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
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