marshalling a hash table in ocaml

Question

I have a hash table in ocaml and I want to store this entire hash table as value field in a Berkeley DB. So I am trying to Marshal the hash table using Marshal.to_string. This returns a string but when I try to unmarshal the same string using Marshal.from_string, an Exception is thrown. Any ideas on what the issue here is?

Answer 1

You have to annotate the type of the value you're unmarshaling. Like so (in top-level):

type t = (string, string) Hashtbl.t;;

let key = "key" in
let t_original : t = Hashtbl.create 1 in
Hashtbl.add t_original key "value";
let t_marshalled = Marshal.to_string t_original [] in
let t_unmarshalled : t = Marshal.from_string t_marshalled 0 in
assert ((Hashtbl.find t_original key) = (Hashtbl.find t_unmarshalled key));;