Create sum type in C implementation of OCaml function

Question

Let's say you had a type declaration:

type foo = Bar | Baz of int

How would you implement a C function to create a Baz? Let's say I declare it like this:

external create_baz : int -> foo = "create_baz"

Then I would need to fill out this code:

CAMLprim value create_baz(value bar) {
  // what do I do here?
}

I understand that this is a rather silly thing to do, but it's just and example of what I'm trying to do.

Answer 1

This is described in Chapter 19 of the OCaml manual .

Basically, constructors are numbered in order, in two separate sequences. Nullary constructors (those taking no values, like Bar ) are numbered in one sequence, and constructors that take a value (like Baz ) are numbered in a second sequence. So both of your constructors are numbered 0.

Nullary constructors are represented by immediate values (a simple bit pattern representing a value like an int). Constructors taking values are represented by pointers to blocks, which have fields that can store the contained values.

So, basically your function wants to make a block of size 1 with tag 0. bar is saved in the block's 0th field (the only field).

It looks something like this:

value create_baz(value bar) {
    // Caller guarantees that bar is an int.
    //
    CAMLparam1(bar);
    CAMLlocal1(result);
    result = caml_alloc(1, 0);
    Store_field(result, 0, bar);
    CAMLreturn(result);
}