[q-lang-users] Subtypes and Interfaces

[Rob H. <hub...@gm...>]

Hello Albert,

This is probably not a good time to pose questions unrelated to the
imminent 7.7 release. Sorry.

Suppose I have (parametrised) types

    public type T1 = private const nil F;
    public type T2 : T1; // a variant with special meaning

I can define constructors and tests for T1:

    public empty_t1 F;
    public is_t1 T;

    empty_t1 F = nil F;
    is_t1 _:T1 = true;
    is_t1 _ = false;

Now I wish to define constructors and tests for T2:

    public empty_t2 F;
    public is_t2 T;
    private adjust F;

    adjust F = (F . fst); // for example
    empty_t2 F = empty_t1 (adjust F); // *this is not quite right*
    is_t2 _:T2 = true;
    is_t2 _ = false;

Thus a T2 is also a T1, but the constructor makes some adjustments to
the interface.

My question is: is there a way to "cast" the result of the constructor
empty_t2 so that the object returned is a T2?

What I want is for these both to return true:

    is_t1 (empty_t2 my_func);
    is_t2 (empty_t2 my_func);

At the moment only the first does.

I want to take advantage of the fact that all the T1 code will work on
T2 objects without having to repeat all the T1 code, and I don't think
I want to wrap the T1 within T2 (in C++ terminology, I want T2 to
inherit from T1 rather than to have a T1 member datum; so T2 "is a"
T1, but not T2 "has a" T1).

I can't write this:

    public type T1 = private const nil F;
    public type T2 : T1 = private const nil F;

(in the same file) as this is a conflict for nil.

Thanks,
Rob.