X-Git-Url: http://git.megacz.com/?a=blobdiff_plain;f=ghc%2Fcompiler%2Ftypes%2FFunDeps.lhs;h=9347f5f665d7c04d7b6387001bae2d0213b3f419;hb=28a464a75e14cece5db40f2765a29348273ff2d2;hp=40e154f93340e1bdfd27eab7104db2b17f503ca0;hpb=788faebb40b51d37e73ed94dfc99460d39a1a811;p=ghc-hetmet.git diff --git a/ghc/compiler/types/FunDeps.lhs b/ghc/compiler/types/FunDeps.lhs index 40e154f..9347f5f 100644 --- a/ghc/compiler/types/FunDeps.lhs +++ b/ghc/compiler/types/FunDeps.lhs @@ -7,21 +7,29 @@ It's better to read it as: "if we know these, then we're going to know these" \begin{code} module FunDeps ( - oclose, grow, improve, checkInstFDs, checkClsFD, pprFundeps + Equation, pprEquation, + oclose, grow, improve, + checkInstCoverage, checkFunDeps, + pprFundeps ) where #include "HsVersions.h" +import Name ( Name, getSrcLoc ) import Var ( TyVar ) import Class ( Class, FunDep, classTvsFds ) -import Type ( Type, ThetaType, PredType(..), predTyUnique, tyVarsOfTypes, tyVarsOfPred ) -import Subst ( mkSubst, emptyInScopeSet, substTy ) -import Unify ( unifyTyListsX, unifyExtendTysX ) -import Outputable ( Outputable, SDoc, interppSP, ptext, empty, hsep, punctuate, comma ) +import Unify ( tcUnifyTys, BindFlag(..) ) +import Type ( substTys, notElemTvSubst ) +import TcType ( Type, PredType(..), tcEqType, + predTyUnique, mkClassPred, tyVarsOfTypes, tyVarsOfPred ) +import InstEnv ( Instance(..), InstEnv, instanceHead, classInstances, + instanceCantMatch, roughMatchTcs ) import VarSet import VarEnv +import Outputable +import Util ( notNull ) import List ( tails ) -import Maybes ( maybeToBool ) +import Maybe ( isJust ) import ListSetOps ( equivClassesByUniq ) \end{code} @@ -118,8 +126,8 @@ oclose preds fixed_tvs \begin{code} grow :: [PredType] -> TyVarSet -> TyVarSet grow preds fixed_tvs - | null pred_sets = fixed_tvs - | otherwise = loop fixed_tvs + | null preds = fixed_tvs + | otherwise = loop fixed_tvs where loop fixed_tvs | new_fixed_tvs `subVarSet` fixed_tvs = fixed_tvs @@ -143,29 +151,40 @@ grow preds fixed_tvs \begin{code} ---------- -type Equation = (TyVarSet, Type,Type) -- These two types should be equal, for some - -- substitution of the tyvars in the tyvar set - -- For example, ({a,b}, (a,Int,b), (Int,z,Bool)) - -- We unify z with Int, but since a and b are quantified we do nothing to them - -- We usually act on an equation by instantiating the quantified type varaibles - -- to fresh type variables, and then calling the standard unifier. - -- - -- INVARIANT: they aren't already equal - - +type Equation = (TyVarSet, [(Type, Type)]) +-- These pairs of types should be equal, for some +-- substitution of the tyvars in the tyvar set +-- INVARIANT: corresponding types aren't already equal + +-- It's important that we have a *list* of pairs of types. Consider +-- class C a b c | a -> b c where ... +-- instance C Int x x where ... +-- Then, given the constraint (C Int Bool v) we should improve v to Bool, +-- via the equation ({x}, [(Bool,x), (v,x)]) +-- This would not happen if the class had looked like +-- class C a b c | a -> b, a -> c + +-- To "execute" the equation, make fresh type variable for each tyvar in the set, +-- instantiate the two types with these fresh variables, and then unify. +-- +-- For example, ({a,b}, (a,Int,b), (Int,z,Bool)) +-- We unify z with Int, but since a and b are quantified we do nothing to them +-- We usually act on an equation by instantiating the quantified type varaibles +-- to fresh type variables, and then calling the standard unifier. + +pprEquation (qtvs, pairs) + = vcat [ptext SLIT("forall") <+> braces (pprWithCommas ppr (varSetElems qtvs)), + nest 2 (vcat [ ppr t1 <+> ptext SLIT(":=:") <+> ppr t2 | (t1,t2) <- pairs])] ---------- -improve :: InstEnv a -- Gives instances for given class - -> [PredType] -- Current constraints - -> [Equation] -- Derived equalities that must also hold - -- (NB the above INVARIANT for type Equation) - -type InstEnv a = Class -> [(TyVarSet, [Type], a)] --- This is a bit clumsy, because InstEnv is really --- defined in module InstEnv. However, we don't want --- to define it (and ClsInstEnv) here because InstEnv --- is their home. Nor do we want to make a recursive --- module group (InstEnv imports stuff from FunDeps). +type Pred_Loc = (PredType, SDoc) -- SDoc says where the Pred comes from + +improve :: (Class -> [Instance]) -- Gives instances for given class + -> [Pred_Loc] -- Current constraints; + -> [(Equation,Pred_Loc,Pred_Loc)] -- Derived equalities that must also hold + -- (NB the above INVARIANT for type Equation) + -- The Pred_Locs explain which two predicates were + -- combined (for error messages) \end{code} Given a bunch of predicates that must hold, such as @@ -199,18 +218,21 @@ NOTA BENE: \begin{code} improve inst_env preds - = [ eqn | group <- equivClassesByUniq predTyUnique preds, + = [ eqn | group <- equivClassesByUniq (predTyUnique . fst) preds, eqn <- checkGroup inst_env group ] ---------- -checkGroup :: InstEnv a -> [PredType] -> [Equation] +checkGroup :: (Class -> [Instance]) + -> [Pred_Loc] + -> [(Equation, Pred_Loc, Pred_Loc)] -- The preds are all for the same class or implicit param -checkGroup inst_env (IParam _ ty : ips) +checkGroup inst_env (p1@(IParam _ ty, _) : ips) = -- For implicit parameters, all the types must match - [(emptyVarSet, ty, ty') | IParam _ ty' <- ips, ty /= ty'] + [ ((emptyVarSet, [(ty,ty')]), p1, p2) + | p2@(IParam _ ty', _) <- ips, not (ty `tcEqType` ty')] -checkGroup inst_env clss@(ClassP cls tys : _) +checkGroup inst_env clss@((ClassP cls _, _) : _) = -- For classes life is more complicated -- Suppose the class is like -- classs C as | (l1 -> r1), (l2 -> r2), ... where ... @@ -224,54 +246,113 @@ checkGroup inst_env clss@(ClassP cls tys : _) -- -- We need to do something very similar comparing each predicate -- with relevant instance decls - pairwise_eqns ++ instance_eqns + + instance_eqns ++ pairwise_eqns + -- NB: we put the instance equations first. This biases the + -- order so that we first improve individual constraints against the + -- instances (which are perhaps in a library and less likely to be + -- wrong; and THEN perform the pairwise checks. + -- The other way round, it's possible for the pairwise check to succeed + -- and cause a subsequent, misleading failure of one of the pair with an + -- instance declaration. See tcfail143.hs for an exmample where (cls_tvs, cls_fds) = classTvsFds cls - cls_inst_env = inst_env cls + instances = inst_env cls -- NOTE that we iterate over the fds first; they are typically -- empty, which aborts the rest of the loop. - pairwise_eqns :: [Equation] + pairwise_eqns :: [(Equation,Pred_Loc,Pred_Loc)] pairwise_eqns -- This group comes from pairwise comparison - = [ eqn | fd <- cls_fds, - ClassP _ tys1 : rest <- tails clss, - ClassP _ tys2 <- rest, - eqn <- checkClsFD emptyVarSet fd cls_tvs tys1 tys2 + = [ (eqn, p1, p2) + | fd <- cls_fds, + p1@(ClassP _ tys1, _) : rest <- tails clss, + p2@(ClassP _ tys2, _) <- rest, + eqn <- checkClsFD emptyVarSet fd cls_tvs tys1 tys2 ] - instance_eqns :: [Equation] + instance_eqns :: [(Equation,Pred_Loc,Pred_Loc)] instance_eqns -- This group comes from comparing with instance decls - = [ eqn | fd <- cls_fds, - (qtvs, tys1, _) <- cls_inst_env, - ClassP _ tys2 <- clss, - eqn <- checkClsFD qtvs fd cls_tvs tys1 tys2 + = [ (eqn, p1, p2) + | fd <- cls_fds, -- Iterate through the fundeps first, + -- because there often are none! + p2@(ClassP _ tys2, _) <- clss, + let rough_tcs2 = trimRoughMatchTcs cls_tvs fd (roughMatchTcs tys2), + ispec@(Instance { is_tvs = qtvs, is_tys = tys1, + is_tcs = mb_tcs1 }) <- instances, + not (instanceCantMatch mb_tcs1 rough_tcs2), + eqn <- checkClsFD qtvs fd cls_tvs tys1 tys2, + let p1 = (mkClassPred cls