Conditional module aliasing

Question

For a class project I am writing a program that evaluates the performance of different implementations of the same abstract data structure. Since I am using identical code to test each of them, I would like to be able to set a module alias depending on user input and just run that module through the testing code.

In other words, I want something like:

let module M = 
  if model = "tree" then TreeModel else
  if model = "hash" then HashModel else
  ListModel
in ...

Is there a way I can make this work, or am I going about this all wrong?

score 13 · Accepted Answer · answered Apr 22 '12 at 19:39

There are no conditionals on the module level, but you can use first-class modules for this:

let m = match model with
  | "tree" -> (module TreeModel : MODEL)
  | "hash" -> (module HashModel : MODEL)
  | "list" -> (module ListModel : MODEL)
in let module M = (val m : MODEL)
in ...

score 0 · Answer 2 · answered Dec 05 '17 at 16:40

0

As a follow-up to @Andreas, at least in recent ocaml versions you can also write:

let (module M : MODEL) = match model with
  | "tree" -> (module TreeModel : MODEL)
  | "hash" -> (module HashModel : MODEL)
  | "list" -> (module ListModel : MODEL)
in ...

answered Dec 05 '17 at 16:40

ansiwen

1,061
8
13

and with this form, you don't have to write the module type information in all match branches: `(module M : MODEL)` is enough and will propagate. – lavi Dec 06 '17 at 09:14
1

Not here. I would get `Error: The signature for this packaged module couldn't be inferred.` without the type annotation. – ansiwen May 29 '18 at 09:39
may be depend on the version of your compiler, but for me this is working: `let (module M : MODEL) = match model with | "tree" -> (module TreeModel) ...` – lavi May 29 '18 at 11:04

Conditional module aliasing

2 Answers2