Why is OCaml's (+) not polymorphic?

Question

I am an OCaml newbie. I like OCaml\'s speed but I don\'t fully understand its design. For example, I would like the + operator to be polymorphic to support inte

Answer 1

Basically, the type systems of SML and OCaml (ignoring the object system and modules) do not support ad hoc polymorphism. This is a design decision. OCaml also, unlike SML, decided against including syntactic sugar for arithmetic.

Some languages in the ML family have extremely limited forms of ad hoc polymorphism in numeric operators. For instance, (+) in Standard ML has a default type of (int, int) -> int, but has type (float, float) -> float if its argument or return type is known to be float. These operators are special in SML, and could not be defined if they were not already built in. It also isn't possible to endow other values with this property. val add = (+) would have type (int, int) -> int. The specialness here is confined to the syntax of the language, there is no support for ad hoc polymorphism in the type system.

OCaml has a handful of operators with special semantics (but numeric operators are not among them), for instance || and && are short-circuit (but become long-circuit if you assign them to an intermediate value)

let long_circuit_or = (||);;
let print_true x = print_string x; true;;
(* just prints "4" *)
print_true "4" || print "5";;
(* prints "45" *)
long_circuit_or (print_true "4") (print_true "5");;