[m-dev.] Trouble with user-defined equality

[Ralph Becket]

Ralph Becket, Thursday, 15 April 2004:
> I'm having trouble making a little non-canonical type example work.
> [...]

I've answered my own question: here's the trick of it in case anyone was
wondering:

:- module noncan.
:- interface.

:- type ch.

:- implementation.
:- import_module bool.

:- type ch ---> z ; s(ch) where equality is eq.

:- pred eq(ch::in, ch::in) is semidet.

eq(A, B) :- promise_only_solution(eq2(A, B)) = yes.

:- pred eq2(ch::in, ch::in, bool::out) is cc_multi.

eq2(z,    z   , yes).
eq2(z,    s(_), no ).
eq2(s(_), z   , no ).
eq2(s(X), s(Y), Eq ) :- eq2(X, Y, Eq).

Essentially, any switch on a non-canonical type must be guaranteed to
succeed, in which case it has mode cc_multi and therefore cannot appear
in context that can fail.

-- Ralph
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