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Sun Mar 7 10:14:27 CET 2010

## Arrows

To understand arrows is to understand their basic combinators[1]. As
a concrete example one could think of arrows as a generalization of
functions.
> instance Arrow (->) where
> arr f = f
> f >>> g = g . f
> first f = \(x,y) -> (f x, y) -- for comparison's sake
> second f = \(x,y) -> ( x, f y) -- like first
> f *** g = \(x,y) -> (f x, g y) -- takes two arrows, and not just one
> f &&& g = \x -> (f x, g x) -- feed the same input into both functions
[1] http://en.wikibooks.org/wiki/Haskell/Understanding_arrows

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