Commit 27759873 authored by simonpj@microsoft.com's avatar simonpj@microsoft.com

Robustify lookupFamInstEnv

Suppose we have
        type family T a :: * -> *
        type instance T Int = []

and now we encounter the type (T Int Bool).  That is perfectly
fine, even though T is over-saturated here.

This patch makes lookupFamInstEnv robust to such over-saturation.
Previously one caller (TcTyFuns.tcUnfoldSynFamInst) dealt with
the over-saturation case, but the others did not. It's better
to desl with the issue at the root, in lookupFamInstEnv itself.
parent e3cb60d2
......@@ -69,21 +69,16 @@ tcUnfoldSynFamInst (TyConApp tycon tys)
| not (isOpenSynTyCon tycon) -- unfold *only* _synonym_ family instances
= return Nothing
| otherwise
= do { -- we only use the indexing arguments for matching,
-- not the additional ones
; maybeFamInst <- tcLookupFamInst tycon idxTys
= do { -- The TyCon might be over-saturated, but that's ok for tcLookupFamInst
; maybeFamInst <- tcLookupFamInst tycon tys
; case maybeFamInst of
Nothing -> return Nothing
Just (rep_tc, rep_tys) -> return $ Just (mkTyConApp rep_tc tys',
mkTyConApp coe_tc tys')
Just (rep_tc, rep_tys) -> return $ Just (mkTyConApp rep_tc rep_tys,
mkTyConApp coe_tc rep_tys)
where
tys' = rep_tys ++ restTys
coe_tc = expectJust "TcTyFuns.tcUnfoldSynFamInst"
(tyConFamilyCoercion_maybe rep_tc)
}
where
n = tyConArity tycon
(idxTys, restTys) = splitAt n tys
tcUnfoldSynFamInst _other = return Nothing
\end{code}
......
......@@ -218,8 +218,25 @@ desugared to
we return the matching instance '(FamInst{.., fi_tycon = :R42T}, Int)'.
Note [Over-saturated matches]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
It's ok to look up an over-saturated type constructor. E.g.
type family F a :: * -> *
type instance F (a,b) = Either (a->b)
The type instance gives rise to a newtype TyCon (at a higher kind
which you can't do in Haskell!):
newtype FPair a b = FP (Either (a->b))
Then looking up (F (Int,Bool) Char) will return a FamInstMatch
(FPair, [Int,Bool,Char])
The "extra" type argument [Char] just stays on the end.
\begin{code}
type FamInstMatch = (FamInst, [Type]) -- Matching type instance
-- See Note [Over-saturated matches]
lookupFamInstEnv :: FamInstEnvs
-> TyCon -> [Type] -- What we are looking for
......@@ -228,13 +245,23 @@ lookupFamInstEnv (pkg_ie, home_ie) fam tys
| not (isOpenTyCon fam)
= []
| otherwise
= home_matches ++ pkg_matches
= ASSERT( n_tys >= arity ) -- Family type applications must be saturated
home_matches ++ pkg_matches
where
rough_tcs = roughMatchTcs tys
all_tvs = all isNothing rough_tcs
home_matches = lookup home_ie
pkg_matches = lookup pkg_ie
-- See Note [Over-saturated matches]
arity = tyConArity fam
n_tys = length tys
extra_tys = drop arity tys
(match_tys, add_extra_tys)
| arity > n_tys = (take arity tys, \res_tys -> res_tys ++ extra_tys)
| otherwise = (tys, \res_tys -> res_tys)
-- The second case is the common one, hence functional representation
--------------
lookup env = case lookupUFM env fam of
Nothing -> [] -- No instances for this class
......@@ -255,8 +282,8 @@ lookupFamInstEnv (pkg_ie, home_ie) fam tys
= find rest
-- Proper check
| Just subst <- tcMatchTys tpl_tvs tpl_tys tys
= (item, substTyVars subst (tyConTyVars tycon)) : find rest
| Just subst <- tcMatchTys tpl_tvs tpl_tys match_tys
= (item, add_extra_tys $ substTyVars subst (tyConTyVars tycon)) : find rest
-- No match => try next
| otherwise
......
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