OCaml cons (::) operator?

Question

In OCaml, is there a way to refer to the cons operator by itself?

For example, I can use (+) and ( * ) as int -> int -> int functions, but I cannot use (::) as a 'a -> 'a list -> 'a list function, as the following example show:

# (+) 3 5;;
- : int = 8
# ( * ) 4 6;;
- : int = 24
# (::) 1 [2;3;4];;
Error: Syntax error: operator expected.

Is there a way to produce a result like (::) other than with fun x y -> x::y? And does anyone know why (::) wasn't implemented in OCaml?

It would work if you surround the arguments with parenthesis, like so: `(::) (1, [2; 3; 4]);;` — dub stylee, Sep 21 '16 at 22:34

camlspotter · Answer 1 · 2014-07-04T04:55:44.783

16

Adding to the answer of @seanmcl,

Actually OCaml supports a prefix form of (::):

# (::)(1, []);;
- : int list = [1]

This is in the uncurried form, corresponding with the fact that all the OCaml variant constructors are not curried and cannot be partially applied. This is handled by a special parsing rule just for (::), which is why you got a rather strange error message Error: Syntax error: operator expected..

Update:

Upcoming OCaml 4.02 removes this parsing rule, therefore this is no longer available.

edited Jul 04 '14 at 04:55

answered Oct 21 '13 at 03:45

camlspotter

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This is still working in 4.11 so did it not get removed after all? – xyz Dec 21 '20 at 11:55

score 14 · Accepted Answer · answered Oct 20 '13 at 14:27

14

No. Cons (::) is a constructor, constructors can not be infix operators. The allowed infix symbols are here:

http://caml.inria.fr/pub/docs/manual-caml-light/node4.9.html

Some workarounds are (as you mention) the verbose

(fun x l -> x :: l)

and defining your own nontraditional infix cons

let (+:) x l = x :: l

answered Oct 20 '13 at 14:27

seanmcl

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For reference, the knowledge is introduced by [http://caml.inria.fr/pub/docs/manual-ocaml-4.00/expr.html#toc50] section Variants. – Gqqnbig Jan 23 '14 at 05:05

score 7 · Answer 3 · answered Aug 21 '18 at 10:38

7

As of Ocaml 4.03, you can now use cons (in the List module). That is, cons x xs is the same as x :: xs.

answered Aug 21 '18 at 10:38

John Wickerson

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score -1 · Answer 4 · answered Jun 19 '22 at 17:18

-1

It's also possible to just define your own cons function:

let cons = fun a list -> a :: list

answered Jun 19 '22 at 17:18

rithvik

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Welcome to SO! I think this answer is subsumed by that of @seanmcl (which shows that you can define your own function) and that of @JohnWickerson (which says that such a function named `cons` already exists in the standard library). – Maëlan Jun 23 '22 at 15:49
True, sorry to reiterate that's my bad – rithvik Jun 30 '22 at 00:46

OCaml cons (::) operator?

4 Answers4