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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).

Examples:

The type Either<String, Int> is the type of values which can be either a String or an Int. The left constructor can be used only on Strings, and the right constructor can be used only on Ints:

>>> let s: Either<String, Int> = .left("foo")
>>> s
s: Either<String, Int> = left {
    left = "foo"
}
>>> let n: Either<String, Int> = .right(3)
>>> n
n: Either<String, Int> = right {
    right = 3
}
>>> type(of: s)
Either<String, Int>.Type = Either<String, Int>
>>> type(of: n)
Either<String, Int>.Type = Either<String, Int>

The map(_:) from our Functor instance will ignore left values, but will apply the supplied function to values contained in a right:

>>> let s: Either<String, Int> = .left("foo")
>>> let n: Either<String, Int> = .right(3)
>>> s.map { $0 * 2 }
Either<String, Int> = left {
    left = "foo"
}
>>> n.map { $0 * 2 }
Either<String, Int> = right {
    right = 6
}

See more

public enum Either<L, R>