How to get the value of a Maybe in Haskell

Question

I'm relatively new to Haskell and began to read "Real World Haskell".

I Just stumbled over the type Maybe and have a question about how to receive the actual value from a Just 1 for example.

I have written the following code:

combine a b c = (eliminate a, eliminate b, eliminate c)                 where eliminate (Just a) = a                       eliminate Nothing = 0 

This works fine if I use:

combine (Just 1) Nothing (Just 2) 

But if I change, for example, 1 to a String it doesn't work.

I think I know why: because eliminate has to give back one type, which is, in this case, an Int. But how can I change eliminate to deal at least with Strings (or maybe with all kind of types)?

Answer 1

From the standard Prelude,

maybe :: b -> (a -> b) -> Maybe a -> b maybe n _ Nothing = n maybe _ f (Just x) = f x 

Given a default value, and a function, apply the function to the value in the Maybe or return the default value.

Your eliminate could be written maybe 0 id, e.g. apply the identity function, or return 0.

From the standard Data.Maybe,

fromJust :: Maybe a -> a fromJust Nothing = error "Maybe.fromJust: Nothing" fromJust (Just x) = x 

This is a partial function (does not return a value for every input, as opposed to a total function, which does), but extracts the value when possible.