[mercury-users] infinite recursion

[Ralph Becket]

> From: Michael Day [mailto:mcda at students.cs.mu.oz.au]
> 
> :- pred a.
> :- mode a is semidet.
> 
> a :- a.
> 
> Operationally, this is an infinite loop, which the compiler 
> is kind enough to warn about. Logically, should a/0 fail?

No: translating the above clause into FOL we have a <= a, and
(a <= a)  <=>  (~a \/ a)  <=>  true

So a/0 should be det and true.  Detecting this is, in general,
undecidable.

Ralph
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