] + } + +-- Note [Rigidity] +-- The red_givens are rigid so far as cmpInst is concerned. +-- There is one case where they are not totally rigid, namely in tcSimplifyIPs +-- let ?x = e in ... +-- Here, the given is (?x::a), where 'a' is not necy a rigid type +-- But that doesn't affect the comparison, which is based only on mame. + +-- Note [RedStack] +-- The red_stack pair (n,insts) pair is just used for error reporting. +-- 'n' is always the depth of the stack. +-- The 'insts' is the stack of Insts being reduced: to produce X +-- I had to produce Y, to produce Y I had to produce Z, and so on. + + +mkRedEnv :: SDoc -> (Inst -> WhatToDo) -> [Inst] -> RedEnv +mkRedEnv doc try_me givens + = RedEnv { red_doc = doc, red_try_me = try_me, + red_givens = givens, red_stack = (0,[]), + red_improve = True } + +mkNoImproveRedEnv :: SDoc -> (Inst -> WhatToDo) -> RedEnv +-- Do not do improvement; no givens +mkNoImproveRedEnv doc try_me + = RedEnv { red_doc = doc, red_try_me = try_me, + red_givens = [], red_stack = (0,[]), + red_improve = True } + data WhatToDo = ReduceMe WantSCs -- Try to reduce this - -- If there's no instance, behave exactly like - -- DontReduce: add the inst to the irreductible ones, - -- but don't produce an error message of any kind. + -- If there's no instance, add the inst to the + -- irreductible ones, but don't produce an error + -- message of any kind. -- It might be quite legitimate such as (Eq a)! - | DontReduceUnlessConstant -- Return as irreducible unless it can - -- be reduced to a constant in one step - - | Free -- Return as free - -reduceMe :: Inst -> WhatToDo -reduceMe inst = ReduceMe AddSCs + | Stop -- Return as irreducible unless it can + -- be reduced to a constant in one step + -- Do not add superclasses; see data WantSCs = NoSCs | AddSCs -- Tells whether we should add the superclasses -- of a predicate when adding it to the avails @@ -1351,323 +1518,75 @@ data WantSCs = NoSCs | AddSCs -- Tells whether we should add the superclasses -- Note [SUPER-CLASS LOOP 1] \end{code} - - -\begin{code} -type Avails = FiniteMap Inst Avail -emptyAvails = emptyFM - -data Avail - = IsFree -- Used for free Insts - | Irred -- Used for irreducible dictionaries, - -- which are going to be lambda bound - - | Given TcId -- Used for dictionaries for which we have a binding - -- e.g. those "given" in a signature - Bool -- True <=> actually consumed (splittable IPs only) - - | Rhs -- Used when there is a RHS - (LHsExpr TcId) -- The RHS - [Inst] -- Insts free in the RHS; we need these too - - | Linear -- Splittable Insts only. - Int -- The Int is always 2 or more; indicates how - -- many copies are required - Inst -- The splitter - Avail -- Where the "master copy" is - - | LinRhss -- Splittable Insts only; this is used only internally - -- by extractResults, where a Linear - -- is turned into an LinRhss - [LHsExpr TcId] -- A supply of suitable RHSs - -pprAvails avails = vcat [sep [ppr inst, nest 2 (equals <+> pprAvail avail)] - | (inst,avail) <- fmToList avails ] - -instance Outputable Avail where - ppr = pprAvail - -pprAvail IsFree = text "Free" -pprAvail Irred = text "Irred" -pprAvail (Given x b) = text "Given" <+> ppr x <+> - if b then text "(used)" else empty -pprAvail (Rhs rhs bs) = text "Rhs" <+> ppr rhs <+> braces (ppr bs) -pprAvail (Linear n i a) = text "Linear" <+> ppr n <+> braces (ppr i) <+> ppr a -pprAvail (LinRhss rhss) = text "LinRhss" <+> ppr rhss -\end{code} - -Extracting the bindings from a bunch of Avails. -The bindings do *not* come back sorted in dependency order. -We assume that they'll be wrapped in a big Rec, so that the -dependency analyser can sort them out later - -The loop startes -\begin{code} -extractResults :: Avails - -> [Inst] -- Wanted - -> TcM (TcDictBinds, -- Bindings - [Inst], -- Irreducible ones - [Inst]) -- Free ones - -extractResults avails wanteds - = go avails emptyBag [] [] wanteds - where - go avails binds irreds frees [] - = returnM (binds, irreds, frees) - - go avails binds irreds frees (w:ws) - = case lookupFM avails w of - Nothing -> pprTrace "Urk: extractResults" (ppr w) $ - go avails binds irreds frees ws - - Just IsFree -> go (add_free avails w) binds irreds (w:frees) ws - Just Irred -> go (add_given avails w) binds (w:irreds) frees ws - - Just (Given id _) -> go avails new_binds irreds frees ws - where - new_binds | id == instToId w = binds - | otherwise = addBind binds w (L (instSpan w) (HsVar id)) - -- The sought Id can be one of the givens, via a superclass chain - -- and then we definitely don't want to generate an x=x binding! - - Just (Rhs rhs ws') -> go (add_given avails w) new_binds irreds frees (ws' ++ ws) - where - new_binds = addBind binds w rhs - - Just (Linear n split_inst avail) -- Transform Linear --> LinRhss - -> get_root irreds frees avail w `thenM` \ (irreds', frees', root_id) -> - split n (instToId split_inst) root_id w `thenM` \ (binds', rhss) -> - go (addToFM avails w (LinRhss rhss)) - (binds `unionBags` binds') - irreds' frees' (split_inst : w : ws) - - Just (LinRhss (rhs:rhss)) -- Consume one of the Rhss - -> go new_avails new_binds irreds frees ws - where - new_binds = addBind binds w rhs - new_avails = addToFM avails w (LinRhss rhss) - - get_root irreds frees (Given id _) w = returnM (irreds, frees, id) - get_root irreds frees Irred w = cloneDict w `thenM` \ w' -> - returnM (w':irreds, frees, instToId w') - get_root irreds frees IsFree w = cloneDict w `thenM` \ w' -> - returnM (irreds, w':frees, instToId w') - - add_given avails w = addToFM avails w (Given (instToId w) True) - - add_free avails w | isMethod w = avails - | otherwise = add_given avails w - -- NB: Hack alert! - -- Do *not* replace Free by Given if it's a method. - -- The following situation shows why this is bad: - -- truncate :: forall a. RealFrac a => forall b. Integral b => a -> b - -- From an application (truncate f i) we get - -- t1 = truncate at f - -- t2 = t1 at i - -- If we have also have a second occurrence of truncate, we get - -- t3 = truncate at f - -- t4 = t3 at i - -- When simplifying with i,f free, we might still notice that - -- t1=t3; but alas, the binding for t2 (which mentions t1) - -- will continue to float out! - -split :: Int -> TcId -> TcId -> Inst - -> TcM (TcDictBinds, [LHsExpr TcId]) --- (split n split_id root_id wanted) returns --- * a list of 'n' expressions, all of which witness 'avail' --- * a bunch of auxiliary bindings to support these expressions --- * one or zero insts needed to witness the whole lot --- (maybe be zero if the initial Inst is a Given) --- --- NB: 'wanted' is just a template - -split n split_id root_id wanted - = go n - where - ty = linearInstType wanted - pair_ty = mkTyConApp pairTyCon [ty,ty] - id = instToId wanted - occ = getOccName id - loc = getSrcLoc id - span = instSpan wanted - - go 1 = returnM (emptyBag, [L span $ HsVar root_id]) - - go n = go ((n+1) `div` 2) `thenM` \ (binds1, rhss) -> - expand n rhss `thenM` \ (binds2, rhss') -> - returnM (binds1 `unionBags` binds2, rhss') - - -- (expand n rhss) - -- Given ((n+1)/2) rhss, make n rhss, using auxiliary bindings - -- e.g. expand 3 [rhs1, rhs2] - -- = ( { x = split rhs1 }, - -- [fst x, snd x, rhs2] ) - expand n rhss - | n `rem` 2 == 0 = go rhss -- n is even - | otherwise = go (tail rhss) `thenM` \ (binds', rhss') -> - returnM (binds', head rhss : rhss') - where - go rhss = mapAndUnzipM do_one rhss `thenM` \ (binds', rhss') -> - returnM (listToBag binds', concat rhss') - - do_one rhs = newUnique `thenM` \ uniq -> - tcLookupId fstName `thenM` \ fst_id -> - tcLookupId sndName `thenM` \ snd_id -> - let - x = mkUserLocal occ uniq pair_ty loc - in - returnM (L span (VarBind x (mk_app span split_id rhs)), - [mk_fs_app span fst_id ty x, mk_fs_app span snd_id ty x]) - -mk_fs_app span id ty var = L span (HsVar id) `mkHsTyApp` [ty,ty] `mkHsApp` (L span (HsVar var)) - -mk_app span id rhs = L span (HsApp (L span (HsVar id)) rhs) - -addBind binds inst rhs = binds `unionBags` unitBag (L (instLocSrcSpan (instLoc inst)) - (VarBind (instToId inst) rhs)) -instSpan wanted = instLocSrcSpan (instLoc wanted) -\end{code} - - %************************************************************************ %* * \subsection[reduce]{@reduce@} %* * %************************************************************************ -When the "what to do" predicate doesn't depend on the quantified type variables, -matters are easier. We don't need to do any zonking, unless the improvement step -does something, in which case we zonk before iterating. - -The "given" set is always empty. - -\begin{code} -simpleReduceLoop :: SDoc - -> (Inst -> WhatToDo) -- What to do, *not* based on the quantified type variables - -> [Inst] -- Wanted - -> TcM ([Inst], -- Free - TcDictBinds, - [Inst]) -- Irreducible - -simpleReduceLoop doc try_me wanteds - = mappM zonkInst wanteds `thenM` \ wanteds' -> - reduceContext doc try_me [] wanteds' `thenM` \ (no_improvement, frees, binds, irreds) -> - if no_improvement then - returnM (frees, binds, irreds) - else - simpleReduceLoop doc try_me (irreds ++ frees) `thenM` \ (frees1, binds1, irreds1) -> - returnM (frees1, binds `unionBags` binds1, irreds1) -\end{code} - - \begin{code} -reduceContext :: SDoc - -> (Inst -> WhatToDo) - -> [Inst] -- Given +reduceContext :: RedEnv -> [Inst] -- Wanted - -> TcM (Bool, -- True <=> improve step did no unification - [Inst], -- Free - TcDictBinds, -- Dictionary bindings - [Inst]) -- Irreducible - -reduceContext doc try_me givens wanteds - = - traceTc (text "reduceContext" <+> (vcat [ + -> TcM (ImprovementDone, + TcDictBinds, -- Dictionary bindings + [Inst]) -- Irreducible + +reduceContext env wanteds + = do { traceTc (text "reduceContext" <+> (vcat [ text "----------------------", - doc, - text "given" <+> ppr givens, + red_doc env, + text "given" <+> ppr (red_givens env), text "wanted" <+> ppr wanteds, text "----------------------" - ])) `thenM_` + ])) -- Build the Avail mapping from "givens" - foldlM addGiven emptyAvails givens `thenM` \ init_state -> + ; init_state <- foldlM addGiven emptyAvails (red_givens env) -- Do the real work - reduceList (0,[]) try_me wanteds init_state `thenM` \ avails -> - - -- Do improvement, using everything in avails - -- In particular, avails includes all superclasses of everything - tcImprove avails `thenM` \ no_improvement -> + ; avails <- reduceList env wanteds init_state - extractResults avails wanteds `thenM` \ (binds, irreds, frees) -> + ; let improved = availsImproved avails + ; (binds, irreds) <- extractResults avails wanteds - traceTc (text "reduceContext end" <+> (vcat [ + ; traceTc (text "reduceContext end" <+> (vcat [ text "----------------------", - doc, - text "given" <+> ppr givens, + red_doc env, + text "given" <+> ppr (red_givens env), text "wanted" <+> ppr wanteds, text "----", text "avails" <+> pprAvails avails, - text "frees" <+> ppr frees, - text "no_improvement =" <+> ppr no_improvement, - text "----------------------" - ])) `thenM_` - - returnM (no_improvement, frees, binds, irreds) - --- reduceContextWithoutImprovement differs from reduceContext --- (a) no improvement --- (b) 'givens' is assumed empty -reduceContextWithoutImprovement doc try_me wanteds - = - traceTc (text "reduceContextWithoutImprovement" <+> (vcat [ - text "----------------------", - doc, - text "wanted" <+> ppr wanteds, + text "improved =" <+> ppr improved, text "----------------------" - ])) `thenM_` + ])) - -- Do the real work - reduceList (0,[]) try_me wanteds emptyAvails `thenM` \ avails -> - extractResults avails wanteds `thenM` \ (binds, irreds, frees) -> + ; return (improved, binds, irreds) } - traceTc (text "reduceContextWithoutImprovement end" <+> (vcat [ - text "----------------------", - doc, - text "wanted" <+> ppr wanteds, - text "----", - text "avails" <+> pprAvails avails, - text "frees" <+> ppr frees, - text "----------------------" - ])) `thenM_` - - returnM (frees, binds, irreds) - -tcImprove :: Avails -> TcM Bool -- False <=> no change --- Perform improvement using all the predicates in Avails -tcImprove avails - = tcGetInstEnvs `thenM` \ inst_envs -> - let - preds = [ (pred, pp_loc) - | (inst, avail) <- fmToList avails, - pred <- get_preds inst avail, - let pp_loc = pprInstLoc (instLoc inst) - ] +tcImproveOne :: Avails -> Inst -> TcM ImprovementDone +tcImproveOne avails inst + | not (isDict inst) = return False + | otherwise + = do { inst_envs <- tcGetInstEnvs + ; let eqns = improveOne (classInstances inst_envs) + (dictPred inst, pprInstArising inst) + [ (dictPred p, pprInstArising p) + | p <- availsInsts avails, isDict p ] -- Avails has all the superclasses etc (good) -- It also has all the intermediates of the deduction (good) -- It does not have duplicates (good) -- NB that (?x::t1) and (?x::t2) will be held separately in avails -- so that improve will see them separate - - -- For free Methods, we want to take predicates from their context, - -- but for Methods that have been squished their context will already - -- be in Avails, and we don't want duplicates. Hence this rather - -- horrid get_preds function - get_preds inst IsFree = fdPredsOfInst inst - get_preds inst other | isDict inst = [dictPred inst] - | otherwise = [] - - eqns = improve get_insts preds - get_insts clas = classInstances inst_envs clas - in - if null eqns then - returnM True - else - traceTc (ptext SLIT("Improve:") <+> vcat (map pprEquationDoc eqns)) `thenM_` - mappM_ unify eqns `thenM_` - returnM False + ; traceTc (text "improveOne" <+> ppr inst) + ; unifyEqns eqns } + +unifyEqns :: [(Equation,(PredType,SDoc),(PredType,SDoc))] + -> TcM ImprovementDone +unifyEqns [] = return False +unifyEqns eqns + = do { traceTc (ptext SLIT("Improve:") <+> vcat (map pprEquationDoc eqns)) + ; mappM_ unify eqns + ; return True } where unify ((qtvs, pairs), what1, what2) = addErrCtxtM (mkEqnMsg what1 what2) $ @@ -1689,78 +1608,47 @@ mkEqnMsg (pred1,from1) (pred2,from2) tidy_env The main context-reduction function is @reduce@. Here's its game plan. \begin{code} -reduceList :: (Int,[Inst]) -- Stack (for err msgs) - -- along with its depth - -> (Inst -> WhatToDo) - -> [Inst] - -> Avails - -> TcM Avails -\end{code} - -@reduce@ is passed - try_me: given an inst, this function returns - Reduce reduce this - DontReduce return this in "irreds" - Free return this in "frees" - - wanteds: The list of insts to reduce - state: An accumulating parameter of type Avails - that contains the state of the algorithm - - It returns a Avails. - -The (n,stack) pair is just used for error reporting. -n is always the depth of the stack. -The stack is the stack of Insts being reduced: to produce X -I had to produce Y, to produce Y I had to produce Z, and so on. - -\begin{code} -reduceList (n,stack) try_me wanteds state - | n > opt_MaxContextReductionDepth - = failWithTc (reduceDepthErr n stack) - - | otherwise - = +reduceList :: RedEnv -> [Inst] -> Avails -> TcM Avails +reduceList env@(RedEnv {red_stack = (n,stk)}) wanteds state + = do { dopts <- getDOpts #ifdef DEBUG - (if n > 8 then - pprTrace "Interesting! Context reduction stack deeper than 8:" - (int n $$ ifPprDebug (nest 2 (pprStack stack))) - else (\x->x)) + ; if n > 8 then + dumpTcRn (hang (ptext SLIT("Interesting! Context reduction stack depth") <+> int n) + 2 (ifPprDebug (nest 2 (pprStack stk)))) + else return () #endif - go wanteds state + ; if n >= ctxtStkDepth dopts then + failWithTc (reduceDepthErr n stk) + else + go wanteds state } where - go [] state = returnM state - go (w:ws) state = reduce (n+1, w:stack) try_me w state `thenM` \ state' -> - go ws state' + go [] state = return state + go (w:ws) state = do { state' <- reduce (env {red_stack = (n+1, w:stk)}) w state + ; go ws state' } -- Base case: we're done! -reduce stack try_me wanted avails +reduce env wanted avails -- It's the same as an existing inst, or a superclass thereof - | Just avail <- isAvailable avails wanted - = if isLinearInst wanted then - addLinearAvailable avails avail wanted `thenM` \ (avails', wanteds') -> - reduceList stack try_me wanteds' avails' - else - returnM avails -- No op for non-linear things + | Just avail <- findAvail avails wanted + = returnM avails | otherwise - = case try_me wanted of { - - ; DontReduceUnlessConstant -> -- It's irreducible (or at least should not be reduced) - -- First, see if