Hi Renah,

don't know if I understood correctly, but this might help:

Long version:

fun' :: MonadIO m => (a -> m [b]) -> [a] -> m [b]
fun' f cs = do
results <- mapM f cs
let merged = concat results
return merged

Shorter version with fmap:

import Control.Applicative ((<$>))

fun :: (MonadIO m, Functor m) => (a -> m [b]) -> [a] -> m [b]
fun f cs = concat <$> mapM f cs

Best regards,

vlatko

-------- Original Message --------
Subject: [Haskell-beginners] combine concatMap and mapM
From: Renah Scarowsky <renahs@suite-sol.com>
To: beginners@haskell.org <beginners@haskell.org>
Date: 17.02.2014 11:38

Hi,

How would I combine the functionalities of concatMap and mapM to define a function like:

Monad m => (a -> m [b]) -> [a] -> m [b]

I need to create a recursive function within the IO monad which results in a list of the original type.

So something like:

goX :: MonadIO m => x -> m [x]

goX = do

….

x includes within it a list of xs, let's call it (children x)

I need to call goX on each of the children and get a modified list of children as result
ys <- ?

return $ x { children = ys } : zs

Thanks,

Renah Scarowsky

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