[Haskell-cafe] Re: (liftM join .) . mapM

On Tue, 2009-12-29 at 18:20 +0000, Conor McBride wrote:

Hi Tony

On 29 Dec 2009, at 12:10, Tony Morris wrote:

...

Can (liftM join .) . mapM be improved? (Monad m) => (a -> m [b]) -> [a] -> m [b]

You can

(a) generalize m from Monad to Applicative (b) generalize [b] to any Monoid (c) generalize [a] to f a for any Foldable f

and write

ala AppLift foldMap

if you happen to have some of my usual kit. See below.

Cheers

Conor

What is benefit of it over: concatMapA f = foldr (liftA2 mappend . f) (pure mempty) Regards