Commit fe61599f authored by Iavor S. Diatchki's avatar Iavor S. Diatchki

Use a version of the coverage condition even with UndecidableInstances.

This fixes bug #1241 and #2247.  When UndecidableInstances are on,
we use the "Liberal Coverage Condition", which is what GHC used to do in
the past.  This is the gist of the check:

class C a b | a -> b
instance theta => C t1 t2

we check that `fvs t2` is a subset of `fd-closure(theta,fvs t1)`.

This is strictly more general than the coverage condition, while
it still guarantees consistency with the FDs of the class.  This
check is completely orthogonal to termination (it by no means guarantees
it).

I am not sure of the role of the "coverage condition" in termination---
the comments suggest that it is important.  This is why, for the moment,
we only use this check when UndecidableInstances are on.
parent 6387eba8
......@@ -827,7 +827,9 @@ checkValidInstance ctxt hs_type ty
-- in the constraint than in the head
; undecidable_ok <- xoptM Opt_UndecidableInstances
; if undecidable_ok
then checkAmbiguity ctxt ty
then do checkAmbiguity ctxt ty
checkTc (checkInstLiberalCoverage clas theta inst_tys)
(instTypeErr clas inst_tys liberal_msg)
else do { checkInstTermination inst_tys theta
; checkTc (checkInstCoverage clas inst_tys)
(instTypeErr clas inst_tys msg) }
......@@ -837,6 +839,10 @@ checkValidInstance ctxt hs_type ty
msg = parens (vcat [ptext (sLit "the Coverage Condition fails for one of the functional dependencies;"),
undecidableMsg])
liberal_msg = vcat
[ ptext $ sLit "Multiple uses of this instance may be inconsistent"
, ptext $ sLit "with the functional dependencies of the class."
]
-- The location of the "head" of the instance
head_loc = case hs_type of
L _ (HsForAllTy _ _ _ (L loc _)) -> loc
......
......@@ -19,7 +19,7 @@ module FunDeps (
FDEq (..),
Equation(..), pprEquation,
oclose, improveFromInstEnv, improveFromAnother,
checkInstCoverage, checkFunDeps,
checkInstCoverage, checkInstLiberalCoverage, checkFunDeps,
pprFundeps
) where
......@@ -145,6 +145,52 @@ oclose preds fixed_tvs
ClassPred cls tys -> [(cls, tys)]
TuplePred ts -> concatMap classesOfPredTy ts
_ -> []
-- An alternative implementation of `oclose`. Differences:
-- 1. The empty set of variables is allowed to determine stuff,
-- 2. We also use equality predicates as FDs.
--
-- I (Iavor) believe that this is the correct implementation of oclose.
-- For 1: the above argument about `t` being monomorphic seems incorrect.
-- The correct behavior is to quantify over `t`, even though we know that
-- it may be instantiated to at most one type. The point is that we might
-- only find out what that type is later, at the class site to the function.
-- In genral, we should be quantifying all variables that are not mentioned
-- in the environment + the variables that are determined by them.
-- For 2: This is just a nicity, but it makes things a bit more general:
-- if we have an assumption `t1 ~ t2`, then we use the fact that if we know
-- `t1` we also know `t2` and the other way.
oclose1 :: [PredType] -> TyVarSet -> TyVarSet
oclose1 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)]
tv_fds = [ (tyVarsOfTypes xs, tyVarsOfTypes ys)
| (xs, ys) <- concatMap deterimned preds
]
deterimned :: PredType -> [([Type],[Type])]
deterimned pred
= case classifyPredType pred of
ClassPred cls tys ->
do let (cls_tvs, cls_fds) = classTvsFds cls
fd <- cls_fds
return (instFD fd cls_tvs tys)
EqPred t1 t2 -> [([t1],[t2]), ([t2],[t1])]
TuplePred ts -> concatMap deterimned ts
_ -> []
\end{code}
......@@ -471,6 +517,23 @@ checkInstCoverage clas inst_taus
fundep_ok fd = tyVarsOfTypes rs `subVarSet` tyVarsOfTypes ls
where
(ls,rs) = instFD fd tyvars inst_taus
checkInstLiberalCoverage :: Class -> [PredType] -> [Type] -> Bool
-- Check that the Liberal Coverage Condition is obeyed in an instance decl
-- For example, if we have:
-- class C a b | a -> b
-- instance theta => C t1 t2
-- Then we require fv(t2) `subset` oclose(fv(t1), theta)
-- This ensures the self-consistency of the instance, but
-- it does not guarantee termination.
-- See Note [Coverage Condition] below
checkInstLiberalCoverage clas theta inst_taus
= all fundep_ok fds
where
(tyvars, fds) = classTvsFds clas
fundep_ok fd = tyVarsOfTypes rs `subVarSet` oclose1 theta (tyVarsOfTypes ls)
where (ls,rs) = instFD fd tyvars inst_taus
\end{code}
Note [Coverage condition]
......
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