Question

I am reading through OCaml lead designer’s 1994 paper on modules, types, and separate compilation. (kindly pointed to me by Norman Ramsey in another question ). I understand that the paper discusses the origins of OCaml’s present module type / signature system. It it, the author proposes opaque interpretation of type declarations in signatures (to allow separate compilation) together with manifest type declarations (for expressiveness). Attempting to put together some examples of my own to demonstrate the kind of problems the OCaml module signature notation is trying to tackle I wrote the following code in two files:

In file ordering.ml (or .mli — I’ve tried both) (file A):

module type ORDERING = sig
 type t
 val isLess : t -> t -> bool
end

and in file useOrdering.ml (file B):

open Ordering
module StringOrdering : ORDERING
  let main () =
    Printf.printf "%b" StringOrdering.isLess "a" "b"
  main ()

The idea being to expect the compiler to complain (when compiling the second file) that not enough type information is available on module StringOrdering to typecheck the StringOrdering.isLess application (and thus motivate the need for the with type syntax).
However, although file A compiles as expected, file B causes the 3.11.2 ocamlc to complain for a syntax error. I understood that signatures were meant to allow someone to write code based on the module signature, without access to the implementation (the module structure).

I confess that I am not sure about the syntax: module A : B which I encountered in this rather old paper on separate compilation but it makes me wonder whether such or similar syntax exists (without involving functors) to allow someone to write code based only on the module type, with the actual module structure provided at linking time, similar to how one can use *.h and *.c files in C/C++. Without such an ability it would seem to be that module types / signatures are basically for sealing / hiding the internals of modules or more explicit type checking / annotations but not for separate / independent compilation.

Actually, looking at the OCaml manual section on modules and separate compilation it seems that my analogy with C compilation units is broken because the OCaml manual defines the OCaml compilation unit to be the A.ml and A.mli duo, whereas in C/C++ the .h files are pasted to the compilation unit of any importing .c file.

Answer 1

The right way to do such a thing is to do the following:

1. In ordering.mli write:

   (* This define the signature *)
   module type ORDERING = sig
     type t
     val isLess : t -> t -> bool
   end
   
   (* This define a module having ORDERING as signature *)
    module StringOrdering : ORDERING
   

2. Compile the file: ocamlc -c ordering.mli

3. In another file, refer to the compiled signature:

   open Ordering
   
   let main () =
     Printf.printf "%b" (StringOrdering.isLess "a" "b")
   
   let () = main ()
   

   When you compile the file, you get the expected type error (ie. string is not compatible with Ordering.StringOrdering.t). If you want to remove the type error, you should add the with type t = string constraint to the definition of StringOrdering in ordering.mli.

So answer to you second question: yes, in bytecode mode the compiler just needs to know about the interfaces your are depending on, and you can choose which implementation to use at link time. By default, that’s not true for native code compilation (because of inter-module optimizations) but you can disable it.