define.c

pry(Module):4> show-method define_method
--
From Ruby Core (C Method):
--
static VALUE
rb_mod_define_method(int argc, VALUE *argv, VALUE mod)
{
    ID id;
    VALUE body;
    int noex = NOEX_PUBLIC;

    if (argc == 1) {
    id = rb_to_id(argv[0]);
    body = rb_block_lambda();
    }
    else if (argc == 2) {
    id = rb_to_id(argv[0]);
    body = argv[1];
    if (!rb_obj_is_method(body) && !rb_obj_is_proc(body)) {
        rb_raise(rb_eTypeError,
             "wrong argument type %s (expected Proc/Method)",
             rb_obj_classname(body));
    }
    }
    else {
    rb_raise(rb_eArgError, "wrong number of arguments (%d for 1)", argc);
    }

    if (rb_obj_is_method(body)) {
    struct METHOD *method = (struct METHOD *)DATA_PTR(body);
    VALUE rclass = method->rclass;
    if (rclass != mod && !RTEST(rb_class_inherited_p(mod, rclass))) {
        if (FL_TEST(rclass, FL_SINGLETON)) {
        rb_raise(rb_eTypeError,
             "can't bind singleton method to a different class");
        }
        else {
        rb_raise(rb_eTypeError,
             "bind argument must be a subclass of %s",
             rb_class2name(rclass));
        }
    }
    rb_method_entry_set(mod, id, &method->me, noex);
    }
    else if (rb_obj_is_proc(body)) {
    rb_proc_t *proc;
    body = proc_dup(body);
    GetProcPtr(body, proc);
    if (BUILTIN_TYPE(proc->block.iseq) != T_NODE) {
        proc->block.iseq->defined_method_id = id;
        proc->block.iseq->klass = mod;
        proc->is_lambda = TRUE;
        proc->is_from_method = TRUE;
    }
    rb_add_method(mod, id, VM_METHOD_TYPE_BMETHOD, (void *)body, noex);
    }
    else {
    /* type error */
    rb_raise(rb_eTypeError, "wrong argument type (expected Proc/Method)");
    }

    return body;
}
=> nil