[Haskell-cafe] rewrite rules

[Sjoerd Visscher]

Ah, thanks.

It turns out that this works:

   transform t l = error "urk"

but this doesn't:

   transform t l = FM $ error "urk"

So it has something to do with the newtype FMList. They are probably  
already gone when rewrite rules fire?

Sjoerd

On Jun 24, 2009, at 6:32 PM, Ryan Ingram wrote:

> Your FMLists are defaulting to Integer, so the rule (which
> specifically mentions Int) doesn't apply.  Simon's code doesn't have
> this problem because of the explicit signature on "upto"; you could do
> the same by limiting "singleton" to Int.
>
>  -- ryan
>
> On Wed, Jun 24, 2009 at 12:44 AM, Sjoerd  
> Visscher<sjoerd at w3future.com> wrote:
>> Thanks for looking into this.
>>
>> Your code does give me 2 firings. But not when I replace [] with  
>> FMList. See
>> the attached code.
>>
>>
>>
>>
>>
>> Sjoerd
>>
>> On Jun 23, 2009, at 5:59 PM, Simon Peyton-Jones wrote:
>>
>>> | 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)))
>>>
>>> That's odd. It works for me.
>>>
>>> Specifically, I compiled the attached code with GHC 6.10, and I  
>>> get two
>>> firings of transform/transform.
>>>
>>> Does that not happen for you?
>>>
>>> Simon
>>>
>>> <Rules.hs>
>>
>> --
>> Sjoerd Visscher
>> sjoerd at w3future.com
>>
>>
>>
>>
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>>
>

--
Sjoerd Visscher
sjoerd at w3future.com