Question

Is it possible to store different types in the same hashtable (Hashtbl) in Ocaml? Are hashtables really restricted to just one type?

Answer 1

Yes, hash tables entries are restricted to one type for each table. This is really a question about the OCaml type sytem and not about hash tables. If it seems odd to require things to be the same type in a hash table, how about in a list?

Without knowing the problem you’re solving, it’s hard to know what to suggest. However, a common thing to do is to create an algebraic type that has one variant for each of the types you’re dealing with:

type alg = A of int | B of float

A value of type (string, alg) Hashtbl.t would store ints and floats, using a string as the lookup key.

# let ht = Hashtbl.create 44;;
val ht : ('_a, '_b) Hashtbl.t = <abstr>
# Hashtbl.add ht "yes" (A 3);;
- : unit = ()
# Hashtbl.add ht "no" (B 1.7);;
- : unit = ()
# ht;;
- : (string, alg) Hashtbl.t = <abstr>
# Hashtbl.find ht "yes";;
- : alg = A 3

After you get used to the flexible and strong typing of OCaml, it’s hard to go back to systems without it.