Result.Extra

Convenience functions for working with Result.

Common Helpers

isOk : Result e a -> Bool

Check whether the result is Ok without unwrapping it.

isErr : Result e a -> Bool

Check whether the result is Err without unwrapping it.

extract : (e -> a) -> Result e a -> a

Turn a Result e a to an a, by applying the conversion function specified to the e.

unwrap : b -> (a -> b) -> Result e a -> b

Convert a Result e a to a b by applying a function if the Result is Ok or using the provided default value if it is an Err.

unpack : (e -> b) -> (a -> b) -> Result e a -> b

Convert a Result e a to a b by applying either the first function if the Result is an Err or the second function if the Result is Ok. Both of these functions must return the same type.

mapBoth : (e -> f) -> (a -> b) -> Result e a -> Result f b

Apply the first argument function to an Err and the second argument function to an Ok of a Result.

combine : List (Result x a) -> Result x (List a)

Combine a list of results into a single result (holding a list).

merge : Result a a -> a

Eliminate Result when error and success have been mapped to the same type, such as a message type.

merge (Ok 4)   == 4
merge (Err -1) == -1

More pragmatically:

type Msg
    = UserTypedInt Int
    | UserInputError String


msgFromInput : String -> Msg
msgFromInput =
    String.toInt
    >> Result.mapError UserInputError
    >> Result.map UserTypedInt
    >> Result.Extra.merge

Applying

singleton : a -> Result e a

Create a singleton from a value to an Result with a Ok of the same type. Also known as pure. You can use the Err constructor for a singleton of the Err variety.

singleton 2 == Ok 2

andMap : Result e a -> Result e (a -> b) -> Result e b

Apply the function that is inside Result to a value that is inside Result. Return the result inside Result. If one of the Result arguments is Err e, return Err e. Also known as apply.

Err "Oh" |> andMap (Err "No!")   == Err "Oh"
Err "Oh" |> andMap (Ok 2)        == Err "Oh"
Ok ((+) 1) |> andMap (Err "No!") == Err "No!"
Ok ((+) 1) |> andMap (Ok 2)      == Ok 3

Alternatives

or : Result e a -> Result e a -> Result e a

Like the Boolean || this will return the first value that is positive (Ok). However, unlike with ||, both values will be computed anyway (there is no short-circuiting).

or (Ok 4)      (Ok 5)      == Ok 4
or (Err "Oh!") (Ok 5)      == Ok 5
or (Ok 4)      (Err "No!") == Ok 4
or (Err "Oh!") (Err "No!") == Err "No!"

As the last example line shows, the second error is returned if both results are erroneous.

orLazy : Result e a -> (() -> Result e a) -> Result e a

Non-strict version of or. The second argument will only be evaluated if the first argument is an Err.

orElseLazy : (() -> Result e a) -> Result e a -> Result e a

Piping-friendly version of orLazy. The first argument will only be evaluated if the second argument is an Err. Example use:

String.toInt "Hello"
|> orElseLazy (\() -> String.toInt "42")

orElse : Result e a -> Result e a -> Result e a

Strict version of orElseLazy (and at the same time, piping-friendly version of or).

orElse (Ok 4)      (Ok 5)      == Ok 5  -- crucial difference from `or`
orElse (Err "Oh!") (Ok 5)      == Ok 5
orElse (Ok 4)      (Err "No!") == Ok 4
orElse (Err "Oh!") (Err "No!") == Err "Oh!"  -- also different from `or`

Also:

String.toInt "Hello"
|> orElse (String.toInt "42")

Modules

Result.Extra

elm-community/result-extra/2.2.0

Result.Extra

Common Helpers

Applying

Alternatives

Modules