Yep, these are called "discriminated unions" in F#. I think most functional programming languages have something similar. The underlying concept is "algebraic

Yep, these are called "discriminated unions" in F#. I think most functional programming languages have something similar. The underlying concept is "algebraic data types": You can compose types as products (tuples or records) and sums (discriminated unions) of simpler types. From there, you can actually use algebra on types, which is pretty mind-blowing. For example, we can define a generic sum type called Either: Either a b = Left a | Right b You can think of Either as the sum of types "a" and "b": a + b. Tuples are product types, so you can use the distributive property of multiplication over addition to establish that: (a, Either b c) = Either (a, b) (a, c) Or algebraically: a * (b + c) = (a * b) + (a * c) This is one of those areas where category theory is actually useful for programmers.