[Haskell-cafe] rewrite rules

[Sjoerd Visscher]

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 #-}
>

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 - 
 > FMList a
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<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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>>
>

--
Sjoerd Visscher
sjoerd at w3future.com