OCaml

OCaml is a functional programming language in the ML family, an extension of the Caml language with object-oriented constructs.

This page aims to cover some of its differences from Haskell.

Syntactic dictionary

Language	Haskell	OCaml	Comments
Anonymous functions	\x y -> ...	fun x y -> ...
Multiple assignments	let x = 4 y = 5 in ...	let x = 4 and y = 5 in ...
Types	Int, Bool, (Double, Char), a	int, bool, float * char, 'a	float is a double type
Type signatures	const :: a -> b -> a	const : 'a -> 'b -> 'a	Signatures usually omitted in OCaml
Type declarations	data A = B Int | C Char Bool x = B 3 y = C 'a' True	type a = B of int | C of char * bool let x = B 3 and y = C ('a', true)
Parametrised types	data D a = D (a -> a) data E a b = L a | R b	type 'a d = D of ('a -> 'a) type ('a, 'b) e = L of 'a | R of 'b
Pattern matching	case x of A x -> ... C a b -> ...	match x with B x -> ... C (a, b) -> ...

Conceptual differences

OCaml is strict by default, although it has some facility for introducing laziness.

OCaml's let is non-recursive by default, but has the form let rec for defining recursive functions.

OCaml is impure: although it makes heavy use of immutable data, it also has mutable references and arrays available, and IO is performed by ordinary functions.