tys1, + ptext SLIT("arising from the instance declaration at") <+> + ppr (getSrcLoc ispec)) ] - - ---------- -checkClsFD :: TyVarSet -- The quantified type variables, which - -- can be instantiated to make the types match +checkClsFD :: TyVarSet -- Quantified type variables; see note below -> FunDep TyVar -> [TyVar] -- One functional dependency from the class -> [Type] -> [Type] -> [Equation] checkClsFD qtvs fd clas_tvs tys1 tys2 --- We use 'unify' even though we are often only matching --- unifyTyListsX will only bind variables in qtvs, so it's OK! - = case unifyTyListsX qtvs ls1 ls2 of - Nothing -> [] - Just unif -> [ (qtvs', substTy full_unif r1, substTy full_unif r2) - | (r1,r2) <- rs1 `zip` rs2, - not (maybeToBool (unifyExtendTysX qtvs unif r1 r2))] +-- 'qtvs' are the quantified type variables, the ones which an be instantiated +-- to make the types match. For example, given +-- class C a b | a->b where ... +-- instance C (Maybe x) (Tree x) where .. +-- +-- and an Inst of form (C (Maybe t1) t2), +-- then we will call checkClsFD with +-- +-- qtvs = {x}, tys1 = [Maybe x, Tree x] +-- tys2 = [Maybe t1, t2] +-- +-- We can instantiate x to t1, and then we want to force +-- (Tree x) [t1/x] :=: t2 +-- +-- This function is also used when matching two Insts (rather than an Inst +-- against an instance decl. In that case, qtvs is empty, and we are doing +-- an equality check +-- +-- This function is also used by InstEnv.badFunDeps, which needs to *unify* +-- For the one-sided matching case, the qtvs are just from the template, +-- so we get matching +-- + = ASSERT2( length tys1 == length tys2 && + length tys1 == length clas_tvs + , ppr tys1 <+> ppr tys2 ) + + case tcUnifyTys bind_fn ls1 ls2 of + Nothing -> [] + Just subst | isJust (tcUnifyTys bind_fn rs1' rs2') + -- Don't include any equations that already hold. + -- Reason: then we know if any actual improvement has happened, + -- in which case we need to iterate the solver + -- In making this check we must taking account of the fact that any + -- qtvs that aren't already instantiated can be instantiated to anything + -- at all + -> [] + + | otherwise -- Aha! A useful equation + -> [ (qtvs', zip rs1' rs2')] + -- We could avoid this substTy stuff by producing the eqn + -- (qtvs, ls1++rs1, ls2++rs2) + -- which will re-do the ls1/ls2 unification when the equation is + -- executed. What we're doing instead is recording the partial + -- work of the ls1/ls2 unification leaving a smaller unification problem where - full_unif = mkSubst emptyInScopeSet unif - -- No for-alls in sight; hmm - - qtvs' = filterVarSet (\v -> not (v `elemSubstEnv` unif)) qtvs + rs1' = substTys subst rs1 + rs2' = substTys subst rs2 + qtvs' = filterVarSet (`notElemTvSubst` subst) qtvs -- qtvs' are the quantified type variables -- that have not been substituted out + -- + -- Eg. class C a b | a -> b + -- instance C Int [y] + -- Given constraint C Int z + -- we generate the equation + -- ({y}, [y], z) where + bind_fn tv | tv `elemVarSet` qtvs = BindMe + | otherwise = Skolem + (ls1, rs1) = instFD fd clas_tvs tys1 (ls2, rs2) = instFD fd clas_tvs tys2 @@ -284,22 +365,125 @@ instFD (ls,rs) tvs tys \end{code} \begin{code} -checkInstFDs :: ThetaType -> Class -> [Type] -> Bool --- Check that functional dependencies are obeyed in an instance decl +checkInstCoverage :: Class -> [Type] -> Bool +-- Check that the Coverage Condition is obeyed in an instance decl -- For example, if we have -- class theta => C a b | a -> b -- instance C t1 t2 --- Then we require fv(t2) `subset` oclose(fv(t1), theta) +-- Then we require fv(t2) `subset` fv(t1) +-- See Note [Coverage Condition] below -checkInstFDs theta clas inst_taus +checkInstCoverage clas inst_taus = all fundep_ok fds where (tyvars, fds) = classTvsFds clas - fundep_ok fd = tyVarsOfTypes rs `subVarSet` oclose theta (tyVarsOfTypes ls) + fundep_ok fd = tyVarsOfTypes rs `subVarSet` tyVarsOfTypes ls where (ls,rs) = instFD fd tyvars inst_taus \end{code} +Note [Coverage condition] +~~~~~~~~~~~~~~~~~~~~~~~~~ +For the coverage condition, we used to require only that + fv(t2) `subset` oclose(fv(t1), theta) + +Example: + class Mul a b c | a b -> c where + (.