[Haskell-cafe] Monadic vs "pure" style (was: pros and cons of sta tic typing and side effects)

[Yitzchak Gale]

On Thu, Sep 01, 2005 at 12:41:06AM -0700, Juan Carlos Arevalo Baeza wrote:
>   You can get the correct order by using lists, but you want to use the 
> "Asc" versions:
> 
> myMapM someAction someMap = do
>    list <- sequence $
>            map (\(k, a) -> someAction a >>= (\b -> return (k,b))) $
>            Map.toAscList someMap
>    return (Map.fromAscList list)
> 
> Should also be O(n).

I like this approach. You want to use
fromDistinctAscList though.

By analogy with the Prelude, I would make it
slightly more flexible as follows:

mySequence :: Monad m => Map.Map k (m a) -> m (Map.Map k a)
mySequence someMap = do
    list <- sequence $ map liftTuple $ Map.toAscList someMap
    return $ Map.fromDistinctAscList list
  where
    liftTuple (x, y) = do
      z <- y
      return (x, z)

myMapM :: Monad m => (a -> m b) -> Map.Map k a -> m (Map.Map k b)
myMapM f = mySequence . Map.map f

myMapM_ :: Monad m => (a -> m b) -> Map.Map k a -> m ()
myMapM_ f = sequence_ . map f . Map.elems

Regards,
Yitz