Types in Ocaml

By Xah Lee. Date: . Last updated: .

Defining Types

Explicit Declaration of Type

You can explicit say what's your function input's type. Usually you don't need to, because ocaml can correctly infer it.

(* A function f, with input n, of type int *)
let f (n:int) = n + 1;;

You can explicitly put type info on any variable in a expression, using this syntax var_name:type_literal.

let x = 5;;
(x:int) + (3:int);;                     (* ⇒ 8 *)

Defining Your Own Type

Remember, Ocaml has these built-in simple types: int, float, bool, char, string, unit.

You can define your own type, by assigning a type expression to a name. The syntax is this: type name = type_expression.

The simplest type is just a arbitrary letter sequence, which is called “type constructor”. A type constructor's first letter must be Capitalized.

(* example of defining types. *)
type t1 = X;;
type t2 = Alice;;

(* above defines a type named “t1”, and values of this type is just the symbol
“X”. And “t2” another type. The items belonging to this type is just the symbol “Alice”.  *)

X;;                                     (* evaluate X *)

Type expression can use the operator |, which means “alternative”, “one of”, or “or”.

type myGirls = Many | Jane | Alice ;;
type mySign = Positive | Negative | Zero;;
type testResult = PassTest | FailTest | Undecided;;