\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}
\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
\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
\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 ...
--
-- 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
\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}
ppr_fd (us, vs) = hsep [interppSP us, ptext SLIT("->"), interppSP vs]
\end{code}
+
projects / ghc-hetmet.git / blobdiff