X-Git-Url: http://git.megacz.com/?p=ghc-hetmet.git;a=blobdiff_plain;f=compiler%2Ftypes%2FFunDeps.lhs;h=6ce932bfe356a0c902d23485add4387bb4ef85fb;hp=511472c66c7b5f823e9012f9898402e00c8ac4d4;hb=50d0293555691012f96259de7f8682b94db58517;hpb=53da379cee909d23b9f785c2250e64cba34ad3b2 diff --git a/compiler/types/FunDeps.lhs b/compiler/types/FunDeps.lhs index 511472c..6ce932b 100644 --- a/compiler/types/FunDeps.lhs +++ b/compiler/types/FunDeps.lhs @@ -9,7 +9,8 @@ It's better to read it as: "if we know these, then we're going to know these" \begin{code} module FunDeps ( - Equation, pprEquation, + FDEq (..), + Equation(..), pprEquation, oclose, improveFromInstEnv, improveFromAnother, checkInstCoverage, checkFunDeps, pprFundeps @@ -140,32 +141,67 @@ oclose preds fixed_tvs %************************************************************************ +Each functional dependency with one variable in the RHS is responsible +for generating a single equality. For instance: + class C a b | a -> b +The constraints ([Wanted] C Int Bool) and [Wanted] C Int alpha + FDEq { fd_pos = 1 + , fd_ty_left = Bool + , fd_ty_right = alpha } +However notice that a functional dependency may have more than one variable +in the RHS which will create more than one FDEq. Example: + class C a b c | a -> b c + [Wanted] C Int alpha alpha + [Wanted] C Int Bool beta +Will generate: + fd1 = FDEq { fd_pos = 1, fd_ty_left = alpha, fd_ty_right = Bool } and + fd2 = FDEq { fd_pos = 2, fd_ty_left = alpha, fd_ty_right = beta } + +We record the paremeter position so that can immediately rewrite a constraint +using the produced FDEqs and remove it from our worklist. + + +INVARIANT: Corresponding types aren't already equal +That is, there exists at least one non-identity equality in FDEqs. + +Assume: + class C a b c | a -> b c + instance C Int x x +And: [Wanted] C Int Bool alpha +We will /match/ the LHS of fundep equations, producing a matching substitution +and create equations for the RHS sides. In our last example we'd have generated: + ({x}, [fd1,fd2]) +where + fd1 = FDEq 1 Bool x + fd2 = FDEq 2 alpha x +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 or generate +a new constraint. In the above example we would generate a new unification +variable 'beta' for x and produce the following constraints: + [Wanted] (Bool ~ beta) + [Wanted] (alpha ~ beta) + +Notice the subtle difference between the above class declaration and: + class C a b c | a -> b, a -> c +where we would generate: + ({x},[fd1]),({x},[fd2]) +This means that the template variable would be instantiated to different +unification variables when producing the FD constraints. + +Finally, the position parameters will help us rewrite the wanted constraint ``on the spot'' + \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. +type Pred_Loc = (PredType, SDoc) -- SDoc says where the Pred comes from -pprEquation :: Equation -> SDoc -pprEquation (qtvs, pairs) - = vcat [ptext (sLit "forall") <+> braces (pprWithCommas ppr (varSetElems qtvs)), - nest 2 (vcat [ ppr t1 <+> ptext (sLit "~") <+> ppr t2 | (t1,t2) <- pairs])] +data Equation + = FDEqn { fd_qtvs :: TyVarSet -- Instantiate these to fresh unification vars + , fd_eqs :: [FDEq] -- and then make these equal + , fd_pred1, fd_pred2 :: Pred_Loc } -- The Equation arose from + -- combining these two constraints + +data FDEq = FDEq { fd_pos :: Int -- We use '0' for the first position + , fd_ty_left :: Type + , fd_ty_right :: Type } \end{code} Given a bunch of predicates that must hold, such as @@ -198,93 +234,97 @@ NOTA BENE: \begin{code} -type Pred_Loc = (PredType, SDoc) -- SDoc says where the Pred comes from +instFD_WithPos :: FunDep TyVar -> [TyVar] -> [Type] -> ([Type], [(Int,Type)]) +-- Returns a FunDep between the types