OCaml: Hash Anything

Tasty

OCaml has a generic marshalling code within it, and it has MD5 Digest code.  In this post I will simply compose the two of them to achieve a hash function which works on any object (a handy magical thing to have):

 % ocaml
        Objective Caml version 3.10.2

 # Marshall.to_string;;
 - : 'a -> Marshal.extern_flags list -> string = <fun>
 # Digest.string;;
 - : string -> Digest.t = <fun>
 # Digest.to_hex;;
 - : Digest.t -> string = <fun>

Just entering the component functions you can see the types of each of them when they are entered into ocaml REPL (in fact, ocaml returns symbolname : type = value on each evaluation).  Note that ;; is needed to get it to evaluate since it takes multi-line input (OCaml does not require indentation for scoping -- a "feature" that some languages make use of):

 # let hash x =
    let m_x = Marshal.to_string x [] in
    let d_m_x = Digest.string m_x
    in  Digest.to_hex d_m_x;;
 val hash : 'a -> string = <fun>

Now we can use this hash on all kinds of things (with any type):

 # hash [1;2;3];;
 - : string = "64cb37afbe72effe97fb4f089a82f9b2"
 # hash 4.5966;;
 - : string = "5eed266efb8593218f1cb1cacf1f8d89"
 # hash "Ocaml rocks!";;
 - : string = "f078e0f37bce6c8a83112f9461bf7544"

It is almost too simple.  Writing a meta-programming monster hash function in C++ is much less enjoyable.  Some have criticized the OCaml library and its incompleteness (compared to Java perhaps) as a reason for its less wide adoption.  Now that OCaml-java is a project, that excuse has less weight. Hopefully some of the parametric polymorphism available to the OCaml bundled libraries finds its way into that project.  As usual, choosing the proper tool for any job is an important step in getting it done.