Question

Are there hashtables in Ocaml that use == instead of = when testing for equality of keys?
For example:

# type foo = A of int;;
# let a = A(1);;
# let b = A(1);;
# a == b;;
- : bool = false
# a = b;;
- : bool = true
# let h = Hashtbl.create 8;;
# Hashtbl.add h a 1;;
# Hashtbl.add h b 2;;
# Hashtbl.find h a;;
- : int = 2
# Hashtbl.find h b;;
- : int = 2

I’d like a hashtable that can distinguish between a and b. Is that possible?

Answer 1

You can use custom hashtables:

module H = Hashtbl.Make(struct
  type t = foo
  let equal = (==)
  let hash = Hashtbl.hash
end)

And then use H instead of Hashtbl in your code.