the inst can be reduced to a constant in one step - try_simple (addIrred AddSCs) -- Assume want superclasses - - ; Free -> -- It's free so just chuck it upstairs - -- First, see if the inst can be reduced to a constant in one step - try_simple addFree + = case red_try_me env wanted of { + ; Stop -> try_simple (addIrred NoSCs) -- See Note [No superclasses for Stop] ; ReduceMe want_scs -> -- It should be reduced - lookupInst wanted `thenM` \ lookup_result -> + reduceInst env avails wanted `thenM` \ (avails, lookup_result) -> case lookup_result of - GenInst wanteds' rhs -> addIrred NoSCs avails wanted `thenM` \ avails1 -> - reduceList stack try_me wanteds' avails1 `thenM` \ avails2 -> - addWanted want_scs avails2 wanted rhs wanteds' - -- Experiment with temporarily doing addIrred *before* the reduceList, + NoInstance -> -- No such instance! + -- Add it and its superclasses + addIrred want_scs avails wanted + + GenInst [] rhs -> addWanted want_scs avails wanted rhs [] + + GenInst wanteds' rhs -> do { avails1 <- addIrred NoSCs avails wanted + ; avails2 <- reduceList env wanteds' avails1 + ; addWanted want_scs avails2 wanted rhs wanteds' } + -- Temporarily do addIrred *before* the reduceList, -- which has the effect of adding the thing we are trying -- to prove to the database before trying to prove the things it -- needs. See note [RECURSIVE DICTIONARIES] @@ -1769,148 +1657,19 @@ reduce stack try_me wanted avails -- the examples in [SUPERCLASS-LOOP] -- So we do an addIrred before, and then overwrite it afterwards with addWanted - SimpleInst rhs -> addWanted want_scs avails wanted rhs [] - - NoInstance -> -- No such instance! - -- Add it and its superclasses - addIrred want_scs avails wanted } where + -- First, see if the inst can be reduced to a constant in one step + -- Works well for literals (1::Int) and constant dictionaries (d::Num Int) + -- Don't bother for implication constraints, which take real work try_simple do_this_otherwise - = lookupInst wanted `thenM` \ lookup_result -> - case lookup_result of - SimpleInst rhs -> addWanted AddSCs avails wanted rhs [] - other -> do_this_otherwise avails wanted + = do { res <- lookupSimpleInst wanted + ; case res of + GenInst [] rhs -> addWanted AddSCs avails wanted rhs [] + other -> do_this_otherwise avails wanted } \end{code} -\begin{code} -------------------------- -isAvailable :: Avails -> Inst -> Maybe Avail -isAvailable avails wanted = lookupFM avails wanted - -- NB 1: the Ord instance of Inst compares by the class/type info - -- *not* by unique. So - -- d1::C Int == d2::C Int - -addLinearAvailable :: Avails -> Avail -> Inst -> TcM (Avails, [Inst]) -addLinearAvailable avails avail wanted - -- avails currently maps [wanted -> avail] - -- Extend avails to reflect a neeed for an extra copy of avail - - | Just avail' <- split_avail avail - = returnM (addToFM avails wanted avail', []) - - | otherwise - = tcLookupId splitName `thenM` \ split_id -> - tcInstClassOp (instLoc wanted) split_id - [linearInstType wanted] `thenM` \ split_inst -> - returnM (addToFM avails wanted (Linear 2 split_inst avail), [split_inst]) - - where - split_avail :: Avail -> Maybe Avail - -- (Just av) if there's a modified version of avail that - -- we can use to replace avail in avails - -- Nothing if there isn't, so we need to create a Linear - split_avail (Linear n i a) = Just (Linear (n+1) i a) - split_avail (Given id used) | not used = Just (Given id True) - | otherwise = Nothing - split_avail Irred = Nothing - split_avail IsFree = Nothing - split_avail other = pprPanic "addLinearAvailable" (ppr avail $$ ppr wanted $$ ppr avails) - -------------------------- -addFree :: Avails -> Inst -> TcM Avails - -- When an Inst is tossed upstairs as 'free' we nevertheless add it - -- to avails, so that any other equal Insts will be commoned up right - -- here rather than also being tossed upstairs. This is really just - -- an optimisation, and perhaps it is more trouble that it is worth, - -- as the following comments show! - -- - -- NB: do *not* add superclasses. If we have - -- df::Floating a - -- dn::Num a - -- but a is not bound here, then we *don't* want to derive - -- dn from df here lest we lose sharing. - -- -addFree avails free = returnM (addToFM avails free IsFree) - -addWanted :: WantSCs -> Avails -> Inst -> LHsExpr TcId -> [Inst] -> TcM Avails -addWanted want_scs avails wanted rhs_expr wanteds - = addAvailAndSCs want_scs avails wanted avail - where - avail = Rhs rhs_expr wanteds - -addGiven :: Avails -> Inst -> TcM Avails -addGiven avails given = addAvailAndSCs AddSCs avails given (Given (instToId given) False) - -- Always add superclasses for 'givens' - -- - -- No ASSERT( not (given `elemFM` avails) ) because in an instance - -- decl for Ord t we can add both Ord t and Eq t as 'givens', - -- so the assert isn't true - -addIrred :: WantSCs -> Avails -> Inst -> TcM Avails -addIrred want_scs avails irred = ASSERT2( not (irred `elemFM` avails), ppr irred $$ ppr avails ) - addAvailAndSCs want_scs avails irred Irred - -addAvailAndSCs :: WantSCs -> Avails -> Inst -> Avail -> TcM Avails -addAvailAndSCs want_scs avails inst avail - | not (isClassDict inst) = return avails_with_inst - | NoSCs <- want_scs = return avails_with_inst - | otherwise = do { traceTc (text "addAvailAndSCs" <+> vcat [ppr inst, ppr deps]) - ; addSCs is_loop avails_with_inst inst } - where - avails_with_inst = addToFM avails inst avail - - is_loop pred = any (`tcEqType` mkPredTy pred) dep_tys - -- Note: this compares by *type*, not by Unique - deps = findAllDeps (unitVarSet (instToId inst)) avail - dep_tys = map idType (varSetElems deps) - - findAllDeps :: IdSet -> Avail -> IdSet - -- Find all the Insts that this one depends on - -- See Note [SUPERCLASS-LOOP 2] - -- Watch out, though. Since the avails may contain loops - -- (see Note [RECURSIVE DICTIONARIES]), so we need to track the ones we've seen so far - findAllDeps so_far (Rhs _ kids) = foldl find_all so_far kids - findAllDeps so_far other = so_far - - find_all :: IdSet -> Inst -> IdSet - find_all so_far kid - | kid_id `elemVarSet` so_far = so_far - | Just avail <- lookupFM avails kid = findAllDeps so_far' avail - | otherwise = so_far' - where - so_far' = extendVarSet so_far kid_id -- Add the new kid to so_far - kid_id = instToId kid - -addSCs :: (TcPredType -> Bool) -> Avails -> Inst -> TcM Avails - -- Add all the superclasses of the Inst to Avails - -- The first param says "dont do this because the original thing - -- depends on this one, so you'd build a loop" - -- Invariant: the Inst is already in Avails. - -addSCs is_loop avails dict - = do { sc_dicts <- newDictsAtLoc (instLoc dict) sc_theta' - ; foldlM add_sc avails (zipEqual "add_scs" sc_dicts sc_sels) } - where - (clas, tys) = getDictClassTys dict - (tyvars, sc_theta, sc_sels, _) = classBigSig clas - sc_theta' = substTheta (zipTopTvSubst tyvars tys) sc_theta - - add_sc avails (sc_dict, sc_sel) - | is_loop (dictPred sc_dict) = return avails -- See Note [SUPERCLASS-LOOP 2] - | is_given sc_dict = return avails - | otherwise = addSCs is_loop avails' sc_dict - where - sc_sel_rhs = mkHsDictApp (mkHsTyApp (L (instSpan dict) (HsVar sc_sel)) tys) [instToId dict] - avails' = addToFM avails sc_dict (Rhs sc_sel_rhs [dict]) - - is_given :: Inst -> Bool - is_given sc_dict = case lookupFM avails sc_dict of - Just (Given _ _) -> True -- Given is cheaper than superclass selection - other -> False -\end{code} - Note [SUPERCLASS-LOOP 2] ~~~~~~~~~~~~~~~~~~~~~~~~ But the above isn't enough. Suppose we are *given* d1:Ord a, @@ -1945,7 +1704,7 @@ At first I had a gross hack, whereby I simply did not add superclass constraints in addWanted, though I did for addGiven and addIrred. This was sub-optimal, becuase it lost legitimate superclass sharing, and it still didn't do the job: I found a very obscure program (now tcrun021) in which improvement meant the -simplifier got two bites a the cherry... so something seemed to be an Irred +simplifier got two bites a the cherry... so something seemed to be an Stop first time, but reducible next time. Now we implement the Right Solution, which is to check for loops directly @@ -1991,6 +1750,402 @@ contributing clauses. %************************************************************************ %* * + Reducing a single constraint +%* * +%************************************************************************ + +\begin{code} +--------------------------------------------- +reduceInst :: RedEnv -> Avails -> Inst -> TcM (Avails, LookupInstResult) +reduceInst env avails (ImplicInst { tci_tyvars = tvs, tci_reft = reft, tci_loc = loc, + tci_given = extra_givens, tci_wanted = wanteds }) + = reduceImplication env avails reft tvs extra_givens wanteds loc + +reduceInst env avails other_inst + = do { result <- lookupSimpleInst other_inst + ; return (avails, result) } +\end{code} + +\begin{code} +--------------------------------------------- +reduceImplication :: RedEnv + -> Avails + -> Refinement -- May refine the givens; often empty + -> [TcTyVar] -- Quantified type variables; all skolems + -> [Inst] -- Extra givens; all rigid + -> [Inst] -- Wanted + -> InstLoc + -> TcM (Avails, LookupInstResult) +\end{code} + +Suppose we are simplifying the constraint + forall bs. extras => wanted +in the context of an overall simplification problem with givens 'givens', +and refinment 'reft'. + +Note that + * The refinement is often empty + + * The 'extra givens' need not mention any of the quantified type variables + e.g. forall {}. Eq a => Eq [a] + forall {}. C Int => D (Tree Int) + + This happens when you have something like + data T a where + T1 :: Eq a => a -> T a + + f :: T a -> Int + f x = ...(case x of { T1 v -> v==v })... + +\begin{code} + -- ToDo: should we instantiate tvs? I think it's not necessary + -- + -- ToDo: what about improvement? There may be some improvement + -- exposed as a result of the simplifications done by reduceList + -- which are discarded if we back off. + -- This is almost certainly Wrong, but we'll fix it when dealing + -- better with equality constraints +reduceImplication env orig_avails reft tvs extra_givens wanteds inst_loc + = do { -- Add refined givens, and the extra givens + (refined_red_givens, avails) + <- if isEmptyRefinement reft then return (red_givens env, orig_avails) + else foldlM (addRefinedGiven reft) ([], orig_avails) (red_givens env) + ; avails <- foldlM addGiven avails extra_givens + + -- Solve the sub-problem + ; let try_me inst = ReduceMe AddSCs -- Note [Freeness and implications] + env' = env { red_givens = refined_red_givens ++ extra_givens + , red_try_me = try_me } + + ; traceTc (text "reduceImplication" <+> vcat + [ ppr orig_avails, + ppr (red_givens env), ppr extra_givens, + ppr reft, ppr wanteds, ppr avails ]) + ; avails <- reduceList env' wanteds avails + + -- Extract the binding + ; (binds, irreds) <- extractResults avails wanteds + + -- We always discard the extra avails we've generated; + -- but we remember if we have done any (global) improvement + ; let ret_avails = updateImprovement orig_avails avails + + ; if isEmptyLHsBinds binds then -- No progress + return (ret_avails, NoInstance) + else do + { (implic_insts, bind) <- makeImplicationBind inst_loc tvs reft extra_givens irreds + -- This binding is useless if the recursive simplification + -- made no progress; but currently we don't try to optimise that + -- case. After all, we only try hard to reduce at top level, or + -- when inferring types. + + ; let dict_ids = map instToId extra_givens + co = mkWpTyLams tvs <.> mkWpLams dict_ids <.> WpLet (binds `unionBags` bind) + rhs = mkHsWrap co payload + loc = instLocSpan inst_loc + payload | isSingleton wanteds = HsVar (instToId (head wanteds)) + | otherwise = ExplicitTuple (map (L loc . HsVar . instToId) wanteds) Boxed + + -- If there are any irreds, we back off and return NoInstance + ; return (ret_avails, GenInst implic_insts (L loc rhs)) + } } +\end{code} + +Note [Freeness and implications] +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +It's hard to say when an implication constraint can be floated out. Consider + forall {} Eq a => Foo [a] +The (Foo [a]) doesn't mention any of the quantified variables, but it +still might be partially satisfied by the (Eq a). + +There is a useful special case when it *is* easy to partition the +constraints, namely when there are no 'givens'. Consider + forall {a}. () => Bar b +There are no 'givens', and so there is no reason to capture (Bar b). +We can let it float out. But if there is even one constraint we +must be much more careful: + forall {a}. C a b => Bar (m b) +because (C a b) might have a superclass (D b), from which we might +deduce (Bar [b]) when m later gets instantiated to []. Ha! + +Here is an even more exotic example + class C a => D a b +Now consider the constraint + forall b. D Int b => C Int +We can satisfy the (C Int) from the superclass of D, so we don't want +to float the (C Int) out, even though it mentions no type variable in +the constraints! + +Note [Pruning the givens in an implication constraint] +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +Suppose we are about to form the implication constraint + forall tvs. Eq a => Ord b +The (Eq a) cannot contribute to the (Ord b), because it has no access to +the type variable 'b'. So we could filter out the (Eq a) from the givens. + +Doing so would be a bit tidier, but all the implication constraints get +simplified away by the optimiser, so it's no great win. So I don't take +advantage of that at the moment. + +If you do, BE CAREFUL of wobbly type variables. + + +%************************************************************************ +%* * + Avails and AvailHow: the pool of evidence +%* * +%************************************************************************ + + +\begin{code} +data Avails = Avails !ImprovementDone !AvailEnv + +type ImprovementDone = Bool -- True <=> some unification has happened + -- so some Irreds might now be reducible + -- keys that are now + +type AvailEnv = FiniteMap Inst AvailHow +data AvailHow + = IsIrred -- Used for irreducible dictionaries, + -- which are going to be lambda bound + + | Given TcId -- Used for dictionaries for which we have a binding + -- e.g. those "given" in a signature + + | Rhs -- Used when there is a RHS + (LHsExpr TcId) -- The RHS + [Inst] -- Insts free in the RHS; we need these too + +instance Outputable Avails where + ppr = pprAvails + +pprAvails (Avails imp avails) + = vcat [ ptext SLIT("Avails") <> (if imp then ptext SLIT("[improved]") else empty) + , nest 2 (vcat [sep [ppr inst, nest 2 (equals <+> ppr avail)] + | (inst,avail) <- fmToList avails ])] + +instance Outputable AvailHow where + ppr = pprAvail + +------------------------- +pprAvail :: AvailHow -> SDoc +pprAvail IsIrred = text "Irred" +pprAvail (Given x) = text "Given" <+> ppr x +pprAvail (Rhs rhs bs) = text "Rhs" <+> ppr rhs <+> braces (ppr bs) + +------------------------- +extendAvailEnv :: AvailEnv -> Inst -> AvailHow -> AvailEnv +extendAvailEnv env inst avail = addToFM env inst avail + +findAvailEnv :: AvailEnv -> Inst -> Maybe AvailHow +findAvailEnv env wanted = lookupFM env wanted + -- NB 1: the Ord instance of Inst compares by the class/type info + -- *not* by unique. So + -- d1::C Int == d2::C Int + +emptyAvails :: Avails +emptyAvails = Avails False emptyFM + +findAvail :: Avails -> Inst -> Maybe AvailHow +findAvail (Avails _ avails) wanted = findAvailEnv avails wanted + +elemAvails :: Inst -> Avails -> Bool +elemAvails wanted (Avails _ avails) = wanted `elemFM` avails + +extendAvails :: Avails -> Inst -> AvailHow -> TcM Avails +-- Does improvement +extendAvails avails@(Avails imp env) inst avail + = do { imp1 <- tcImproveOne avails inst -- Do any improvement + ; return (Avails (imp || imp1) (extendAvailEnv env inst avail)) } + +availsInsts :: Avails -> [Inst] +availsInsts (Avails _ avails) = keysFM avails + +availsImproved (Avails imp _) = imp + +updateImprovement :: Avails -> Avails -> Avails +-- (updateImprovement a1 a2) sets a1's improvement flag from a2 +updateImprovement (Avails _ avails1) (Avails imp2 _) = Avails imp2 avails1 +\end{code} + +Extracting the bindings from a bunch of Avails. +The bindings do *not* come back sorted in dependency order. +We assume that they'll