X-Git-Url: http://git.megacz.com/?a=blobdiff_plain;f=ghc%2Fcompiler%2Ftypes%2FFunDeps.lhs;h=79f62fb4c7cb78229f7cf8c74e952c2791e22fd1;hb=9af77fa423926fbda946b31e174173d0ec5ebac8;hp=c1db64ebab408d5852dcf4702b0080b4c9fed543;hpb=7007351bb709611fbb259aae2eb286d107355486;p=ghc-hetmet.git diff --git a/ghc/compiler/types/FunDeps.lhs b/ghc/compiler/types/FunDeps.lhs index c1db64e..79f62fb 100644 --- a/ghc/compiler/types/FunDeps.lhs +++ b/ghc/compiler/types/FunDeps.lhs @@ -7,67 +7,349 @@ It's better to read it as: "if we know these, then we're going to know these" \begin{code} module FunDeps ( - oclose, - instantiateFdClassTys, - tyVarFunDep, - pprFundeps + Equation, pprEquation, pprEquationDoc, + oclose, grow, improve, checkInstFDs, checkClsFD, pprFundeps ) where #include "HsVersions.h" -import Class ( classTvsFds ) -import Type ( tyVarsOfType ) -import Outputable ( interppSP, ptext, empty, hsep, punctuate, comma ) -import UniqSet ( elementOfUniqSet, addOneToUniqSet, - uniqSetToList, unionManyUniqSets ) -import List ( elemIndex ) +import Name ( getSrcLoc ) +import Var ( Id, TyVar ) +import Class ( Class, FunDep, classTvsFds ) +import Subst ( mkSubst, emptyInScopeSet, substTy ) +import TcType ( Type, ThetaType, SourceType(..), PredType, + predTyUnique, mkClassPred, tyVarsOfTypes, tyVarsOfPred, + unifyTyListsX, unifyExtendTysX, tcEqType + ) +import PprType ( ) +import VarSet +import VarEnv +import Outputable +import List ( tails ) +import Maybes ( maybeToBool ) +import ListSetOps ( equivClassesByUniq ) \end{code} +%************************************************************************ +%* * +\subsection{Close type variables} +%* * +%************************************************************************ + +(oclose preds tvs) closes the set of type variables tvs, +wrt functional dependencies in preds. The result is a superset +of the argument set. For example, if we have + class C a b | a->b where ... +then + oclose [C (x,y) z, C (x,p) q] {x,y} = {x,y,z} +because if we know x and y then that fixes z. + +Using oclose +~~~~~~~~~~~~ +oclose is used + +a) When determining ambiguity. The type + forall a,b. C a b => a +is not ambiguous (given the above class decl for C) because +a determines b. + +b) When generalising a type T. Usually we take FV(T) \ FV(Env), +but in fact we need + FV(T) \ (FV(Env)+) +where the '+' is the oclosure operation. Notice that we do not +take FV(T)+. This puzzled me for a bit. Consider + + f = E + +and suppose e have that E :: C a b => a, and suppose that b is +free in the environment. Then we quantify over 'a' only, giving +the type forall a. C a b => a. Since a->b but we don't have b->a, +we might have instance decls like + instance C Bool Int where ... + instance C Char Int where ... +so knowing that b=Int doesn't fix 'a'; so we quantify over it. + + --------------- + A WORRY: ToDo! + --------------- +If we have class C a b => D a b where .... + class D a b | a -> b where ... +and the preds are [C (x,y) z], then we want to see the fd in D, +even though it is not explicit in C, giving [({x,y},{z})] + +Similarly for instance decls? E.g. Suppose we have + instance C a b => Eq (T a b) where ... +and we infer a type t with constraints Eq (T a b) for a particular +expression, and suppose that 'a' is free in the environment. +We could generalise to + forall b. Eq (T a b) => t +but if we reduced the constraint, to C a b, we'd see that 'a' determines +b, so that a better type might be + t (with free constraint C a b) +Perhaps it doesn't matter, because we'll still force b to be a +particular type at the call sites. Generalising over too many +variables (provided we don't shadow anything by quantifying over a +variable that is actually free in the envt) may postpone errors; it +won't hide them altogether. + + +\begin{code} +oclose :: [PredType] -> TyVarSet -> TyVarSet +oclose preds fixed_tvs + | null tv_fds = fixed_tvs -- Fast