[Haskell-beginners] (no subject)

[Tony Morris]

On 03/09/12 13:32, Dennis Raddle wrote:
> I wanted to run the map function from Data.Map, let's call it M.map,
> but inside a monad transformer stack including the Error monad.
>
> M.map has this type:
>
> M.map :: (Ord k) => (a -> b)  -> Map k a  -> Map k b
>
> However, I want to use a mapping function that has type
>
> (Monad m) => a -> m b
>
> (i.e. errors could be thrown during the computation, a log could be
> written, etc)
>
> I wrote the following. Any comments on this way of doing things?
>
> mapMapM :: (Monad m, Ord k) => (a -> m b) -> Map k a -> m (Map k b)
> mapMapM g mapIn = do
>   let h (k,a) = do
>         b <- g a
>         return (k,b)
>   y <- mapM h (M.toAscList mapIn)
>   return $ M.fromAscList y
>
>
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You probably want to look at Data.Traversable#traverse.


-- 
Tony Morris
http://tmorris.net/


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