be wrapped in a big Rec, so that the +dependency analyser can sort them out later + +\begin{code} +extractResults :: Avails + -> [Inst] -- Wanted + -> TcM ( TcDictBinds, -- Bindings + [Inst]) -- Irreducible ones + +extractResults (Avails _ avails) wanteds + = go avails emptyBag [] wanteds + where + go :: AvailEnv -> TcDictBinds -> [Inst] -> [Inst] + -> TcM (TcDictBinds, [Inst]) + go avails binds irreds [] + = returnM (binds, irreds) + + go avails binds irreds (w:ws) + = case findAvailEnv avails w of + Nothing -> pprTrace "Urk: extractResults" (ppr w) $ + go avails binds irreds ws + + Just IsIrred -> go (add_given avails w) binds (w:irreds) ws + + Just (Given id) + | id == instToId w + -> go avails binds irreds ws + -- The sought Id can be one of the givens, via a superclass chain + -- and then we definitely don't want to generate an x=x binding! + + | otherwise + -> go avails (addBind binds w (nlHsVar id)) irreds ws + + Just (Rhs rhs ws') -> go (add_given avails w) new_binds irreds (ws' ++ ws) + where + new_binds = addBind binds w rhs + + add_given avails w = extendAvailEnv avails w (Given (instToId w)) + +addBind binds inst rhs = binds `unionBags` unitBag (L (instSpan inst) + (VarBind (instToId inst) rhs)) +\end{code} + + +Note [No superclasses for Stop] +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +When we decide not to reduce an Inst -- the 'WhatToDo' --- we still +add it to avails, so that any other equal Insts will be commoned up +right here. However, we do *not* add superclasses. If we have + df::Floating a + dn::Num a +but a is not bound here, then we *don't* want to derive dn from df +here lest we lose sharing. + +\begin{code} +addWanted :: WantSCs -> Avails -> Inst -> LHsExpr TcId -> [Inst] -> TcM Avails +addWanted want_scs avails wanted rhs_expr wanteds + = addAvailAndSCs want_scs avails wanted avail + where + avail = Rhs rhs_expr wanteds + +addGiven :: Avails -> Inst -> TcM Avails +addGiven avails given = addAvailAndSCs AddSCs avails given (Given (instToId given)) + -- Always add superclasses for 'givens' + -- + -- No ASSERT( not (given `elemAvails` avails) ) because in an instance + -- decl for Ord t we can add both Ord t and Eq t as 'givens', + -- so the assert isn't true + +addRefinedGiven :: Refinement -> ([Inst], Avails) -> Inst -> TcM ([Inst], Avails) +addRefinedGiven reft (refined_givens, avails) given + | isDict given -- We sometimes have 'given' methods, but they + -- are always optional, so we can drop them + , let pred = dictPred given + , isRefineablePred pred -- See Note [ImplicInst rigidity] + , Just (co, pred) <- refinePred reft pred + = do { new_given <- newDictBndr (instLoc given) pred + ; let rhs = L (instSpan given) $ + HsWrap (WpCo co) (HsVar (instToId given)) + ; avails <- addAvailAndSCs AddSCs avails new_given (Rhs rhs [given]) + ; return (new_given:refined_givens, avails) } + -- ToDo: the superclasses of the original given all exist in Avails + -- so we could really just cast them, but it's more awkward to do, + -- and hopefully the optimiser will spot the duplicated work + | otherwise + = return (refined_givens, avails) +\end{code} + +Note [ImplicInst rigidity] +~~~~~~~~~~~~~~~~~~~~~~~~~~ +Consider + C :: forall ab. (Eq a, Ord b) => b -> T a + + ...(case x of C v -> )... + +From the case (where x::T ty) we'll get an implication constraint + forall b. (Eq ty, Ord b) => +Now suppose itself has an implication constraint +of form + forall c. => +Then, we can certainly apply the refinement to the Ord b, becuase it is +existential, but we probably should not apply it to the (Eq ty) because it may +be wobbly. Hence the isRigidInst + +@Insts@ are ordered by their class/type info, rather than by their +unique. This allows the context-reduction mechanism to use standard finite +maps to do their stuff. It's horrible that this code is here, rather +than with the Avails handling stuff in TcSimplify + +\begin{code} +addIrred :: WantSCs -> Avails -> Inst -> TcM Avails +addIrred want_scs avails irred = ASSERT2( not (irred `elemAvails` avails), ppr irred $$ ppr avails ) + addAvailAndSCs want_scs avails irred IsIrred + +addAvailAndSCs :: WantSCs -> Avails -> Inst -> AvailHow -> TcM Avails +addAvailAndSCs want_scs avails inst avail + | not (isClassDict inst) = extendAvails avails inst avail + | NoSCs <- want_scs = extendAvails avails inst avail + | otherwise = do { traceTc (text "addAvailAndSCs" <+> vcat [ppr inst, ppr deps]) + ; avails' <- extendAvails avails inst avail + ; addSCs is_loop avails' inst } + where + is_loop pred = any (`tcEqType` mkPredTy pred) dep_tys + -- Note: this compares by *type*, not by Unique + deps = findAllDeps (unitVarSet (instToId inst)) avail + dep_tys = map idType (varSetElems deps) + + findAllDeps :: IdSet -> AvailHow -> IdSet + -- Find all the Insts that this one depends on + -- See Note [SUPERCLASS-LOOP 2] + -- Watch out, though. Since the avails may contain loops + -- (see Note [RECURSIVE DICTIONARIES]), so we need to track the ones we've seen so far + findAllDeps so_far (Rhs _ kids) = foldl find_all so_far kids + findAllDeps so_far other = so_far + + find_all :: IdSet -> Inst -> IdSet + find_all so_far kid + | kid_id `elemVarSet` so_far = so_far + | Just avail <- findAvail avails kid = findAllDeps so_far' avail + | otherwise = so_far' + where + so_far' = extendVarSet so_far kid_id -- Add the new kid to so_far + kid_id = instToId kid + +addSCs :: (TcPredType -> Bool) -> Avails -> Inst -> TcM Avails + -- Add all the superclasses of the Inst to Avails + -- The first param says "dont do this because the original thing + -- depends on this one, so you'd build a loop" + -- Invariant: the Inst is already in Avails. + +addSCs is_loop avails dict + = ASSERT( isDict dict ) + do { sc_dicts <- newDictBndrs (instLoc dict) sc_theta' + ; foldlM add_sc avails (zipEqual "add_scs" sc_dicts sc_sels) } + where + (clas, tys) = getDictClassTys dict + (tyvars, sc_theta, sc_sels, _) = classBigSig clas + sc_theta' = substTheta (zipTopTvSubst tyvars tys) sc_theta + + add_sc avails (sc_dict, sc_sel) + | is_loop (dictPred sc_dict) = return avails -- See Note [SUPERCLASS-LOOP 2] + | is_given sc_dict = return avails + | otherwise = do { avails' <- extendAvails avails sc_dict (Rhs sc_sel_rhs [dict]) + ; addSCs is_loop avails' sc_dict } + where + sc_sel_rhs = L (instSpan dict) (HsWrap co_fn (HsVar sc_sel)) + co_fn = WpApp (instToId dict) <.> mkWpTyApps tys + + is_given :: Inst -> Bool + is_given sc_dict = case findAvail avails sc_dict of + Just (Given _) -> True -- Given is cheaper than superclass selection + other -> False +\end{code} + +%************************************************************************ +%* * \section{tcSimplifyTop: defaulting} %* * %************************************************************************ @@ -2012,103 +2167,44 @@ It's OK: the final zonking stage should zap y to (), which is fine. \begin{code} tcSimplifyTop, tcSimplifyInteractive :: [Inst] -> TcM TcDictBinds tcSimplifyTop wanteds - = tc_simplify_top doc False {- Not interactive loop -} AddSCs wanteds + = tc_simplify_top doc False wanteds where doc = text "tcSimplifyTop" tcSimplifyInteractive wanteds - = tc_simplify_top doc True {- Interactive loop -} AddSCs wanteds + = tc_simplify_top doc True wanteds where - doc = text "tcSimplifyTop" + doc = text "tcSimplifyInteractive" -- The TcLclEnv should be valid here, solely to improve -- error message generation for the monomorphism restriction -tc_simplify_top doc is_interactive want_scs wanteds - = do { lcl_env <- getLclEnv - ; traceTc (text "tcSimplifyTop" <+> ppr (lclEnvElts lcl_env)) - - ; let try_me inst = ReduceMe want_scs - ; (frees, binds, irreds) <- simpleReduceLoop doc try_me wanteds - - ; let - -- First get rid of implicit parameters - (non_ips, bad_ips) = partition isClassDict irreds - - -- All the non-tv or multi-param ones are definite errors - (unary_tv_dicts, non_tvs) = partition is_unary_tyvar_dict non_ips - bad_tyvars = unionVarSets (map tyVarsOfInst non_tvs) - - -- Group by type variable - tv_groups = equivClasses cmp_by_tyvar unary_tv_dicts - - -- Pick the ones which its worth trying to disambiguate - -- namely, the ones whose type variable isn't bound - -- up with one of the non-tyvar classes - (default_gps, non_default_gps) = partition defaultable_group tv_groups - defaultable_group ds - = not (bad_tyvars `intersectsVarSet` tyVarsOfInst (head ds)) - && defaultable_classes (map get_clas ds) - defaultable_classes clss - | is_interactive = any isInteractiveClass clss - | otherwise = all isStandardClass clss && any isNumericClass clss - - isInteractiveClass cls = isNumericClass cls - || (classKey cls `elem` [showClassKey, eqClassKey, ordClassKey]) - -- In interactive mode, we default Show a to Show () - -- to avoid graututious errors on "show []" - - - -- Collect together all the bad guys - bad_guys = non_tvs ++ concat non_default_gps - (ambigs, no_insts) = partition isTyVarDict bad_guys - -- If the dict has no type constructors involved, it must be ambiguous, - -- except I suppose that another error with fundeps maybe should have - -- constrained those type variables - - -- Report definite errors - ; ASSERT( null frees ) - groupErrs (addNoInstanceErrs Nothing []) no_insts - ; strangeTopIPErrs bad_ips - - -- Deal with ambiguity errors, but only if - -- if there has not been an error so far: - -- errors often give rise to spurious ambiguous Insts. - -- For example: - -- f = (*) -- Monomorphic - -- g :: Num a => a -> a - -- g x = f x x - -- Here, we get a complaint when checking the type signature for g, - -- that g isn't polymorphic enough; but then we get another one when - -- dealing with the (Num a) context arising from f's definition; - -- we try to unify a with Int (to default it), but find that it's - -- already been unified with the rigid variable from g's type sig - ; binds_ambig <- ifErrsM (returnM []) $ - do { -- Complain about the ones that don't fall under - -- the Haskell rules for disambiguation - -- This group includes both non-existent instances - -- e.g. Num (IO a) and Eq (Int -> Int) - -- and ambiguous dictionaries - -- e.g. Num a - addTopAmbigErrs ambigs - - -- Disambiguate the ones that look feasible - ; mappM disambigGroup default_gps } - - ; return (binds `unionBags` unionManyBags binds_ambig) } - ----------------------------------- -d1 `cmp_by_tyvar` d2 = get_tv d1 `compare` get_tv d2 - -is_unary_tyvar_dict :: Inst -> Bool -- Dicts of form (C a) - -- Invariant: argument is a ClassDict, not IP or method -is_unary_tyvar_dict d = case getDictClassTys d of - (_, [ty]) -> tcIsTyVarTy ty - other -> False - -get_tv d = case getDictClassTys d of - (clas, [ty]) -> tcGetTyVar "tcSimplify" ty -get_clas d = case getDictClassTys d of - (clas, _) -> clas +tc_simplify_top doc interactive wanteds + = do { wanteds <- mapM zonkInst wanteds + ; mapM_ zonkTopTyVar (varSetElems (tyVarsOfInsts wanteds)) + + ; (irreds1, binds1) <- topCheckLoop doc wanteds + + ; if null irreds1 then + return binds1 + else do + -- OK, so there are some errors + { -- Use the defaulting rules to do extra unification + -- NB: irreds are already zonked + ; extended_default <- if interactive then return True + else doptM Opt_ExtendedDefaultRules + ; disambiguate extended_default irreds1 -- Does unification + ; (irreds2, binds2) <- topCheckLoop doc irreds1 + + -- Deal with implicit parameter + ; let (bad_ips, non_ips) = partition isIPDict irreds2 + (ambigs, others) = partition isTyVarDict non_ips + + ; topIPErrs bad_ips -- Can arise from f :: Int -> Int + -- f x = x + ?y + ; addNoInstanceErrs others + ; addTopAmbigErrs ambigs + + ; return (binds1 `unionBags` binds2) }} \end{code} If a dictionary constrains a type variable which is @@ -2144,88 +2240,90 @@ Since we're not using the result of @foo@, the result if (presumably) @void@. \begin{code} -disambigGroup :: [Inst] -- All standard classes of form (C a) - -> TcM TcDictBinds +disambiguate :: Bool -> [Inst] -> TcM () + -- Just does unification to fix the default types + -- The Insts are assumed to be pre-zonked +disambiguate extended_defaulting insts + | null defaultable_groups + = do { traceTc (text "disambigutate" <+> vcat [ppr unaries, ppr bad_tvs, ppr defaultable_groups]) + ; return () } + | otherwise + = do { -- Figure out what default types to use + mb_defaults <- getDefaultTys + ; default_tys <- case mb_defaults of + Just tys -> return tys + Nothing -> -- No use-supplied default; + -- use [Integer, Double] + do { integer_ty <- tcMetaTy integerTyConName + ; checkWiredInTyCon doubleTyCon + ; return [integer_ty, doubleTy] } + ; traceTc (text "disambigutate" <+> vcat [ppr unaries, ppr bad_tvs, ppr defaultable_groups]) + ; mapM_ (disambigGroup default_tys) defaultable_groups } + where + unaries :: [(Inst,Class, TcTyVar)] -- (C tv) constraints + bad_tvs :: TcTyVarSet -- Tyvars mentioned by *other* constraints + (unaries, bad_tvs) = getDefaultableDicts insts -disambigGroup dicts - = -- THE DICTS OBEY THE DEFAULTABLE CONSTRAINT - -- SO, TRY DEFAULT TYPES IN ORDER - - -- Failure here is caused by there being no type in the - -- default list which can satisfy all the ambiguous classes. - -- For example, if Real a is reqd, but the only type in the - -- default list is Int. - get_default_tys `thenM` \ default_tys -> - let - try_default [] -- No defaults work, so fail - = failM - - try_default (default_ty : default_tys) - = tryTcLIE_ (try_default default_tys) $ -- If default_ty fails, we try - -- default_tys instead - tcSimplifyDefault theta `thenM` \ _ -> - returnM default_ty - where - theta = [mkClassPred clas [default_ty] | clas <- classes] - in - -- See if any default works - tryM (try_default default_tys) `thenM` \ mb_ty -> - case mb_ty of - Left _ -> bomb_out - Right chosen_default_ty -> choose_default chosen_default_ty + -- Group by type variable + defaultable_groups :: [[(Inst,Class,TcTyVar)]] + defaultable_groups = filter defaultable_group (equivClasses cmp_tv unaries) + cmp_tv (_,_,tv1) (_,_,tv2) = tv1 `compare` tv2 + + defaultable_group :: [(Inst,Class,TcTyVar)] -> Bool + defaultable_group ds@((_,_,tv):_) + = not (isImmutableTyVar tv) -- Note [Avoiding spurious errors] + && not (tv `elemVarSet` bad_tvs) + && defaultable_classes [c | (_,c,_) <- ds] + defaultable_group [] = panic "defaultable_group" + + defaultable_classes clss + | extended_defaulting = any isInteractiveClass clss + | otherwise = all isStandardClass clss && any isNumericClass clss + + -- In interactive mode, or with -fextended-default-rules, + -- we default Show a to Show () to avoid graututious errors on "show []" + isInteractiveClass cls + = isNumericClass cls + || (classKey cls `elem` [showClassKey, eqClassKey, ordClassKey]) + + +disambigGroup :: [Type] -- The default types + -> [(Inst,Class,TcTyVar)] -- All standard classes of form (C a) + -> TcM () -- Just does unification, to fix the default types + +disambigGroup default_tys dicts + = try_default default_tys where - tyvar = get_tv (head dicts) -- Should be non-empty - classes = map get_clas dicts - - choose_default default_ty -- Commit to tyvar = default_ty - = -- Bind the type variable - unifyType default_ty (mkTyVarTy tyvar) `thenM_` - -- and reduce the context, for real this time - simpleReduceLoop (text "disambig" <+> ppr dicts) - reduceMe dicts `thenM` \ (frees, binds, ambigs) -> - WARN( not (null frees && null ambigs), ppr frees $$ ppr ambigs ) - warnDefault dicts default_ty `thenM_` - returnM binds - - bomb_out = addTopAmbigErrs dicts `thenM_` - returnM emptyBag - -get_default_tys - = do { mb_defaults <- getDefaultTys - ; case mb_defaults of - Just tys -> return tys - Nothing -> -- No use-supplied default; - -- use [Integer, Double] - do { integer_ty <- tcMetaTy integerTyConName - ; checkWiredInTyCon doubleTyCon - ; return [integer_ty, doubleTy] } } + (_,_,tyvar) = head dicts -- Should be non-empty + classes = [c | (_,c,_) <- dicts] + + try_default [] = return () + try_default (default_ty : default_tys) + = tryTcLIE_ (try_default default_tys) $ + do { tcSimplifyDefault [mkClassPred clas [default_ty] | clas <- classes] + -- This may fail; then the tryTcLIE_ kicks in + -- Failure here is caused by there being no type in the + -- default list which can satisfy all the ambiguous classes. + -- For example, if Real a is reqd, but the only