Union types and Intersection types

Question

What are the various use cases for union types and intersection types? There has been lately a lot of buzz about these type system features, yet somehow I have never felt ne

Answer 1

Union types are useful for typing dynamic languages or otherwise allowing more flexibility in the types passed around than most static languages allow. For example, consider this:

var a;
if (condition) {
  a = "string";
} else {
  a = 123;
}

If you have union types, it's easy to type a as int | string.

One use for intersection types is to describe an object that implements multiple interfaces. For example, C# allows multiple interface constraints on generics:

interface IFoo {
  void Foo();
}

interface IBar {
  void Bar();
}

void Method(T arg) where T : IFoo, IBar {
  arg.Foo();
  arg.Bar();
}

Here, arg's type is the intersection of IFoo and IBar. Using that, the type-checker knows both Foo() and Bar() are valid methods on it.