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Maybe

Use Maybe when a value may be absent and the absent case needs no error information.

Type and constructors​

Maybe​

data Maybe a = Nothing | Just a.

Maybe(a) contains either no value or one value of type a.

Nothing​

The expected type determines the parameter of Nothing.

Just​

Both constructors are available to patterns.

Transforming​

maybeMap​

maybeMap :: (a -> b) -> Maybe(a) -> Maybe(b).

Calls the function for Just; Nothing passes through unchanged.

maybeAndThen​

maybeAndThen :: (a -> Maybe(b)) -> Maybe(a) -> Maybe(b).

Calls the function for Just without nesting its Maybe result. Nothing skips the function.

maybeFilter​

maybeFilter :: (a -> Bool) -> Maybe(a) -> Maybe(a).

Keeps a present value when the predicate returns True; otherwise returns Nothing. An absent value never calls the predicate.

These transformations are O(1) apart from callback work.

Defaults and alternatives​

maybeWithDefault​

maybeWithDefault :: a -> Maybe(a) -> a.

Returns the first argument only for Nothing.

maybeOrElse​

maybeOrElse :: Maybe(a) -> Maybe(a) -> Maybe(a).

Returns the second argument when it is Just; otherwise returns the fallback passed first.

Inspection​

maybeIsJust​

maybeIsJust :: Maybe(a) -> Bool.

maybeIsNothing​

maybeIsNothing :: Maybe(a) -> Bool.

Conversion​

maybeToList​

maybeToList :: Maybe(a) -> [a].

Maps Nothing to [] and Just value to [value].

maybeFromList​

maybeFromList :: [a] -> Maybe(a).

Uses the first list value, or returns Nothing for an empty list. The remainder is ignored.

Use Result instead when the absent branch should carry an error.