[mercury-users] Unique mode problem: what's the difference?

[Fergus Henderson]

On 16-Nov-1999, Ralph Becket <rbeck at microsoft.com> wrote:
> I have a bug in some code using mostly unique modes (it's a term
> unifier.)  I'm not entirely sure what is going wrong...
> unify(T1, free, _T2, functor(Name2, Arity2, Args2)) -->
>         \+ occurs(T1, Args2),
>         tr_store__set_mutvar(T1, functor(Name2, Arity2, Args2)).
> 
> I get the following error report
...
> logic.m:077: In clause for `unify(in, in, in, in, mdi, muo)':
> logic.m:077:   in argument 3 of call to predicate `logic:occurs/4':
> logic.m:077:   unique-mode error: the called procedure would clobber
> logic.m:077:   its argument, but variable `DCG_0' is still live.

At first glance, that one looks like a bug in the compiler's
mode analysis of mostly-unique modes.  The compiler is being
overly conservative in its analysis.

> At first blush it seems DCG0 is being destroyed in the condition of the if
> by the call to occurs/4 -

Yes, that's what the compiler thinks...

> but that occurs in a negated context, so surely it
> isn't destroyed?

You are correct, and the compiler ought to take this into account.

> If I rewrite the offending clause to be
> 
> unify(T1, free, _T2, functor(Name2, Arity2, Args2)) -->
>         ( if occurs(T1, Args) then
>                 { fail }
>           else
>                 tr_store__set_mutvar(T1, functor(Name2, Arity2, Args2))
>         ).
> 
> then the error message goes away.

Great -- in that case, please use that as a work-around for now.

P.S.
I've managed to reproduce the problem here, so there is no need
for you to send a bug report.

-- 
Fergus Henderson <fjh at cs.mu.oz.au>  |  "I have always known that the pursuit
WWW: <http://www.cs.mu.oz.au/~fjh>  |  of excellence is a lethal habit"
PGP: finger fjh at 128.250.37.3        |     -- the last words of T. S. Garp.
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