Data.Either.Compat

base-compat-0.12.1: A compatibility layer for base

Safe Haskell | Safe-Inferred
Language | Haskell2010

Data.Either.Compat

Synopsis

- module Data.Either
- isLeft :: Either a b -> Bool
- isRight :: Either a b -> Bool
- fromLeft :: a -> Either a b -> a
- fromRight :: b -> Either a b -> b

# Documentation

module Data.Either

isLeft :: Either a b -> Bool #

Return `True` if the given value is a `Left`-value, `False` otherwise.

#### Examples

Expand

Basic usage:

    >>> isLeft (Left "foo")
    True
    >>> isLeft (Right 3)
    False

Assuming a `Left` value signifies some sort of error, we can use `isLeft` to write a very simple error-reporting function that does absolutely nothing in the case of success, and outputs "ERROR" if any error occurred.

This example shows how `isLeft` might be used to avoid pattern matching when one does not care about the value contained in the constructor:

    >>> import Control.Monad ( when )
    >>> let report e = when (isLeft e) $ putStrLn "ERROR"
    >>> report (Right 1)
    >>> report (Left "parse error")
    ERROR

Since: base-4.7.0.0

isRight :: Either a b -> Bool #

Return `True` if the given value is a `Right`-value, `False` otherwise.

#### Examples

Expand

Basic usage:

    >>> isRight (Left "foo")
    False
    >>> isRight (Right 3)
    True

Assuming a `Left` value signifies some sort of error, we can use `isRight` to write a very simple reporting function that only outputs "SUCCESS" when a computation has succeeded.

This example shows how `isRight` might be used to avoid pattern matching when one does not care about the value contained in the constructor:

    >>> import Control.Monad ( when )
    >>> let report e = when (isRight e) $ putStrLn "SUCCESS"
    >>> report (Left "parse error")
    >>> report (Right 1)
    SUCCESS

Since: base-4.7.0.0

fromLeft :: a -> Either a b -> a #

Return the contents of a `Left`-value or a default value otherwise.

#### Examples

Expand

Basic usage:

    >>> fromLeft 1 (Left 3)
    3
    >>> fromLeft 1 (Right "foo")
    1

Since: base-4.10.0.0

fromRight :: b -> Either a b -> b #

Return the contents of a `Right`-value or a default value otherwise.

#### Examples

Expand

Basic usage:

    >>> fromRight 1 (Right 3)
    3
    >>> fromRight 1 (Left "foo")
    1

Since: base-4.10.0.0