accompanied along with their +-- position (<=0) in the types argument list. +instFD_WithPos (ls,rs) tvs tys + = (map (snd . lookup) ls, map lookup rs) + where + ind_tys = zip [0..] tys + env = zipVarEnv tvs ind_tys + lookup tv = lookupVarEnv_NF env tv -improveFromInstEnv :: (Class -> [Instance]) - -> Pred_Loc - -> [(Equation,Pred_Loc,Pred_Loc)] --- Improvement from top-level instances -improveFromInstEnv _inst_env pred - = improveOne _inst_env pred [] -- TODO: Refactor to directly use instance_eqnd? +zipAndComputeFDEqs :: (Type -> Type -> Bool) -- Discard this FDEq if true + -> [Type] + -> [(Int,Type)] + -> [FDEq] +-- Create a list of FDEqs from two lists of types, making sure +-- that the types are not equal. +zipAndComputeFDEqs discard (ty1:tys1) ((i2,ty2):tys2) + | discard ty1 ty2 = zipAndComputeFDEqs discard tys1 tys2 + | otherwise = FDEq { fd_pos = i2 + , fd_ty_left = ty1 + , fd_ty_right = ty2 } : zipAndComputeFDEqs discard tys1 tys2 +zipAndComputeFDEqs _ _ _ = [] + +-- Improve a class constraint from another class constraint +-- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +improveFromAnother :: Pred_Loc -- Template item (usually given, or inert) + -> Pred_Loc -- Workitem [that can be improved] + -> [Equation] +-- Post: FDEqs always oriented from the other to the workitem +-- Equations have empty quantified variables +improveFromAnother pred1@(ClassP cls1 tys1, _) pred2@(ClassP cls2 tys2, _) + | tys1 `lengthAtLeast` 2 && cls1 == cls2 + = [ FDEqn { fd_qtvs = emptyVarSet, fd_eqs = eqs, fd_pred1 = pred1, fd_pred2 = pred2 } + | let (cls_tvs, cls_fds) = classTvsFds cls1 + , fd <- cls_fds + , let (ltys1, rs1) = instFD fd cls_tvs tys1 + (ltys2, irs2) = instFD_WithPos fd cls_tvs tys2 + , tcEqTypes ltys1 ltys2 -- The LHSs match + , let eqs = zipAndComputeFDEqs tcEqType rs1 irs2 + , not (null eqs) ] + +improveFromAnother _ _ = [] + + +-- Improve a class constraint from instance declarations +-- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +pprEquation :: Equation -> SDoc +pprEquation (FDEqn { fd_qtvs = qtvs, fd_eqs = pairs }) + = vcat [ptext (sLit "forall") <+> braces (pprWithCommas ppr (varSetElems qtvs)), + nest 2 (vcat [ ppr t1 <+> ptext (sLit "~") <+> ppr t2 | (FDEq _ t1 t2) <- pairs])] -improveFromAnother :: Pred_Loc +improveFromInstEnv :: (InstEnv,InstEnv) -> Pred_Loc - -> [(Equation, Pred_Loc, Pred_Loc)] --- Improvement from another local (given or wanted) constraint -improveFromAnother pred1 pred2 - = improveOne (\_ -> []) pred1 [pred2] -- TODO: Refactor to directly use pairwise_eqns? - - -improveOne :: (Class -> [Instance]) -- Gives instances for given class - -> Pred_Loc -- Do improvement triggered by this - -> [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) --- Just do improvement triggered by a single, distinguised predicate - -improveOne _inst_env pred@(IParam ip ty, _) preds - = [ ((emptyVarSet, [(ty,ty2)]), pred, p2) - | p2@(IParam ip2 ty2, _) <- preds - , ip==ip2 - , not (ty `tcEqType` ty2)] - -improveOne inst_env pred@(ClassP cls tys, _) preds + -> [Equation] -- Needs to be an Equation because + -- of quantified variables +-- Post: Equations oriented from the template (matching instance) to the workitem! +improveFromInstEnv _inst_env (pred,_loc) + | not (isClassPred pred) + = panic "improveFromInstEnv: not a class predicate" +improveFromInstEnv inst_env pred@(ClassP cls tys, _) | tys `lengthAtLeast` 2 - = 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 example - where - (cls_tvs, cls_fds) = classTvsFds cls - instances = inst_env cls - rough_tcs = roughMatchTcs tys - - -- 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, pred, p2) - | fd <- cls_fds - , p2@(ClassP cls2 tys2, _) <- preds - , cls == cls2 - , eqn <- checkClsFD emptyVarSet fd