haskell - Using the Maybe Monad in "reverse"

Question

Let's say I have a number of functions:

f :: a -> Maybe a
g :: a -> Maybe a
h :: a -> Maybe a

And I want to compose them in the following way: If f returns Nothing, compute g. If g returns Nothing, compute h. If any of them compute Just a, stop the chain. And the whole composition (h . g . f) should of course return Maybe a.

This is the reverse of the typical use of the Maybe monad, where typically you stop computing if Nothing is returned.

What's the Haskell idiom for chaining computations like this?

question from:https://stackoverflow.com/questions/5606228/using-the-maybe-monad-in-reverse

Answer 1

mplus is exactly what you're looking for, part of the MonadPlus typeclass. Here's its definition:

instance MonadPlus Maybe where
   mzero = Nothing

   Nothing `mplus` ys  = ys
   xs      `mplus` _ys = xs

To use it in your case:

combined x = (f x) `mplus` (g x) `mplus` (h x)