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);
}