escape hatch for common case + | otherwise = loop fixed_tvs + where + loop fixed_tvs + | new_fixed_tvs `subVarSet` fixed_tvs = fixed_tvs + | otherwise = loop new_fixed_tvs + where + new_fixed_tvs = foldl extend fixed_tvs tv_fds + + extend fixed_tvs (ls,rs) | ls `subVarSet` fixed_tvs = fixed_tvs `unionVarSet` rs + | otherwise = fixed_tvs + + tv_fds :: [(TyVarSet,TyVarSet)] + -- In our example, tv_fds will be [ ({x,y}, {z}), ({x,p},{q}) ] + -- Meaning "knowing x,y fixes z, knowing x,p fixes q" + tv_fds = [ (tyVarsOfTypes xs, tyVarsOfTypes ys) + | ClassP cls tys <- preds, -- Ignore implicit params + let (cls_tvs, cls_fds) = classTvsFds cls, + fd <- cls_fds, + let (xs,ys) = instFD fd cls_tvs tys + ] +\end{code} + +\begin{code} +grow :: [PredType] -> TyVarSet -> TyVarSet +grow preds fixed_tvs + | null preds = fixed_tvs + | otherwise = loop fixed_tvs + where + loop fixed_tvs + | new_fixed_tvs `subVarSet` fixed_tvs = fixed_tvs + | otherwise = loop new_fixed_tvs + where + new_fixed_tvs = foldl extend fixed_tvs pred_sets + + extend fixed_tvs pred_tvs + | fixed_tvs `intersectsVarSet` pred_tvs = fixed_tvs `unionVarSet` pred_tvs + | otherwise = fixed_tvs + + pred_sets = [tyVarsOfPred pred | pred <- preds] +\end{code} + +%************************************************************************ +%* * +\subsection{Generate equations from functional dependencies} +%* * +%************************************************************************ + + +\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 + -- + + +pprEquationDoc (eqn, doc) = vcat [pprEquation eqn, nest 2 doc] + +pprEquation (qtvs, t1, t2) = ptext SLIT("forall") <+> braces (pprWithCommas ppr (varSetElems qtvs)) + <+> ppr t1 <+> ptext SLIT(":=:") <+> ppr t2 + +---------- +improve :: InstEnv Id -- Gives instances for given class + -> [(PredType,SDoc)] -- Current constraints; doc says where they come from + -> [(Equation,SDoc)] -- Derived equalities that must also hold + -- (NB the above INVARIANT for type Equation) + -- The SDoc explains why the equation holds (for error messages) + +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). +\end{code} + +Given a bunch of predicates that must hold, such as + + C Int t1, C Int t2, C Bool t3, ?x::t4, ?x::t5 + +improve figures out what extra equations must hold. +For example, if we have + + class C a b | a->b where ... + +then improve will return + + [(t1,t2), (t4,t5)] + +NOTA BENE: + + * improve does not iterate. It's possible that when we make + t1=t2, for example, that will in turn trigger a new equation. + This would happen if we also had + C t1 t7, C t2 t8 + If t1=t2, we also get t7=t8. + + improve does *not* do this extra step. It relies on the caller + doing so. + + * The equations unify types that are not already equal. So there + is no effect iff the result of improve is empty + + + +\begin{code} +improve inst_env preds + = [ eqn | group <- equivClassesByUniq (predTyUnique . fst) preds, + eqn <- checkGroup inst_env group ] + +---------- +checkGroup :: InstEnv Id -> [(PredType,SDoc)] -> [(Equation, SDoc)] + -- The preds are all for the same class or implicit param + +checkGroup inst_env (p1@(IParam _ ty, _) : ips) + = -- For implicit parameters, all the types must match + [ ((emptyVarSet, ty, ty'), mkEqnMsg p1 p2) + | p2@(IParam _ ty', _) <- ips, not (ty `tcEqType` ty')] + +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 ... + -- Then FOR EACH PAIR (ClassP c tys1, ClassP c tys2) in the list clss + -- we check whether + -- U l1[tys1/as] = U l2[tys2/as] + -- (where U is a unifier) + -- + -- If so, we return the pair + -- U r1[tys1/as] = U l2[tys2/as] + -- + -- We need to do something very similar comparing each predicate + -- with relevant instance decls + pairwise_eqns ++ instance_eqns + + where + (cls_tvs, cls_fds) = classTvsFds cls + cls_inst_env = 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,SDoc)] + pairwise_eqns -- This group comes from pairwise comparison + = [ (eqn, mkEqnMsg 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,SDoc)] + instance_eqns -- This group comes from comparing with instance decls + = [ (eqn, mkEqnMsg p1 p2) + | fd <- cls_fds, + (qtvs, tys1, dfun_id) <- cls_inst_env, + let p1 = (mkClassPred cls tys1, + ptext SLIT("arising from the instance declaration at") <+> ppr (getSrcLoc dfun_id)), + p2@(ClassP _ tys2, _) <- clss, + eqn <- checkClsFD qtvs fd cls_tvs tys1 tys2 + ] + +mkEqnMsg (pred1,from1) (pred2,from2) + = vcat [ptext SLIT("When using functional dependencies to combine"), + nest 2 (sep [ppr pred1 <> comma, nest 2 from1]), + nest 2 (sep [ppr pred2 <> comma, nest 2 from2])] + +---------- +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 +-- '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 + +-- 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 -> -- pprTrace "checkFD" (vcat [ppr_fd fd, + -- ppr (varSetElems qtvs) <+> (ppr ls1 $$ ppr ls2), + -- ppr unif]) $ + [ (qtvs', substTy full_unif r1, substTy full_unif r2) + | (r1,r2) <- rs1 `zip` rs2, + not (maybeToBool (unifyExtendTysX qtvs unif r1 r2))] + -- Don't include any equations that already hold + -- taking account of the fact that any qtvs that aren't + -- already instantiated can be instantiated to anything at all + -- NB: qtvs, not qtvs' because unifyExtendTysX only tries to + -- look template tyvars up in the substitution + where + full_unif = mkSubst emptyInScopeSet unif + -- No for-alls in sight; hmm + + qtvs' = filterVarSet (\v -> not (v `elemSubstEnv` unif)) qtvs + -- qtvs' are the quantified type variables + -- that have not been substituted out + where + (ls1, rs1) = instFD fd clas_tvs tys1 + (ls2, rs2) = instFD fd clas_tvs tys2 + +instFD :: FunDep TyVar -> [TyVar] -> [Type] -> FunDep Type +instFD (ls,rs) tvs tys + = (map lookup ls, map lookup rs) + where + env = zipVarEnv tvs tys + lookup tv = lookupVarEnv_NF env tv +\end{code} + \begin{code} -oclose fds vs = - case oclose1 fds vs of - (vs', False) -> vs' - (vs', True) -> oclose fds vs' - -oclose1 [] vs = (vs, False) -oclose1 (fd@(ls, rs):fds) vs = - if osubset ls vs then - (vs'', b1 || b2) - else - vs'b1 - where - vs'b1@(vs', b1) = oclose1 fds vs - (vs'', b2) = ounion rs vs' - -osubset [] vs = True -osubset (u:us) vs = if u `elementOfUniqSet` vs then osubset us vs else False - -ounion [] ys = (ys, False) -ounion (x:xs) ys = - if x `elementOfUniqSet` ys then (ys', b) else (addOneToUniqSet ys' x, True) - where - (ys', b) = ounion xs ys - -instantiateFdClassTys clas ts = - map (lookupInstFundep tyvars ts) fundeps - where - (tyvars, fundeps) = classTvsFds clas - lookupInstFundep tyvars ts (us, vs) = - (lookupInstTys tyvars ts us, lookupInstTys tyvars ts vs) -lookupInstTys tyvars ts = map (lookupInstTy tyvars ts) -lookupInstTy tyvars ts u = ts !! i - where Just i = elemIndex u tyvars - -tyVarFunDep fdtys = - map (\(xs, ys) -> (unionMap getTyVars xs, unionMap getTyVars ys)) fdtys - where - getTyVars ty = tyVarsOfType ty - unionMap f xs = uniqSetToList (unionManyUniqSets (map f xs)) +checkInstFDs :: ThetaType -> Class -> [Type] -> Bool +-- Check that functional dependencies are 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) + +checkInstFDs theta clas inst_taus + = all fundep_ok fds + where + (tyvars, fds) = classTvsFds clas + fundep_ok fd = tyVarsOfTypes rs `subVarSet` oclose theta (tyVarsOfTypes ls) + where + (ls,rs) = instFD fd tyvars inst_taus +\end{code} +%************************************************************************ +%* * +\subsection{Miscellaneous} +%* * +%************************************************************************ + +\begin{code} +pprFundeps :: Outputable a => [FunDep a] -> SDoc pprFundeps [] = empty pprFundeps fds = hsep (ptext SLIT("|") : punctuate comma (map ppr_fd fds)) ppr_fd (us, vs) = hsep [interppSP us, ptext SLIT("->"), interppSP vs] - \end{code}