[Haskell-cafe] Re: Implicit parameters and Arrows/Yampa?

[Claude Heiland-Allen]

ChrisK wrote:
> Could I has one question?  What is the purpose of the "stream" function 
> in the ArrowLoop instance?  Is it just to catch an unexpected [] at 
> runtime?

>> instance ArrowLoop SF where
>>    loop (SF f) = SF $ \as ->
>>        let (bs,cs) = unzip (f (zip as (stream cs))) in bs
>>      where stream ~(x:xs) = x:stream xs
> 
> It looks like stream is (almost) an identity which would crash at 
> runtime if it encountered a [].  In particular it is equivalent to
> 
>  >      where stream xs = head xs:stream (tail xs)


Not quite that, from a blog post of mine a while back:

[snip]

But then the trouble arrived - I suddenly noticed that the paper was 
using infinite sequences, not the finite sequences I was using myself. 
Trying to implement an ArrowLoop as per the paper led to horrible grief:

-- instance ArrowLoop SF where
--  loop (SF f) = SF (loop (unzip . f . uncurry zip))

The problem is that this is far too strict - on non-empty input it 
caused stack overflow crashes, which isn't exactly what I wanted. I 
found the solution in Programming With Arrows [1] (page 17), which 
involves some subtlety with lazy patterns:

instance ArrowLoop SF where
   loop (SF f) = SF $ \as ->
       let (bs,cs) = unzip (f (zip as (stream cs))) in bs
     where stream ~(x:xs) = x:stream xs

[unsnip]

http://www.haskell.org/arrows/biblio.html#Hug05 [1]


Hope this helps,


Claude
-- 
http://claudiusmaximus.goto10.org