OCaml boolean expression [[]] == [[]]

Question

I have a function that return [[]], and I want to test the result as unit test. But I found that the expression [[]] == [[]] return false. Here a simple test code:

# [[]] == [[]];;
- : bool = false

Can someone explain me why this expression is evaluated as false?

Thanks.

Answer 1

Use = since you have structural equality for comparing two values:

# [[]] = [[]];;
- : bool = true

Because == is reference equality, it only returns true if you refer to the same memory location:

let a = [[]]
let b = a

# b == a;;
- : bool = true

Answer 2

The == operator in OCaml means "physical equality". However, you have two (physically) different lists. Probably, you want "structural equality", which is tested by =.