[mercury-users] Possible state var / uniqueness bug Re: Request for comments on this code

[Peter Moulder]

On Mon, Jul 10, 2006 at 04:18:59PM +1000, Julien Fischer wrote:
> 
> On Mon, 10 Jul 2006, Peter Moulder wrote:
> 
> > Is this a compiler bug?  Or how should tst2.m be written such that it
> > uses state-variable syntax for its io vars, and has an I/O call as the
> > condition of an if-then-else?
> 
> It's a compiler bug - you aren't supposed be allowed to do I/O in the
> condition of an if-then-else.

I don't see this restriction mentioned in (mercury_ref)Goals, or any of
the subsections of (mercury_ref)Unique modes.  I suggest that these are
the obvious places to look for a person wondering "is this valid Mercury
code".

Furthermore, it isn't clear to me why it should be forbidden (except
perhaps for stylistic reasons, at least if an alternative like `&&'
existed).  The compiler knows that the condition can't fail, so I
believe it's just a case of `CONDGOAL && THENGOAL', i.e. declaratively
equivalent to `CONDGOAL, THENGOAL'.

Btw, I haven't tested this code in current 0.13 betas; from memory,
Julien mentioned in a June 23 post to this list that these versions
include a fix for a mode analysis bug.

pjrm.
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