Data.Either

The Either type represents values with two possibilities: a value of type Either a b is either Left a or Right b. The Either type is sometimes used to represent a value which is either correct or an error; by convention, the Left constructor is used to hold an error value and the Right constructor is used to hold a correct value (mnemonic: “right” also means “correct”).

Constructors

• Data.Either.Left x
• Data.Either.Right x

Operations

either :: (Either a b, Func, Func) -> c

Case analysis for the Either type. If the value is Left a, apply the first function to a; if it is Right b, apply the second function to b.

<?php
  Either (null) -> either (
    # Left a
    function ($a) {
      return Int (0);
    },
    # Right b
    function ($b) {
      return Int ($b);
    });

isLeft :: Either a b -> Bool

Return Bool (True) if the given value is a Left-value, Bool (False) otherwise.

<?php
  Either (null) -> isLeft (); # Bool (True)

isRight :: Either a b -> Bool

Return Bool (True) if the given value is a Left-value, Bool (False) otherwise.

<?php
  Either (Int (10)) -> isRight (); # Bool (True)