\begin{code}
module FunDeps (
Equation, pprEquation, pprEquationDoc,
- oclose, grow, improve, checkInstFDs, checkClsFD, pprFundeps
+ oclose, grow, improve,
+ checkInstFDs, checkFunDeps,
+ pprFundeps
) where
#include "HsVersions.h"
-import Name ( getSrcLoc )
-import Var ( Id, TyVar )
+import Name ( Name, getSrcLoc )
+import Var ( TyVar )
import Class ( Class, FunDep, classTvsFds )
import Unify ( tcUnifyTys, BindFlag(..) )
import Type ( substTys, notElemTvSubst )
-import TcType ( Type, ThetaType, PredType(..), tcEqType,
+import TcType ( Type, ThetaType, 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 )
nest 2 (vcat [ ppr t1 <+> ptext SLIT(":=:") <+> ppr t2 | (t1,t2) <- pairs])]
----------
-improve :: InstEnv Id -- Gives instances for given class
+improve :: (Class -> [Instance]) -- 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 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
eqn <- checkGroup inst_env group ]
----------
-checkGroup :: InstEnv Id -> [(PredType,SDoc)] -> [(Equation, SDoc)]
+checkGroup :: (Class -> [Instance])
+ -> [(PredType,SDoc)]
+ -> [(Equation, SDoc)]
-- The preds are all for the same class or implicit param
checkGroup inst_env (p1@(IParam _ ty, _) : ips)
--
-- 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.
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)),
+ | fd <- cls_fds, -- Iterate through the fundeps first,
+ -- because there often are none!
p2@(ClassP _ tys2, _) <- clss,
- eqn <- checkClsFD qtvs fd cls_tvs tys1 tys2
+ 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))
]
mkEqnMsg (pred1,from1) (pred2,from2)
(ls,rs) = instFD fd tyvars inst_taus
\end{code}
+
+%************************************************************************
+%* *
+ 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