On Sat, Jul 28, 2012 at 8:00 AM, Thiago Negri <evohunz@gmail.com> wrote:

I'm solving this exercise:
http://www.haskell.org/haskellwiki/All_About_Monads#Exercise_4:_Using_the_Monad_class_constraint

I'm missing a function to transform a Maybe a into a MonadPlus m => m a.
I did search on Hoogle with no luck.

There is no standard definition for the "g" function I'm defining?

g :: (MonadPlus m) => Maybe a -> m a
g Nothing = mzero
g (Just a) = return a

I doubt there is a "standard" named function for this, since conversions between monads are typically not "unique" and there are lots and lots of pairs of monads you can convert between.

You can define g in terms of Data.Maybe.maybe as:

g = maybe mzero return