Does != have meaning in OCaml?

Question

It seems to be an equivalency comparison for some types, but not strings.

# 3 != 3;;
- : bool = false
# 3 != 2;;
- : bool = true

This is as

Answer 1

A quick explanation about == and != in OCaml in addition to all the correct answers that have already been provided:

1/ == and != expose implementation details that you really don't want to know about. Example:

# let x = Some [] ;;
val x : 'a list option = Some []
# let t = Array.create 1 x ;;
val t : '_a list option array = [|Some []|]
# x == t.(0) ;;
- : bool = true

So far, so good: x and t.(0) are physically equal because t.(0) contains a pointer to the same block that x is pointing to. This is what basic knowledge of the implementation dictates. BUT:

# let x = 1.125 ;;
val x : float = 1.125
# let t = Array.create 1 x ;;
val t : float array = [|1.125|]
# x == t.(0) ;;
- : bool = false

What you are seeing here are the results of an otherwise useful optimization involving floats.

2/ On the other hand, there is a safe way to use ==, and that is as a quick but incomplete way to check for structural equality.

If you are writing an equality function on binary trees

let equal t1 t2 =
  match ...

checking t1 and t2 for physical equality is a quick way to detect that they are obviously structurally equal, without even having to recurse and read them. That is:

let equal t1 t2 =
  if t1 == t2
  then true
  else 
    match ...

And if you keep in mind that in OCaml the “boolean or” operator is “lazy”,

let equal t1 t1 =
  (t1 == t2) ||
  match ...