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 expected.

# "odp" = "odp";; - : bool = true # "odp" != "odp";; - : bool = true # "odp" <> "odp";; - : bool = false 

Why does "odp" != "odp" evaluate to true? What is it actually doing? Shouldn't it generate a type error?

Answer 1

you have experienced the difference between structural and physical equality.

<> is to = (structural equality) as != is to == (physical equality)

"odg" = "odg"  (* true  *) "odg" == "odg" (* false *) 

is false because each is instantiated in different memory locations, doing:

let v = "odg" v == v (* true *) v = v  (* true *) 

Most of the time you'll want to use = and <>.

edit about when structural and physical equality are equivalent:

You can use the what_is_it function and find out all the types that would be equal both structurally and physically. As mentioned in the comments below, and in the linked article, characters, integers, unit, empty list, and some instances of variant types will have this property.

Answer 2

The opposite for != operator is == operator, not the = one.

# "a" != "a" ;; - : bool = true # "a" == "a" ;; - : bool = false 

The == operator is a "physical equality". When you type "a" == "a", you compare two different instances of strings that happen to look alike, so the operator returns false. While having one instance makes it return true:

# let str = "a"   in str == str ;; - : bool = true # let str = "a"   in str != str ;; - : bool = false