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

[Maciej Piechotka]

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