Either

Either represents a value of two possible types: Left or Right. Example: compute() function, which results either in an integer value (in the case of success) or an error message of type string (in the case of failure).

/** @var $value Either<string,int> */

$value = compute().map(fn($i) => $i*2);

If the result of compute() is 1, the value is Right(2). If the result of compute() is error, the value is Left('error').

By convention the success case is Right and the failure is Left.

Projecting Left Either to a Right have no effect on the left value. Similarly, projecting Right Either to a 'Left` have no effect one the right value.

Construction

There are two named constructors available: left and right

$either = Either::left('error');

$either = Either::right('success');