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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
  x = 4
  y = 5
 in ...
let x = 4
and y = 5
 in ...
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