How can I simulate Haskell's “Either a b” in Java

Question

How can I write a typesafe Java method that returns either something of class a or something of class b? For example:

public ... either(boolean b) {
  if (b)         


        

Answer 1

There is a stand-alone implementation of Either for Java 8 in a small library, "ambivalence": http://github.com/poetix/ambivalence

It is closest to the Scala standard implementation - for example, it provides left and right projections for map and hashMap operations.

There is no direct access to the left or right values; rather, you join the two types by providing lambdas to map them into a single result type:

Either either1 = Either.ofLeft("foo");
Either either2 = Either.ofRight(23);
String result1 = either1.join(String::toUpperCase, Object::toString);
String result2 = either2.join(String::toUpperCase, Object::toString);

You can get it from Maven central:

    com.codepoetics
    ambivalence
    0.2