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's let is non-recursive and strict by default, but has keywords rec (as in let rec) and lazy to introduce the Haskell behaviour.

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.