[Haskell-beginners] (no subject)
tonymorris at gmail.com
Mon Sep 3 05:34:42 CEST 2012
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
> Beginners mailing list
> Beginners at haskell.org
You probably want to look at Data.Traversable#traverse.
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