[Haskell-cafe] *almost* composition in writer monad
miguelimo38 at yandex.ru
Wed Mar 4 12:29:48 EST 2009
Isn't that "sequence" in State monad?
On 4 Mar 2009, at 19:37, Edsko de Vries wrote:
> Does this function remind anybody of anything? It seems like I'm
> missing an obvious abstraction:
> composeWriter :: [a -> (a, b)] -> a -> (a, [b])
> composeWriter  a
> = (a, )
> composeWriter (f:fs) a
> = let (a', b) = f a
> (final_a, bs) = composeWriter fs a'
> in (final_a, b:bs)
> It's almost but not quite composition for functions in the Writer
> monad; the difference is that every function has exactly one b
> result, rather than a list of them.
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