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.

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.

Syntactic dictionary

	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 double-precision
Type signatures	const :: a -> b -> a	const : 'a -> 'b -> 'a	Signatures usually omitted in OCaml implementations, may be provided separately in interface files
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 DList a = MkDList ([a] -> [a]) data Either a b = Left a | Right b	type 'a dlist = MkDList of ('a list -> 'a list) type ('a, 'b) either = Left of 'a | Right of 'b
Pattern matching	case x of B x | x > 0 -> ... | otherwise -> ... C a b -> ... case Left () of Left x -> x Right x -> x	match x with B x when x > 0 -> ... | B x -> ... | C (a, b) -> ... match Left () with Left x | Right x -> x