Picking which ocaml module to use with command line parameter

Question

In my code I've module M = Implementation1 and then I reference M, instead of Implementation1. The problem is, I've to recompile my program to change Implementation1 to Implementation2. I'd like to control which implementation to use from with a command line parameter. Is that possible?

Is the situation simpler, when all implementations share signature?

Your question is not clear enough. Are you dealing with only two implementations known statically, in which case you should look for [first class modules](http://caml.inria.fr/pub/docs/manual-ocaml-400/manual021.html#toc81), or with implementations that may be added later in which case you need to look for [dynamic linking](http://caml.inria.fr/pub/docs/manual-ocaml/libref/Dynlink.html). — Daniel Bünzli, Jan 13 '14 at 15:08

score 7 · Accepted Answer · answered Jan 14 '14 at 01:41

Since both implementation are known statically you can use first class modules. There are quite a few different possibilities on how to structure your program, here's one that minimizes toplevel reference cells and toplevel effectful statements:

module type M = sig
  val test : unit -> unit
end

module M1 : M = struct
  let test () = Printf.printf "Implementation 1\n%!" 
end

module M2 : M = struct
  let test () = Printf.printf "Implementation 2\n%!"
end

let test m = 
  let module M = (val m : M) in 
  (* If other modules of your program depend on the implementation of 
     M, functorize them over sig M and instantiate them with M here. *)
  M.test ()

let main () = 
  let exec = Filename.basename Sys.executable_name in 
  let usage = Printf.sprintf
      "Usage: %s [OPTION]...\n\
       Program synopsis.\n\
       Options:" exec 
  in
  let m = ref (module M1 : M) in
  let options = [
    "-m1", Arg.Unit (fun () -> m := (module M1 : M)), 
    " Use implementation 1 (default)"; 
    "-m2", Arg.Unit (fun () -> m := (module M2 : M)),
    " Use implementation 2"; ]
  in
  let anon _ = raise (Arg.Bad "no positional argument supported") in 
  Arg.parse (Arg.align options) anon usage; 
  test !m

let () = main ()

nlucaroni · Answer 2 · 2014-01-13T16:33:34.930

For most versions of OCaml you can do this with camlp4.

IFDEF USE_IMP1 THEN
  module M = Implementation1
ELSE
  module M = Implementation2
END

Then passing the pre-processing options like the following will choose the correct one to incorporate in the build.

-pp "camlp4of -UUSE_IMP1" //to undefine USE_IMP1
-pp "camlp4of -DUSE_IMP1" //to define USE_IMP2

For versions of OCaml >= 4.00.0 you can use the First-Class modules and an expression like,

module Choose = (val (if use_impl1 then (module Impl_1) else (module Impl_2)) : IMP)

to use an if statement to choose the module as determined by the use_impl1 value.

Picking which ocaml module to use with command line parameter

2 Answers2

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