Commit 5a3ada9c authored by chak@cse.unsw.edu.au.'s avatar chak@cse.unsw.edu.au.
Browse files

Overlap check for type families

- If two "type instance"s overlap, they right-hand sides must be syntactically
  equal under the overlap substitution.  (Ie, we admit limited overlap, but
  require the system to still be confluent.)
parent 78f4da28
......@@ -180,19 +180,27 @@ checkForConflicts inst_envs famInst
; let { matches = lookupFamInstEnvUnify inst_envs fam tys'
; conflicts = [ conflictingFamInst
| match@(conflictingFamInst, _) <- matches
, conflicting fam tys' tycon match
| match@((conflictingFamInst, _), _) <- matches
, conflicting tycon match
]
}
; unless (null conflicts) $
conflictInstErr famInst (head conflicts)
}
where
-- In the case of data/newtype instances, any overlap is a conflict (as
-- these instances imply injective type mappings).
conflicting _ _ tycon _ | isAlgTyCon tycon = True
conflicting fam tys' tycon (subst, cFamInst) | otherwise =
panic "FamInst.checkForConflicts: overlap check for indexed synonyms is still missing"
-- * In the case of data family instances, any overlap is fundamentally a
-- conflict (as these instances imply injective type mappings).
-- * In the case of type family instances, overlap is admitted as long as
-- the right-hand sides of the overlapping rules coincide under the
-- overlap substitution. We require that they are syntactically equal;
-- anything else would be difficult to test for at this stage.
conflicting tycon1 ((famInst2, _), subst)
| isAlgTyCon tycon1 = True
| otherwise = not (rhs1 `tcEqType` rhs2)
where
tycon2 = famInstTyCon famInst2
rhs1 = substTy subst $ synTyConType tycon1
rhs2 = substTy subst $ synTyConType tycon2
conflictInstErr famInst conflictingFamInst
= addFamInstLoc famInst $
......
......@@ -277,7 +277,7 @@ indexed synonyms and we don't want to slow that down by needless unification.
\begin{code}
lookupFamInstEnvUnify :: (FamInstEnv, FamInstEnv) -> TyCon -> [Type]
-> [(FamInstMatch)]
-> [(FamInstMatch, TvSubst)]
lookupFamInstEnvUnify (pkg_ie, home_ie) fam tys
| not (isOpenTyCon fam)
= []
......@@ -318,7 +318,7 @@ lookupFamInstEnvUnify (pkg_ie, home_ie) fam tys
case tcUnifyTys bind_fn tpl_tys tys of
Just subst -> let rep_tys = substTyVars subst (tyConTyVars tycon)
in
(item, rep_tys) : find rest
((item, rep_tys), subst) : find rest
Nothing -> find rest
-- See explanation at @InstEnv.bind_fn@.
......
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