[Haskell-cafe] adding the elements of two lists
wren ng thornton
wren at freegeek.org
Mon Mar 26 19:00:34 CEST 2012
On 3/26/12 8:16 AM, Jake McArthur wrote:
> This is interesting because it seems to be a counterexample to the claim
> that you can lift any Num through an Applicative (ZipList, in this case).
> It seems like maybe that only works in general for monoids instead of rings?
I'm not so sure about that. The Applicative structure of ZipLists is
specifically defined for infinite lists (cf., pure = repeat). And in the
case of infinite lists the (+) = zipWith(+) definition works just fine,
since we don't have to worry about truncation. I wasn't aware that Num
was supposed to be liftable over any Applicative, but this doesn't seem
like a counterexample...
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