[mercury-users] Beginner question

[Sandy Harris]

The boolean library module has:

:- type bool ---> no ; yes.

:- pred bool__or(bool, bool, bool).
:- mode bool__or(in, in, out) is det.

:- pred bool__and(bool, bool, bool).
:- mode bool__and(in, in, out) is det.

So far so good, but what other mode declarations could be added?
One could just say:

:- mode bool__or(in, out, in) is nondet.
:- mode bool__and(in, out, in) is nondet.

but that seems to lose some information. Can you instead say
something like:

:- mode bool__and(no, out, yes) is failure.
:- mode bool_or(yes, out, no) is failure.

since, in C notation, there is no x such that 0 & x == 1
or 1 | x == 0

and then have:

:- mode bool__and(yes, out, in) is det.
:- mode bool_or(no, out, in) is det.

since x & 1 == x and x | 0 == x for all x.

:- mode bool__and(no, out, no) is multi.
:- mode bool__or(yes, out, yes) is multi.

since 0 & x == 0 and 1 | x == 1 for any x.

Methinks adding this detail might help with complex inferences
about booleans, but I'm not sure how to do it or even if it is
possible.

XOR has more convenient modes:

:- pred bool__xor(bool, bool, bool).
:- mode bool__xor(in, in, out) is det.
:- mode bool__xor(in, out, in) is det.
:- mode bool__xor(out, in, in) is det.
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