[Haskell-cafe] mixing map and mapM ?

[wren ng thornton]

Pierre-Etienne Meunier wrote:
> This way :
> 
> do
> 	times<-mapM PF.getFileStatus filenames >>= return.(map PF.modificationTime)
> 
> Or also :
> 
> do
> 	times<-mapM (PF.getFileStatus >>= (return.(PF.modificationTime))) filenames
> 	let sorted=...
> 
> I do not know exactly how ghc compiles the IO monad, but it seems to me that the latter would allocate a little less.

FWIW, (a >>= (return . f)) == (liftM f a) ~= (fmap f a)

Where available, the fmap version is the most efficient. The liftM 
function can be less efficient since it's defined generically (namely 
with the bind/return definition above), whereas fmap can take advantage 
of knowing the specific monad it's working on. But then, not everyone 
defines Functor instances for their monads...

-- 
Live well,
~wren