[EMAIL PROTECTED] wrote:
>
> I want to propose the following function slide, which is like map, but
> depends not on one value of
> a list, but on several consecutive ones.
>
> slide :: ([a] -> b) -> [a] -> [b]
> slide f [] = []
> slide f xs = f xs : slide f (tail xs)
The function is interesting from the theoretical side since it is an instance of
*redecoration*. Written as
slide :: [a] -> ([a] -> b) -> [b] ,
one sees a similarity with the monadic multiplication (or substitution, here for the
case of lists)
>>= :: [a] -> (a -> [b]) -> [b] .
Indeed, slide is the multiplication operation of lists viewed as a *comonad* and hence
the dual of ">>=".
Read more in
T Uustalu, V Vene. The dual of substitution is redecoration. In K Hammond, S Curtis,
eds, Trends in Functional Programming 3, pp 99-110. Intellect,
Bristol / Portland, OR, 2002. - .ps.gz, 46K (© Intellect)
http://www.cs.ioc.ee/~tarmo/papers/sfp01-book.ps.gz ,
if you want to be bothered with categorical nonsence ;-)
Cheers,
Andreas
--
Andreas Abel --<>-- What if Jesus is right?
Theoretical Computer Science, University of Munich
http://www.tcs.informatik.uni-muenchen.de/~abel/
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