*almost* composition in writer monad
Hi, 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. Edsko
Isn't that "sequence" in State monad? On 4 Mar 2009, at 19:37, Edsko de Vries wrote:
Hi,
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.
Edsko
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Doh, yes, of course. I had a feeling I was missing something obvious :) Thanks :) On 4 Mar 2009, at 17:29, Miguel Mitrofanov wrote:
Isn't that "sequence" in State monad?
On 4 Mar 2009, at 19:37, Edsko de Vries wrote:
Hi,
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.
Edsko
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participants (2)
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Edsko de Vries -
Miguel Mitrofanov