*.) :: a -> b -> c + + instance Mul Int Int Int where (.*.) = (*) + instance Mul Int Float Float where x .*. y = fromIntegral x * y + instance Mul a b c => Mul a [b] [c] where x .*. v = map (x.*.) v + +In the third instance, it's not the case that fv([c]) `subset` fv(a,[b]). +But it is the case that fv([c]) `subset` oclose( theta, fv(a,[b]) ) + +But it is a mistake to accept the instance because then this defn: + f = \ b x y -> if b then x .*. [y] else y +makes instance inference go into a loop, because it requires the constraint + Mul a [b] b + + +%************************************************************************ +%* * + Check that a new instance decl is OK wrt fundeps +%* * +%************************************************************************ + +Here is the bad case: + class C a b | a->b where ... + instance C Int Bool where ... + instance C Int Char where ... + +The point is that a->b, so Int in the first parameter must uniquely +determine the second. In general, given the same class decl, and given + + instance C s1 s2 where ... + instance C t1 t2 where ... + +Then the criterion is: if U=unify(s1,t1) then U(s2) = U(t2). + +Matters are a little more complicated if there are free variables in +the s2/t2. + + class D a b c | a -> b + instance D a b => D [(a,a)] [b] Int + instance D a b => D [a] [b] Bool + +The instance decls don't overlap, because the third parameter keeps +them separate. But we want to make sure that given any constraint + D s1 s2 s3 +if s1 matches + + +\begin{code} +checkFunDeps :: (InstEnv, InstEnv) -> Instance + -> Maybe [Instance] -- Nothing <=> ok + -- Just dfs <=> conflict with dfs +-- Check wheher adding DFunId would break functional-dependency constraints +-- Used only for instance decls defined in the module being compiled +checkFunDeps inst_envs ispec + | null bad_fundeps = Nothing + | otherwise = Just bad_fundeps + where + (ins_tvs, _, clas, ins_tys) = instanceHead ispec + ins_tv_set = mkVarSet ins_tvs + cls_inst_env = classInstances inst_envs clas + bad_fundeps = badFunDeps cls_inst_env clas ins_tv_set ins_tys + +badFunDeps :: [Instance] -> Class + -> TyVarSet -> [Type] -- Proposed new instance type + -> [Instance] +badFunDeps cls_insts clas ins_tv_set ins_tys + = [ ispec | fd <- fds, -- fds is often empty + let trimmed_tcs = trimRoughMatchTcs clas_tvs fd rough_tcs, + ispec@(Instance { is_tcs = mb_tcs, is_tvs = tvs, + is_tys = tys }) <- cls_insts, + -- Filter out ones that can't possibly match, + -- based on the head of the fundep + not (instanceCantMatch trimmed_tcs mb_tcs), + notNull (checkClsFD (tvs `unionVarSet` ins_tv_set) + fd clas_tvs tys ins_tys) + ] + where + (clas_tvs, fds) = classTvsFds clas + rough_tcs = roughMatchTcs ins_tys + +trimRoughMatchTcs :: [TyVar] -> FunDep TyVar -> [Maybe Name] -> [Maybe Name] +-- Computing rough_tcs for a particular fundep +-- class C a b c | a c -> b where ... +-- For each instance .... => C ta tb tc +-- we want to match only on the types ta, tb; so our +-- rough-match thing must similarly be filtered. +-- Hence, we Nothing-ise the tb type right here +trimRoughMatchTcs clas_tvs (ltvs,_) mb_tcs + = zipWith select clas_tvs mb_tcs + where + select clas_tv mb_tc | clas_tv `elem` ltvs = mb_tc + | otherwise = Nothing +\end{code} + + %************************************************************************ %* * \subsection{Miscellaneous} @@ -313,3 +497,4 @@ pprFundeps fds = hsep (ptext SLIT("|") : punctuate comma (map ppr_fd fds)) ppr_fd (us, vs) = hsep [interppSP us, ptext SLIT("->"), interppSP vs] \end{code} +