[Haskell-beginners] Idiomatic way of working with Either and lists

[Emmanuel Surleau]

On Sat, Jan 26, 2013 at 05:41:39PM +0100, Daniel Fischer wrote:
> On Saturday 26 January 2013, 17:26:26, Emmanuel Surleau wrote:
> > Hi,
> > 
> > I'm trying to figure out the best way to work with Either and lists.
> > 
> > I have a function f which goes:
> > 
> > f :: a -> Either SomeError b
> > 
> > Now, I'd like to do:
> > 
> > applyF :: [a] -> (a -> Either SomeError b) -> Either SomeError [b]
> > 
> > That is, map f over a list and return either a list or the first error
> > found. The function should stop at the first error. I have implemented
> > applyF with a fold, but I'm sure there must be something like this already,
> > being a fairly generic pattern.
> 
> Since (Either e) is a Monad,
> 
> mapM :: Monad m => (a -> m b) -> [a] -> m [b]
> 
> does what you want. If your GHC is <= 7.4.2 (and >= 7.0.1), you will need to
> 
> import Control.Monad.Instances
> 
> The version with the argument order you gave is
> 
> forM :: Monad m => [a] -> (a -> m b) -> m [b]
> forM = flip mapM

Brilliant. Thank you very much, that's much better than my fold.

Cheers,

Emm