[mercury-users] semidet vs unique updating

[Ralph Becket]

Michael Day, Wednesday, 17 October 2001:
> 
> > A better solution would be to simply return an extra boolean result
> > indicating success or failure.
> 
> That is:
> 
> :- pred queue__get(queue(T), maybe(T), queue(T)).
> :- mode queue__get(di, uo, uo).
> 
> Quite possible. I've only been kicking and screaming so much because it
> seems that I should be able to mix unique objects with semidet code. The
> declaration above is lying, or at least misleading, in that the queue
> appears to be modified regardless of the outcome, and that if you called
> it enough times it might return yes(T), when it never will, and making it
> semidet would make that point quite clear. And would allow it to be used
> in if statements, dammit.

What you really want in this case is

:- func queue__get(queue(T)) = maybe(T).
:- mode queue__get(ui) = out is det.

which could be used in if-then-elses.

I'm sure ui modes will work sooner or later.

- Ralph
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