How was `val hash : 'a -> int` was implemented in OCaml?

Question

In OCaml, Hashtbl can hash any thing to an int

Hashtbl.hash x associates a nonnegative integer to any value of any type. It is guaranteed that if x = y or Pervasives.compare xy = 0, then hash x = hash y. Moreover, hash always terminates, even on cyclic structures.

I mean, in Java , we have hashCode() for every object which returns an integer and Java's Hashtable can hash that integer.

But how did OCaml achieve that to hash anything?

Answer 1

It's not tricky. Hashtbl.hash just traverses data in a manner similar to the way the garbage collector does it. It travels a fixed distance into the linked structure, which avoids failing when there are cycles. It doesn't know anything about the high-level types of what it encounters, it just hashes the primitive values it reaches.

You can see the code in byterun/hash.c in the OCaml source distribution.

Update

It occurs to me you might have been asking how Hashtbl.hash was implemented in OCaml . The answer is that it can't be implemented in OCaml (without cheating) because it violates parametricity. The only possible pure functions of type 'a -> int are ones that return a constant value. The intuition is that such a function can't use any information about its argument because it is defined for all possible types.

Hashtbl.hash is one of a few OCaml functions that violate parametricity. They exist in OCaml because they're extremely handy. Another notorious one is the polymorphic comparison compare (and the associated = operator).