Re: [Haskell-cafe] rewrite rules

On Jun 22, 2009, at 6:38 PM, Ryan Ingram wrote:

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 #-}

FMList a

Yes, this is the reason: Inferred type is less polymorphic than expected Quantified type variable `m' is mentioned in the environment: f :: (a -> m) -> b -> m (bound at Data/FMList.hs:124:29) In the first argument of `transform', namely `f' In the expression: transform f (transform g l) When checking the transformation rule "transform/transform" This is the function: transform :: (forall m. Monoid m => (a -> m) -> (b -> m)) -> FMList b - transform t l = FM $ \f -> unFM l (t f) I'll have to clean things up before the core output becomes manageable. Sjoerd

-- ryan

On Mon, Jun 22, 2009 at 2:41 AM, Sjoerd Visscher 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@w3future.com

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-- Sjoerd Visscher sjoerd@w3future.com