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| -rw-r--r-- | docs/users_guide/exts/rewrite_rules.rst | 25 |
1 files changed, 25 insertions, 0 deletions
diff --git a/docs/users_guide/exts/rewrite_rules.rst b/docs/users_guide/exts/rewrite_rules.rst index b384faeb9e..7ac93bb21d 100644 --- a/docs/users_guide/exts/rewrite_rules.rst +++ b/docs/users_guide/exts/rewrite_rules.rst @@ -204,6 +204,31 @@ From a semantic point of view: (b) large or a redex, then it would not be substituted, and the rule would not fire. +- GHC will never match a forall'd variable in a template with an expression + which contains locally bound variables. For example, it is permitted to write + a rule which contains a case expression:: + + {-# RULES + "test/case-tup" forall (x :: (Int, Int)) (y :: Int) (z :: Int). + test (case x of (l, r) -> y) z = case x of (l, r) -> test y z + #-} + + But the rule will not match when ``y`` contains either of ``l`` or ``r`` because + they are locally bound. Therefore the following application will fail to trigger + the rule:: + + prog :: (Int, Int) -> (Int, Int) + prog x = test (case x of (p, q) -> p) 0 + + because ``y`` would have to match against ``p`` (which is locally bound) + but it will fire for:: + + prog :: (Int, Int) -> (Int, Int) + prog x = test (case x of (p, q) -> 0) 0 + + because ``y`` can match against ``0``. + + .. _rules-inline: How rules interact with ``INLINE``/``NOINLINE`` pragmas |