Functions in Data.Maybe

Posted on December 12, 2013

Most introductions to monads in Haskell talk about Maybe. Here are some of the handy functions in that module for operating on the type. maybe (the first function below) is should be available in ghci right away in Prelude, but the others require that you load the Data.Maybe module in ghci:

λ> :m +Data.Maybe

maybe either applies a function to a Maybe or returns a default value. It’s type signature looks like this:

maybe :: b -> (a -> b) -> Maybe a -> b

λ> let x = Just 40
λ> let y = Nothing
λ> maybe 30 (*2) x -- x is Just 40, so it gets applied to (*2)
λ> maybe 30 (*2) y -- y is Nothing, we end up with the default value

isJust is straightforward. Given a Maybe argument, it returns true if it’s Just:

isJust :: Maybe a -> Bool

λ> isJust x
λ> isJust y

isNothing: not hard to figure what this does:

isNothing :: Maybe a -> Bool

λ> isNothing x
λ> isNothing y

fromJust is a relatively unsafe way to get at the value of a Maybe:

fromJust :: Maybe a -> a

λ> fromJust x
λ> fromJust y
*** Exception: Maybe.fromJust: Nothing -- woops. use a case expression in real code

fromMaybe is similar to maybe, but instead of a function argument being applied to the value, you simply get the value in your Maybe or the default you pass in:

fromMaybe :: a -> Maybe a -> a

λ> fromMaybe 30 x
λ> fromMaybe 30 y

maybeToList will plop your Maybe’s value into a list and return it:

maybeToList :: Maybe a -> [a]

λ> maybeToList x
λ> maybeToList y
[] -- always get a value, it just happens to be an empty list if we pass in Nothing

listToMaybe goes the other way:

listToMaybe :: [a] -> Maybe a

λ> listToMaybe [2]
Just 2
λ> listToMaybe $ maybeToList x
Just 40
λ> listToMaybe $ maybeToList y
λ> listToMaybe [20,40,60] -- listToMaybe will drop all but the first element
Just 20
λ> listToMaybe "foo"
Just 'f'

catMaybes takes a list of Maybes and returns a list of the values for any Just value, but drops out your Nothings:

catMaybes :: [Maybe a] -> [a]

λ> catMaybes [x, y, (Just 21), Nothing, Just(22)]

mapMaybe lets you apply a function that accepts and returns a Maybe to a list of Maybes, but returns a list of the values (and skips the Nothings):

mapMaybe :: (a -> Maybe b) -> [a] -> [b]

λ> let f x = (+2) <$> x -- or fmap (+2) x
λ> f (Just 30)
Just 32
λ> mapMaybe f [(Just 20), Nothing, x]