cls_tvs tys tys2 - ] - - instance_eqns :: [(Equation,Pred_Loc,Pred_Loc)] - instance_eqns -- This group comes from comparing with instance decls - = [ (eqn, p_inst, pred) - | fd <- cls_fds -- Iterate through the fundeps first, + = [ FDEqn { fd_qtvs = qtvs, fd_eqs = eqs, fd_pred1 = p_inst, fd_pred2=pred } + | fd <- cls_fds -- Iterate through the fundeps first, -- because there often are none! - , let trimmed_tcs = trimRoughMatchTcs cls_tvs fd rough_tcs + , let trimmed_tcs = trimRoughMatchTcs cls_tvs fd rough_tcs -- Trim the rough_tcs based on the head of the fundep. -- Remember that instanceCantMatch treats both argumnents -- symmetrically, so it's ok to trim the rough_tcs, -- rather than trimming each inst_tcs in turn - , ispec@(Instance { is_tvs = qtvs, is_tys = tys_inst, - is_tcs = inst_tcs }) <- instances - , not (instanceCantMatch inst_tcs trimmed_tcs) - , eqn <- checkClsFD qtvs fd cls_tvs tys_inst tys - , let p_inst = (mkClassPred cls tys_inst, - sep [ ptext (sLit "arising from the dependency") <+> quotes (pprFunDep fd) - , ptext (sLit "in the instance declaration at") - <+> ppr (getSrcLoc ispec)]) - ] - -improveOne _ _ _ - = [] + , ispec@(Instance { is_tvs = qtvs, is_tys = tys_inst, + is_tcs = inst_tcs }) <- instances + , not (instanceCantMatch inst_tcs trimmed_tcs) + , let p_inst = (mkClassPred cls tys_inst, + sep [ ptext (sLit "arising from the dependency") <+> quotes (pprFunDep fd) + , ptext (sLit "in the instance declaration at") + <+> ppr (getSrcLoc ispec)]) + , (qtvs, eqs) <- checkClsFD qtvs fd cls_tvs tys_inst tys -- NB: orientation + , not (null eqs) + ] + where + (cls_tvs, cls_fds) = classTvsFds cls + instances = classInstances inst_env cls + rough_tcs = roughMatchTcs tys +improveFromInstEnv _ _ = [] checkClsFD :: TyVarSet -- Quantified type variables; see note below -> FunDep TyVar -> [TyVar] -- One functional dependency from the class -> [Type] -> [Type] - -> [Equation] + -> [(TyVarSet, [FDEq])] checkClsFD qtvs fd clas_tvs tys1 tys2 -- 'qtvs' are the quantified type variables, the ones which an be instantiated @@ -313,52 +353,69 @@ checkClsFD qtvs fd clas_tvs tys1 tys2 length tys1 == length clas_tvs , ppr tys1 <+> ppr tys2 ) - case tcUnifyTys bind_fn ls1 ls2 of + case tcUnifyTys bind_fn ltys1 ltys2 of Nothing -> [] - Just subst | isJust (tcUnifyTys bind_fn rs1' rs2') - -- Don't include any equations that already hold. + Just subst | isJust (tcUnifyTys bind_fn rtys1' rtys2') + -- 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 + -- 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')] + -- NB: We can't do this 'is-useful-equation' check element-wise + -- because of: + -- class C a b c | a -> b c + -- instance C Int x x + -- [Wanted] C Int alpha Int + -- We would get that x -> alpha (isJust) and x -> Int (isJust) + -- so we would produce no FDs, which is clearly wrong. + -> [] + + | otherwise + -> [(qtvs', fdeqs)] -- 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 + where + rtys1' = map (substTy subst) rtys1 + irs2' = map (\(i,x) -> (i,substTy subst x)) irs2 + rtys2' = map snd irs2' + + fdeqs = zipAndComputeFDEqs (\_ _ -> False) rtys1' irs2' + -- Don't discard anything! + -- We could discard equal types but it's an overkill to call + -- tcEqType again, since we know for sure that /at least one/ + -- equation in there is useful) + 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) + -- 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 + (ltys1, rtys1) = instFD fd clas_tvs tys1 + (ltys2, irs2) = instFD_WithPos fd clas_tvs tys2 +\end{code} + +\begin{code} instFD :: FunDep TyVar -> [TyVar] -> [Type] -> FunDep Type +-- A simpler version of instFD_WithPos to be used in checking instance coverage etc. 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