[mercury-users] inferred determinism

[Simon Taylor]

On 27-Feb-2002, Mark Brown <dougl at cs.mu.OZ.AU> wrote:
> > On Tue, Feb 26, 2002 at 04:56:43PM EST, Fergus Henderson wrote:
> > > On 26-Feb-2002, Michael Day <mikeday at bigpond.net.au> wrote:
> > > > 
> > > > The compiler infers a determinism of nondet for the predicate foo below,
> > > > unless the call to `zero' in the first clause is replaced by the literal
> > > > value 0. But it's an output argument, so why should it care?
> > > > 
> > > > :- pred foo(int, list(T)).
> > > > :- mode foo(out, in) is det.
> > > > 
> > > > foo(zero, []).
> > > > foo(N, [_|Xs]) :- foo(N0, Xs), N = N0+1.
> > > > 
> > > > :- func zero = int.
> > > > zero = 0.
> > > 
> > > Switch detection will not look past function calls.
> > > It only looks for unifications that occur before the
> > > first function call or predicate call in the clause.
> > > (One reason for this is that moving unifications across function
> > > calls could change whether or not the program terminates;
> > > e.g. consider the case where `zero' loops.)
> > 
> > This argument only applies in the case of the strict sequential model.
> 
> The switch detection rules are used to determine whether or not a Mercury
> program is legal, and the legality of a Mercury program should not depend
> on which operational semantics is being used.  Hence that argument is
> still valid.
> 
> Having said that, perhaps the definition of "minimally reordered" (in
> the 'Semantics' chapter of the reference manual) could be changed to
> accommodate this?  For example, in addition to the other reorderings
> allowed in the strict sequential model, we could allow function calls to
> be delayed until after test unifications if this is necessary for switch
> detection.  Then the argument above would no longer apply, although
> there may yet be other reasons not to allow such switches.

This was discussed late last year, it just hasn't been implemented yet.
http://www.cs.mu.oz.au/research/mercury/mailing-lists/mercury-developers/mercury-developers.0112/0063.html

Simon.
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