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Result

Import Result for operations with an explicit success or error branch.

Type and constructors​

Result​

data Result e a = Err e | Ok a.

Result(e, a) contains an error of type e or a successful value of type a.

Err​

The error branch.

Ok​

The success branch. Both constructors are available to patterns.

Transforming​

resultMap​

resultMap :: (a -> b) -> Result(e, a) -> Result(e, b).

Transforms the value inside Ok and preserves Err unchanged.

resultMapError​

resultMapError :: (e -> f) -> Result(e, a) -> Result(f, a).

Transforms the value inside Err and preserves Ok unchanged.

resultAndThen​

resultAndThen :: (a -> Result(e, b)) -> Result(e, a) -> Result(e, b).

Calls the function for Ok and returns its result without nesting. Err skips the function and passes through.

resultRecover​

resultRecover :: (e -> Result(f, a)) -> Result(e, a) -> Result(f, a).

Calls the recovery function for Err. Ok skips recovery and keeps its value. The recovery may change the error type.

All transformation operations are O(1) apart from the callback.

Defaults and inspection​

resultWithDefault​

resultWithDefault :: a -> Result(e, a) -> a.

Uses the first argument only for Err.

resultIsOk​

resultIsOk :: Result(e, a) -> Bool.

resultIsErr​

resultIsErr :: Result(e, a) -> Bool.

Conversion​

resultToMaybe​

resultToMaybe :: Result(e, a) -> Maybe(a).

Converts Ok value to Just value and discards an error as Nothing.

resultErrorToMaybe​

resultErrorToMaybe :: Result(e, a) -> Maybe(e).

Converts Err error to Just error and discards a success as Nothing.

resultFromMaybe​

resultFromMaybe :: e -> Maybe(a) -> Result(e, a).

Converts Just value to Ok value. Nothing becomes Err containing the error supplied first.

Conversions are O(1). Use Maybe when absence needs no error value.