type in the + -- default list is Int. + + -- After this we can't fail + ; warnDefault dicts default_ty + ; unifyType default_ty (mkTyVarTy tyvar) } \end{code} -[Aside - why the defaulting mechanism is turned off when - dealing with arguments and results to ccalls. - -When typechecking _ccall_s, TcExpr ensures that the external -function is only passed arguments (and in the other direction, -results) of a restricted set of 'native' types. - -The interaction between the defaulting mechanism for numeric -values and CC & CR can be a bit puzzling to the user at times. -For example, - - x <- _ccall_ f - if (x /= 0) then - _ccall_ g x - else - return () - -What type has 'x' got here? That depends on the default list -in operation, if it is equal to Haskell 98's default-default -of (Integer, Double), 'x' has type Double, since Integer -is not an instance of CR. If the default list is equal to -Haskell 1.4's default-default of (Int, Double), 'x' has type -Int. - -End of aside] +Note [Avoiding spurious errors] +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +When doing the unification for defaulting, we check for skolem +type variables, and simply don't default them. For example: + f = (*) -- Monomorphic + g :: Num a => a -> a + g x = f x x +Here, we get a complaint when checking the type signature for g, +that g isn't polymorphic enough; but then we get another one when +dealing with the (Num a) context arising from f's definition; +we try to unify a with Int (to default it), but find that it's +already been unified with the rigid variable from g's type sig %************************************************************************ @@ -2245,62 +2343,57 @@ a,b,c are type variables. This is required for the context of instance declarations. \begin{code} -tcSimplifyDeriv :: TyCon +tcSimplifyDeriv :: InstOrigin -> [TyVar] -> ThetaType -- Wanted -> TcM ThetaType -- Needed +-- Given instance (wanted) => C inst_ty +-- Simplify 'wanted' as much as possible +-- The inst_ty is needed only for the termination check -tcSimplifyDeriv tc tyvars theta - = tcInstTyVars tyvars `thenM` \ (tvs, _, tenv) -> +tcSimplifyDeriv orig tyvars theta + = do { (tvs, _, tenv) <- tcInstTyVars tyvars -- The main loop may do unification, and that may crash if -- it doesn't see a TcTyVar, so we have to instantiate. Sigh -- ToDo: what if two of them do get unified? - newDicts DerivOrigin (substTheta tenv theta) `thenM` \ wanteds -> - simpleReduceLoop doc reduceMe wanteds `thenM` \ (frees, _, irreds) -> - ASSERT( null frees ) -- reduceMe never returns Free - - doptM Opt_GlasgowExts `thenM` \ gla_exts -> - doptM Opt_AllowUndecidableInstances `thenM` \ undecidable_ok -> - let - tv_set = mkVarSet tvs - - (bad_insts, ok_insts) = partition is_bad_inst irreds - is_bad_inst dict - = let pred = dictPred dict -- reduceMe squashes all non-dicts - in isEmptyVarSet (tyVarsOfPred pred) - -- Things like (Eq T) are bad - || (not gla_exts && not (isTyVarClassPred pred)) - - simpl_theta = map dictPred ok_insts - weird_preds = [pred | pred <- simpl_theta - , not (tyVarsOfPred pred `subVarSet` tv_set)] - -- Check for a bizarre corner case, when the derived instance decl should - -- have form instance C a b => D (T a) where ... - -- Note that 'b' isn't a parameter of T. This gives rise to all sorts - -- of problems; in particular, it's hard to compare solutions for - -- equality when finding the fixpoint. So I just rule it out for now. - - rev_env = zipTopTvSubst tvs (mkTyVarTys tyvars) + ; wanteds <- newDictBndrsO orig (substTheta tenv theta) + ; (irreds, _) <- topCheckLoop doc wanteds + + ; let (dicts, non_dicts) = partition isDict irreds + -- Exclude implication consraints + ; addNoInstanceErrs non_dicts -- I'm not sure if these can really happen + + ; let rev_env = zipTopTvSubst tvs (mkTyVarTys tyvars) + simpl_theta = substTheta rev_env (map dictPred dicts) -- This reverse-mapping is a Royal Pain, -- but the result should mention TyVars not TcTyVars - head_ty = TyConApp tc (map TyVarTy tvs) - in - - addNoInstanceErrs Nothing [] bad_insts `thenM_` - mapM_ (addErrTc . badDerivedPred) weird_preds `thenM_` - checkAmbiguity tvs simpl_theta tv_set `thenM_` - -- Check instance termination as for user-declared instances. - -- unless we had -fallow-undecidable-instances (which risks - -- non-termination in the 'deriving' context-inference fixpoint - -- loop). - ifM (gla_exts && not undecidable_ok) - (checkInstTermination simpl_theta [head_ty]) `thenM_` - returnM (substTheta rev_env simpl_theta) + ; return simpl_theta } where - doc = ptext SLIT("deriving classes for a data type") + doc = ptext SLIT("deriving classes for a data type") \end{code} +Note [Deriving context] +~~~~~~~~~~~~~~~~~~~~~~~ +With -fglasgow-exts, we allow things like (C Int a) in the simplified +context for a derived instance declaration, because at a use of this +instance, we might know that a=Bool, and have an instance for (C Int +Bool) + +We nevertheless insist that each predicate meets the termination +conditions. If not, the deriving mechanism generates larger and larger +constraints. Example: + data Succ a = S a + data Seq a = Cons a (Seq (Succ a)) | Nil deriving Show + +Note the lack of a Show instance for Succ. First we'll generate + instance (Show (Succ a), Show a) => Show (Seq a) +and then + instance (Show (Succ (Succ a)), Show (Succ a), Show a) => Show (Seq a) +and so on. Instead we want to complain of no instance for (Show (Succ a)). + + + @tcSimplifyDefault@ just checks class-type constraints, essentially; used with \tr{default} declarations. We are only interested in whether it worked or not. @@ -2310,10 +2403,9 @@ tcSimplifyDefault :: ThetaType -- Wanted; has no type variables in it -> TcM () tcSimplifyDefault theta - = newDicts DefaultOrigin theta `thenM` \ wanteds -> - simpleReduceLoop doc reduceMe wanteds `thenM` \ (frees, _, irreds) -> - ASSERT( null frees ) -- try_me never returns Free - addNoInstanceErrs Nothing [] irreds `thenM_` + = newDictBndrsO DefaultOrigin theta `thenM` \ wanteds -> + topCheckLoop doc wanteds `thenM` \ (irreds, _) -> + addNoInstanceErrs irreds `thenM_` if null irreds then returnM () else @@ -2358,7 +2450,7 @@ groupErrs report_err (inst:insts) -- Add the "arising from..." part to a message about bunch of dicts addInstLoc :: [Inst] -> Message -> Message -addInstLoc insts msg = msg $$ nest 2 (pprInstLoc (instLoc (head insts))) +addInstLoc insts msg = msg $$ nest 2 (pprInstArising (head insts)) addTopIPErrs :: [Name] -> [Inst] -> TcM () addTopIPErrs bndrs [] @@ -2368,14 +2460,15 @@ addTopIPErrs bndrs ips where (tidy_env, tidy_ips) = tidyInsts ips mk_msg ips = vcat [sep [ptext SLIT("Implicit parameters escape from"), - nest 2 (ptext SLIT("the monomorphic top-level binding(s) of") + nest 2 (ptext SLIT("the monomorphic top-level binding") + <> plural bndrs <+> ptext SLIT("of") <+> pprBinders bndrs <> colon)], nest 2 (vcat (map ppr_ip ips)), monomorphism_fix] - ppr_ip ip = pprPred (dictPred ip) <+> pprInstLoc (instLoc ip) + ppr_ip ip = pprPred (dictPred ip) <+> pprInstArising ip -strangeTopIPErrs :: [Inst] -> TcM () -strangeTopIPErrs dicts -- Strange, becuase addTopIPErrs should have caught them all +topIPErrs :: [Inst] -> TcM () +topIPErrs dicts = groupErrs report tidy_dicts where (tidy_env, tidy_dicts) = tidyInsts dicts @@ -2383,54 +2476,59 @@ strangeTopIPErrs dicts -- Strange, becuase addTopIPErrs should have caught them mk_msg dicts = addInstLoc dicts (ptext SLIT("Unbound implicit parameter") <> plural tidy_dicts <+> pprDictsTheta tidy_dicts) -addNoInstanceErrs :: Maybe SDoc -- Nothing => top level - -- Just d => d describes the construct - -> [Inst] -- What is given by the context or type sig - -> [Inst] -- What is wanted +addNoInstanceErrs :: [Inst] -- Wanted (can include implications) -> TcM () -addNoInstanceErrs mb_what givens [] - = returnM () -addNoInstanceErrs mb_what givens dicts - = -- Some of the dicts are here because there is no instances - -- and some because there are too many instances (overlap) - tcGetInstEnvs `thenM` \ inst_envs -> - let - (tidy_env1, tidy_givens) = tidyInsts givens - (tidy_env2, tidy_dicts) = tidyMoreInsts