In my language, types consist of two parts, say class and role. I can define the individual subclass and subrole relations, say <class: and <role: by storing instances using where store.

Now, I would like to define the combined subtype relation between two types (class1, role1) <: (class2, role2), which requires class1 <class: class2 and role1 <role: role2. How can I do that?

Asked by Michael on 12 August 2015 at 11:20

I think this should work:

(class1,role1) <: (class2,role2)
  where class1 <class: class2
    and role1 <role: role2

Answered by Jeff Smits on 12 August 2015 at 18:50

Thank you for your answer. I found more information about custom relation definitions on the TS bug tracker. Is this really the only “documentation” about this feature?..

Then, unfortunately, it doesn’t work. I got it running like this:

Assign(l, r) :-
    where (
        l : tl and tl => RoleType(rl, cl) and
        r : tr and tr => RoleType(rr, cr) and
        rr <role: rl and cr <class: cl
    )
        else error $[right-hand side type [tr] is not a subtype of [tl]] on r

but when I change it to this:

RoleType(r1, c1) <: RoleType(r2, c2)
    where r1 <role: r2 and c1 <class: c2

Assign(l, r) :-
    where (
        l : tl and
        r : tr and
        tr <: tl
    )
        else error $[right-hand side type [tr] is not a subtype of [tl]] on r

then I get subtyping errors everywhere.

Any ideas?

Answered by Michael on 13 August 2015 at 11:27

No idea. All I know is custom relations like this don’t always work, and I suspect that’s why they aren’t documented. It’s probably just work in progress.

Answered by Jeff Smits on 13 August 2015 at 17:14