Commit c276b9b6 authored by simonpj@microsoft.com's avatar simonpj@microsoft.com
Browse files

Fix a bug in rule matching

The rule matcher uses a "rough-match" pre-filter, which was being too 
aggressive.  The case looked like this:

	rule:	f True
	expr:	case e of x { True -> f x }

Jues because x doesn't immediately look like True, we shouldn't say
"can't match", but that is exactly what ruleCantMatch was doing.
parent 64f00b23
......@@ -115,8 +115,13 @@ ruleCantMatch :: [Maybe Name] -> [Maybe Name] -> Bool
-- definitely can't match 'tpl' by instantiating 'tpl'.
-- It's only a one-way match; unlike instance matching we
-- don't consider unification
--
-- Notice that there is no case
-- ruleCantMatch (Just n1 : ts) (Nothing : as) = True
-- Reason: a local variable 'v' in the actuals might
-- have an unfolding which is a global.
-- This quite often happens with case scrutinees.
ruleCantMatch (Just n1 : ts) (Just n2 : as) = n1 /= n2 || ruleCantMatch ts as
ruleCantMatch (Just n1 : ts) (Nothing : as) = True
ruleCantMatch (t : ts) (a : as) = ruleCantMatch ts as
ruleCantMatch ts as = False
\end{code}
......@@ -403,10 +408,10 @@ match menv subst@(tv_subst, id_subst) (Var v1) e2
other -> Nothing
| otherwise -- v1 is not a template variable
= case e2 of
Var v2 | v1' == rnOccR rn_env v2 -> Just subst
other -> Nothing
| -- v1 is not a template variable; check for an exact match with e2
Var v2 <- e2, v1' == rnOccR rn_env v2
= Just subst
where
rn_env = me_env menv
v1' = rnOccL rn_env v1
......
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