fn: add new Either[L, R] type

This implements a union of two values. Uses the existing option under
the hood, and adds a simple helper function.

fn/either.go

@@ -0,0 +1,48 @@
+package fn
+
+// Either is a type that can be either left or right.
+type Either[L any, R any] struct {
+	left  Option[L]
+	right Option[R]
+}
+
+// NewLeft returns an Either with a left value.
+func NewLeft[L any, R any](l L) Either[L, R] {
+	return Either[L, R]{left: Some(l), right: None[R]()}
+}
+
+// NewRight returns an Either with a right value.
+func NewRight[L any, R any](r R) Either[L, R] {
+	return Either[L, R]{left: None[L](), right: Some(r)}
+}
+
+// WhenLeft executes the given function if the Either is left.
+func (e Either[L, R]) WhenLeft(f func(L)) {
+	e.left.WhenSome(f)
+}
+
+// WhenRight executes the given function if the Either is right.
+func (e Either[L, R]) WhenRight(f func(R)) {
+	e.right.WhenSome(f)
+}
+
+// IsLeft returns true if the Either is left.
+func (e Either[L, R]) IsLeft() bool {
+	return e.left.IsSome()
+}
+
+// IsRight returns true if the Either is right.
+func (e Either[L, R]) IsRight() bool {
+	return e.right.IsSome()
+}
+
+// MapLeft maps the left value of the Either to a new value.
+func MapLeft[L any, R any, O any](f func(L) O) func(Either[L, R]) Option[O] {
+	return func(e Either[L, R]) Option[O] {
+		if e.IsLeft() {
+			return MapOption(f)(e.left)
+		}
+
+		return None[O]()
+	}
+}