OCaml Variants and Algebraic Data Types

1. Variants

A variant is a data type representing a value that is one of several possibilities. At their simplest, variants are like enums.

  type day = Sun | Mon | Tue | Wed | Thu | Fri | Sat

  let d = Tue

The individual names of the values of a variants are called constructors in OCaml. We can perform pattern matching on variants.

  let int_of_day d =
    match d with
    | Sun -> 1
    | Mon -> 2
    | Tue -> 3
    | Wed -> 4
    | Thu -> 5
    | Fri -> 6
    | Sat -> 7

Suppose there are 2 types defined, with overlapped constructor names.

  type T1 = C | D
  type T2 = D | E

  let x = D

Then, the type defined later wins, which means that, variable x is of type T2.