Represent values that may or may not exist. It can be useful if you have a record field that is only filled in sometimes. Or if a function takes a value sometimes, but does not absolutely need it.

-- A person, but maybe we do not know their age.
type alias Person =
    { name : String
    , age : Maybe Int
    }


tom = { name = "Tom", age = Just 42 }
sue = { name = "Sue", age = Nothing }

Provide a default value, turning an optional value into a normal value. This comes in handy when paired with functions like Dict.get which gives back a Maybe.

withDefault 100 (Just 42)   -- 42
withDefault 100 Nothing     -- 100


withDefault "unknown" (Dict.get "Tom" Dict.empty)   -- "unknown"

Note: This can be overused! Many cases are better handled by a case expression. And if you end up using withDefault a lot, it can be a good sign that a custom type will clean your code up quite a bit!

Transform a Maybe value with a given function:

map sqrt (Just 9) == Just 3
map sqrt Nothing  == Nothing


map sqrt (String.toFloat "9") == Just 3
map sqrt (String.toFloat "x") == Nothing

Apply a function if all the arguments are Just a value.

map2 (+) (Just 3) (Just 4) == Just 7
map2 (+) (Just 3) Nothing == Nothing
map2 (+) Nothing (Just 4) == Nothing


map2 (+) (String.toInt "1") (String.toInt "123") == Just 124
map2 (+) (String.toInt "x") (String.toInt "123") == Nothing
map2 (+) (String.toInt "1") (String.toInt "1.3") == Nothing

Chain together many computations that may fail. It is helpful to see its definition:

andThen : (a -> Maybe b) -> Maybe a -> Maybe b
andThen callback maybe =
    case maybe of
        Just value ->
            callback value


        Nothing ->
            Nothing

This means we only continue with the callback if things are going well. For example, say you need to parse some user input as a month:

parseMonth : String -> Maybe Int
parseMonth userInput =
    String.toInt userInput
      |> andThen toValidMonth


toValidMonth : Int -> Maybe Int
toValidMonth month =
    if 1 <= month && month <= 12 then
        Just month
    else
        Nothing

In the parseMonth function, if String.toInt produces Nothing (because the userInput was not an integer) this entire chain of operations will short-circuit and result in Nothing. If toValidMonth results in Nothing, again the chain of computations will result in Nothing.