tidy_env1 dicts - - -- Run through the dicts, generating a message for each - -- overlapping one, but simply accumulating all the - -- no-instance ones so they can be reported as a group - (overlap_doc, no_inst_dicts) = foldl check_overlap (empty, []) tidy_dicts - check_overlap (overlap_doc, no_inst_dicts) dict - | not (isClassDict dict) = (overlap_doc, dict : no_inst_dicts) - | otherwise - = case lookupInstEnv inst_envs clas tys of +addNoInstanceErrs insts + = do { let (tidy_env, tidy_insts) = tidyInsts insts + ; reportNoInstances tidy_env Nothing tidy_insts } + +reportNoInstances + :: TidyEnv + -> Maybe (InstLoc, [Inst]) -- Context + -- Nothing => top level + -- Just (d,g) => d describes the construct + -- with givens g + -> [Inst] -- What is wanted (can include implications) + -> TcM () + +reportNoInstances tidy_env mb_what insts + = groupErrs (report_no_instances tidy_env mb_what) insts + +report_no_instances tidy_env mb_what insts + = do { inst_envs <- tcGetInstEnvs + ; let (implics, insts1) = partition isImplicInst insts + (insts2, overlaps) = partitionWith (check_overlap inst_envs) insts1 + ; traceTc (text "reportNoInstnces" <+> vcat + [ppr implics, ppr insts1, ppr insts2]) + ; mapM_ complain_implic implics + ; mapM_ (\doc -> addErrTcM (tidy_env, doc)) overlaps + ; groupErrs complain_no_inst insts2 } + where + complain_no_inst insts = addErrTcM (tidy_env, mk_no_inst_err insts) + + complain_implic inst -- Recurse! + = reportNoInstances tidy_env + (Just (tci_loc inst, tci_given inst)) + (tci_wanted inst) + + check_overlap :: (InstEnv,InstEnv) -> Inst -> Either Inst SDoc + -- Right msg => overlap message + -- Left inst => no instance + check_overlap inst_envs wanted + | not (isClassDict wanted) = Left wanted + | otherwise + = case lookupInstEnv inst_envs clas tys of -- The case of exactly one match and no unifiers means -- a successful lookup. That can't happen here, becuase -- dicts only end up here if they didn't match in Inst.lookupInst #ifdef DEBUG - ([m],[]) -> pprPanic "addNoInstanceErrs" (ppr dict) + ([m],[]) -> pprPanic "reportNoInstance" (ppr wanted) #endif - ([], _) -> (overlap_doc, dict : no_inst_dicts) -- No match - res -> (mk_overlap_msg dict res $$ overlap_doc, no_inst_dicts) + ([], _) -> Left wanted -- No match + res -> Right (mk_overlap_msg wanted res) where - (clas,tys) = getDictClassTys dict - in - - -- Now generate a good message for the no-instance bunch - mk_probable_fix tidy_env2 no_inst_dicts `thenM` \ (tidy_env3, probable_fix) -> - let - no_inst_doc | null no_inst_dicts = empty - | otherwise = vcat [addInstLoc no_inst_dicts heading, probable_fix] - heading | null givens = ptext SLIT("No instance") <> plural no_inst_dicts <+> - ptext SLIT("for") <+> pprDictsTheta no_inst_dicts - | otherwise = sep [ptext SLIT("Could not deduce") <+> pprDictsTheta no_inst_dicts, - nest 2 $ ptext SLIT("from the context") <+> pprDictsTheta tidy_givens] - in - -- And emit both the non-instance and overlap messages - addErrTcM (tidy_env3, no_inst_doc $$ overlap_doc) - where + (clas,tys) = getDictClassTys wanted + mk_overlap_msg dict (matches, unifiers) = vcat [ addInstLoc [dict] ((ptext SLIT("Overlapping instances for") <+> pprPred (dictPred dict))), @@ -2448,31 +2546,46 @@ addNoInstanceErrs mb_what givens dicts where ispecs = [ispec | (_, ispec) <- matches] - mk_probable_fix tidy_env dicts - = returnM (tidy_env, sep [ptext SLIT("Possible fix:"), nest 2 (vcat fixes)]) + mk_no_inst_err insts + | null insts = empty + + | Just (loc, givens) <- mb_what, -- Nested (type signatures, instance decls) + not (isEmptyVarSet (tyVarsOfInsts insts)) + = vcat [ addInstLoc insts $ + sep [ ptext SLIT("Could not deduce") <+> pprDictsTheta insts + , nest 2 $ ptext SLIT("from the context") <+> pprDictsTheta givens] + , show_fixes (fix1 loc : fixes2) ] + + | otherwise -- Top level + = vcat [ addInstLoc insts $ + ptext SLIT("No instance") <> plural insts + <+> ptext SLIT("for") <+> pprDictsTheta insts + , show_fixes fixes2 ] + where - fixes = add_ors (fix1 ++ fix2) - - fix1 = case mb_what of - Nothing -> [] -- Top level - Just what -> -- Nested (type signatures, instance decls) - [ sep [ ptext SLIT("add") <+> pprDictsTheta dicts, - ptext SLIT("to the") <+> what] ] - - fix2 | null instance_dicts = [] - | otherwise = [ ptext SLIT("add an instance declaration for") - <+> pprDictsTheta instance_dicts ] - instance_dicts = [d | d <- dicts, isClassDict d, not (isTyVarDict d)] + fix1 loc = sep [ ptext SLIT("add") <+> pprDictsTheta insts + <+> ptext SLIT("to the context of"), + nest 2 (ppr (instLocOrigin loc)) ] + -- I'm not sure it helps to add the location + -- nest 2 (ptext SLIT("at") <+> ppr (instLocSpan loc)) ] + + fixes2 | null instance_dicts = [] + | otherwise = [sep [ptext SLIT("add an instance declaration for"), + pprDictsTheta instance_dicts]] + instance_dicts = [d | d <- insts, isClassDict d, not (isTyVarDict d)] -- Insts for which it is worth suggesting an adding an instance declaration -- Exclude implicit parameters, and tyvar dicts - add_ors :: [SDoc] -> [SDoc] -- The empty case should not happen - add_ors [] = [ptext SLIT("[No suggested fixes]")] -- Strange - add_ors (f1:fs) = f1 : map (ptext SLIT("or") <+>) fs + show_fixes :: [SDoc] -> SDoc + show_fixes [] = empty + show_fixes (f:fs) = sep [ptext SLIT("Possible fix:"), + nest 2 (vcat (f : map (ptext SLIT("or") <+>) fs))] addTopAmbigErrs dicts -- Divide into groups that share a common set of ambiguous tyvars - = mapM report (equivClasses cmp [(d, tvs_of d) | d <- tidy_dicts]) + = ifErrsM (return ()) $ -- Only report ambiguity if no other errors happened + -- See Note [Avoiding spurious errors] + mapM_ report (equivClasses cmp [(d, tvs_of d) | d <- tidy_dicts]) where (tidy_env, tidy_dicts) = tidyInsts dicts @@ -2483,7 +2596,7 @@ addTopAmbigErrs dicts report :: [(Inst,[TcTyVar])] -> TcM () report pairs@((inst,tvs) : _) -- The pairs share a common set of ambiguous tyvars = mkMonomorphismMsg tidy_env tvs `thenM` \ (tidy_env, mono_msg) -> - setSrcSpan (instLocSrcSpan (instLoc inst)) $ + setSrcSpan (instSpan inst) $ -- the location of the first one will do for the err message addErrTcM (tidy_env, msg $$ mono_msg) where @@ -2492,6 +2605,7 @@ addTopAmbigErrs dicts pprQuotedList tvs <+> in_msg, nest 2 (pprDictsInFull dicts)] in_msg = text "in the constraint" <> plural dicts <> colon + report [] = panic "addTopAmbigErrs" mkMonomorphismMsg :: TidyEnv -> [TcTyVar] -> TcM (TidyEnv, Message) @@ -2506,7 +2620,7 @@ mkMonomorphismMsg tidy_env inst_tvs mk_msg [] = ptext SLIT("Probable fix: add a type signature that fixes these type variable(s)") -- This happens in things like -- f x = show (read "foo") - -- whre monomorphism doesn't play any role + -- where monomorphism doesn't play any role mk_msg docs = vcat [ptext SLIT("Possible cause: the monomorphism restriction applied to the following:"), nest 2 (vcat docs), monomorphism_fix @@ -2516,25 +2630,21 @@ monomorphism_fix = ptext SLIT("Probable fix:") <+> (ptext SLIT("give these definition(s) an explicit type signature") $$ ptext SLIT("or use -fno-monomorphism-restriction")) -warnDefault dicts default_ty +warnDefault ups default_ty = doptM Opt_WarnTypeDefaults `thenM` \ warn_flag -> - addInstCtxt (instLoc (head dicts)) (warnTc warn_flag warn_msg) + addInstCtxt (instLoc (head (dicts))) (warnTc warn_flag warn_msg) where + dicts = [d | (d,_,_) <- ups] + -- Tidy them first (_, tidy_dicts) = tidyInsts dicts warn_msg = vcat [ptext SLIT("Defaulting the following constraint(s) to type") <+> quotes (ppr default_ty), pprDictsInFull tidy_dicts] --- Used for the ...Thetas variants; all top level -badDerivedPred pred - = vcat [ptext SLIT("Can't derive instances where the instance context mentions"), - ptext SLIT("type variables that are not data type parameters"), - nest 2 (ptext SLIT("Offending constraint:") <+> ppr pred)] - reduceDepthErr n stack = vcat [ptext SLIT("Context reduction stack overflow; size =") <+> int n, - ptext SLIT("Use -fcontext-stack20 to increase stack size to (e.g.) 20"), + ptext SLIT("Use -fcontext-stack=N to increase stack size to N"), nest 4 (pprStack stack)] pprStack stack = vcat (map pprInstInFull stack)