Allow RULES for higher-ranked terms

[rl]

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 #-}

[simonpj]

Good idea. Fixed by

Tue Nov 17 12:54:17 GMT 2009  simonpj@microsoft.com
  * Improvement to typecheck higher-rank rules better
  
  See Note [Typechecking rules] in TcRules.  
  Suggested by Roman

    M ./compiler/typecheck/TcRules.lhs -5 +20

Simon