#3670: Allow RULES for higher-ranked terms
-----------------------------+----------------------------------------------
Reporter: rl | Owner:
Type: feature request | Status: new
Priority: normal | Component: Compiler
Version: 6.13 | Severity: normal
Keywords: | Testcase:
Os: Unknown/Multiple | Architecture: Unknown/Multiple
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Here is a small code sample:
{{{
foo :: (forall m. m a -> m b) -> m a -> m b
foo f = f
bar :: (forall m. m a -> m a) -> m a -> m a
bar f = f
}}}
I'd like to specialise `foo` to `bar` whenever possible but there seems to
be no way of doing so. This doesn't work:
{{{
{-# RULES "foo/bar" foo = bar #-}
}}}
GHC complains:
{{{
Cannot match a monotype with `(forall (m1 :: * -> *). m1 a -> m1 b)
-> m a
-> m b'
}}}
Adding a signature to the rhs of the rule doesn't help. GHC doesn't accept
signatures in the lhs. The following works, of course, but it's not as
general:
{{{
{-# RULES "foo/bar" forall (f :: (forall m. m a -> m a)). foo f = bar f
#-}
}}}
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/3670>
GHC <http://www.haskell.org/ghc/>
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