[Haskell-beginners] Handling Maybes

[Christopher Howard]

Since I started into Haskell, I frequently run into a scenario like
this: some function returns a Maybe value. If it is a Nothing, I want to
do one thing, but if it isn't, I want to do something with the value
encapsulated by the Maybe. But before I can use that value (say, to
print it) I've got to pull it out of the Maybe in a way that satisfies
the compiler. So one option is to use a separate function with pattern
matching, for example:

handleMaybeVal Nothing = do someAction
                            someOtherActionEtc
handleMaybeVal (Just a) = putStrLn ("The string in reverse is " ++
                                   (reverse a))

Or I can use the maybe function with a conditional

main = do val <- someIOFunc -- val is a Maybe String
          if val == Nothing
            then do someAction
              someOtherActionEtc
            else putStrLn ("The string in reverse is " ++
                          ((reverse . unpack) val))
  where unpack a = maybe
                     (error "unreachable error")
                     (\b -> b) a

The second option I like better, for some reason, but it is a bit
dangerous, as I have to double check my code to make sure the default
value passed to "maybe" cannot possibly be evaluated. (Alternatively, I
could pass in a regular default value, like a blank string, though that
is actually more dangerous in my mind because a bad value might be used
silently.)

Anyway: how do you pros go about handling situations like these?

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