[Haskell-cafe] rewrite rules

[Ryan Ingram]

Not 100% sure (especially without source/core), but my guess is that
the higher-rank types make the rule unlikely to fire.

Try -ddump-simpl to see the core output, and look for places where you
expect the rule to fire.  I suspect you will find that the types of f
and g are not "forall" at that point in the code, but have already
been specialized.

Is there a reason you cannot use this simpler rule?

{-# RULES "transform/tranform" forall f g l. transform f (transform g
l) = transform (g.f) l #-}

  -- ryan

On Mon, Jun 22, 2009 at 2:41 AM, Sjoerd Visscher<sjoerd at w3future.com> wrote:
> Hi all,
>
> I have a rewrite rule as follows:
>
> {-# RULES
> "transform/transform" forall (f::forall m. Monoid m => (a -> m) -> (b -> m))
>                             (g::forall m. Monoid m => (b -> m) -> (c -> m))
>                             (l::FMList c). transform f (transform g l) =
> transform (g.f) l
>  #-}
>
> It fires on this code:
>
>  print $ transform (. (*2)) (transform (. (+1)) (upto 10))
>
> But it doesn't fire on this code:
>
>  print $ map (*2) (map (+1) (upto 10)))
>
> with
>
>  map g x = transform (. g) x
>
> and with or without {-# INLINE map #-}.
>
> What am I doing wrong?
>
> --
> Sjoerd Visscher
> sjoerd at w3future.com
>
>
>
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