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Today's Topics:
1. (no subject) (Dennis Raddle)
2. Re: (no subject) (Tony Morris)
----------------------------------------------------------------------
Message: 1
Date: Sun, 2 Sep 2012 20:32:13 -0700
From: Dennis Raddle <dennis.rad...@gmail.com>
Subject: [Haskell-beginners] (no subject)
To: Haskell Beginners <beginners@haskell.org>
Message-ID:
<CAKxLvop6q48RC-YWhXS5vgGNu=zz8yz1dtvy56kj_cf_lef...@mail.gmail.com>
Content-Type: text/plain; charset="iso-8859-1"
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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Message: 2
Date: Mon, 03 Sep 2012 13:34:42 +1000
From: Tony Morris <tonymor...@gmail.com>
Subject: Re: [Haskell-beginners] (no subject)
To: beginners@haskell.org
Message-ID: <50442552.7080...@gmail.com>
Content-Type: text/plain; charset="iso-8859-1"
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@haskell.org
> http://www.haskell.org/mailman/listinfo/beginners
You probably want to look at Data.Traversable#traverse.
--
Tony Morris
http://tmorris.net/
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