base-compat-0.8.2: A compatibility layer for base

Safe Haskell | Safe
Language | Haskell98

Data.Either.Compat

Synopsis

- isLeft :: Either a b -> Bool
- isRight :: Either a b -> Bool

# Documentation

isLeft :: Either a b -> Bool

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

#### Examples

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: 4.7.0.0

isRight :: Either a b -> Bool

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

#### Examples

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: 4.7.0.0