Types in Ocaml

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Xah Lee, 2009-03

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›)”.*)

(* examples: *)
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 call “type constructor”. A type constructor's first letter must be Capitalized.

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

(* above defines “t1” to be the name of a type that's just the symbol
“X”.  and “t2” is the name of the type of 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;;

Here are some examples: