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?
Answer
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.