X-Git-Url: http://git.megacz.com/?a=blobdiff_plain;f=ghc%2Fcompiler%2Ftypes%2FFunDeps.lhs;fp=ghc%2Fcompiler%2Ftypes%2FFunDeps.lhs;h=0000000000000000000000000000000000000000;hb=0065d5ab628975892cea1ec7303f968c3338cbe1;hp=9347f5f665d7c04d7b6387001bae2d0213b3f419;hpb=28a464a75e14cece5db40f2765a29348273ff2d2;p=ghc-hetmet.git diff --git a/ghc/compiler/types/FunDeps.lhs b/ghc/compiler/types/FunDeps.lhs deleted file mode 100644 index 9347f5f..0000000 --- a/ghc/compiler/types/FunDeps.lhs +++ /dev/null @@ -1,500 +0,0 @@ -% -% (c) The GRASP/AQUA Project, Glasgow University, 2000 -% -\section[FunDeps]{FunDeps - functional dependencies} - -It's better to read it as: "if we know these, then we're going to know these" - -\begin{code} -module FunDeps ( - 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 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 Maybe ( isJust ) -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 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])] - ----------- -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 - - 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 :: (Class -> [Instance]) - -> [Pred_Loc] - -> [(Equation, Pred_Loc, Pred_Loc)] - -- 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')]), 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 - - 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 - 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,Pred_Loc,Pred_Loc)] - pairwise_eqns -- This group comes from pairwise comparison - = [ (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,Pred_Loc,Pred_Loc)] - instance_eqns -- This group comes from comparing with instance decls - = [ (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 -- 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 --- --- 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 - 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 - -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} -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` fv(t1) --- See Note [Coverage Condition] below - -checkInstCoverage clas inst_taus - = all fundep_ok fds - where - (tyvars, fds) = classTvsFds clas - 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} -%* * -%************************************************************************ - -\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} -