desperately seeking RULES help

[Lennart Augustsson]

Interesting.  The problem seems to be that GHC always inlines toInt
and fromInt early, but this means that the rewrite rule no longer
applies.
And, of course, it doesn't inline toInt and fromInt in the rewrite rule.
I have no idea if you can write a rule that will actually work,
because after toInt and fromInt have been inlined you can no longer
write rules that apply, since the types involve dictionaries and the
terms pattern match on dictionaries.

  -- Lennart

2008/6/7 Conal Elliott <conal at conal.net>:
> I'm trying to do some fusion in ghc, and I'd greatly appreciate help with
> the code below (which is simplified from fusion on linear maps).  I've tried
> every variation I can think of, and always something prevents the fusion.
>
> Help, please!  Thanks, - Conal
>
>
> {-# OPTIONS_GHC -O2 -Wall -fglasgow-exts -ddump-simpl -ddump-simpl-stats #-}
> -- {-# OPTIONS_GHC -ddump-simpl-iterations #-}
>
> module F where
>
> -- | Domain of a linear map.
> class AsInt a where
>   toInt   :: a -> Int
>   fromInt :: Int -> a
>
> {-# RULES
> "toInt/fromInt"   forall m. toInt (fromInt m) = m
>  #-}
>
> {-# INLINE onInt #-}
> onInt :: AsInt a => (Int -> Int) -> (a -> a)
> onInt f = fromInt . f . toInt
>
> test :: AsInt a => (Int -> Int) -> (Int -> Int) -> (a -> a)
> test h g = onInt h . onInt g
>
> -- The desired result:
> --
> --   test h g
> --     == onInt h . onInt g
> --     == (fromInt . h . toInt) . (fromInt . g . toInt)
> --     == \ a -> (fromInt . h . toInt) ((fromInt . g . toInt) a)
> --     == \ a -> (fromInt . h . toInt) (fromInt (g (toInt a)))
> --     == \ a -> fromInt (h (toInt (fromInt (g (toInt a)))))
> --     == \ a -> fromInt (h (g (toInt a)))
>
>
>
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