Maybe.md

Module Elm.Maybe

This library fills a bunch of important niches in Elm. A Maybe can help you with optional arguments, error handling, and records with optional fields.

This is implemented in terms of Purescript's Data.Maybe, so you can use functions from there on Maybe values as well.

withDefault

withDefault :: forall a. a -> Maybe a -> a

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"

Equivalent to Purescript's 'fromMaybe`.

Re-exported from Data.Foldable:

oneOf

oneOf :: forall f g a. Foldable f => Plus g => f (g a) -> g a

Combines a collection of elements using the Alt operation.

Re-exported from Data.Maybe:

Maybe

data Maybe a
  = Nothing
  | Just a

The Maybe type is used to represent optional values and can be seen as something like a type-safe null, where Nothing is null and Just x is the non-null value x.

Instances

Functor Maybe
Apply Maybe
Applicative Maybe
Alt Maybe
Plus Maybe
Alternative Maybe
Bind Maybe
Monad Maybe
MonadZero Maybe
Extend Maybe
Invariant Maybe
(Semigroup a) => Semigroup (Maybe a)
(Semigroup a) => Monoid (Maybe a)
(Eq a) => Eq (Maybe a)
Eq1 Maybe
(Ord a) => Ord (Maybe a)
Ord1 Maybe
(Bounded a) => Bounded (Maybe a)
(Show a) => Show (Maybe a)

Re-exported from Elm.Apply:

map5

map5 :: forall w a b c d e f. Apply w => (a -> b -> c -> d -> e -> f) -> w a -> w b -> w c -> w d -> w e -> w f

Map a function of five arguments over some container type.

The equivalent of Purescript's lift5.

map4

map4 :: forall w a b c d e. Apply w => (a -> b -> c -> d -> e) -> w a -> w b -> w c -> w d -> w e

Map a function of four arguments over some container type.

The equivalent of Purescript's lift4.

map3

map3 :: forall w a b c d. Apply w => (a -> b -> c -> d) -> w a -> w b -> w c -> w d

Map a function of three arguments over some container type.

The equivalent of Purescript's lift3.

map2

map2 :: forall w a b c. Apply w => (a -> b -> c) -> w a -> w b -> w c

Map a function of two arguments over some container type.

The equivalent of Purescript's lift2.

Re-exported from Elm.Bind:

andThen

andThen :: forall m a b. Bind m => (a -> m b) -> m a -> m b

Given some computation, chain its result with another computation.

Equivalent to Purescript's bind.

The order of the arguments was flipped in Elm 0.18.

Re-exported from Prelude:

map

map :: forall a b f. Functor f => (a -> b) -> f a -> f b