[mercury-users] Unique and mostly-unique modes

[Ralph Becket]

Peter Hawkins, Thursday,  8 September 2005:
> 
> Making Mercury/cs/test.c
> test.m:015: In clause for `read(out, mdi, muo)':
> test.m:015:   mode error: argument 3 had the wrong instantiatedness.
> test.m:015:   Final instantiatedness of `HeadVar__3' was
> test.m:015:   `unique(test.socket(ground))',
> test.m:015:   expected final instantiatedness was `mostly_unique'.
> 
> Surely something unique qualifies as being mostly_unique as well?

I would have thought so.

> test.m:018: In clause for `write(in, di, uo)':
> test.m:018:   mode error: argument 3 did not get sufficiently instantiated.
> test.m:018:   Final instantiatedness of `HeadVar__3' was
> test.m:018:   `unique(test.socket(ground))',
> test.m:018:   expected final instantiatedness was `unique'.
> 
> How is this incorrect? Surely unique(socket(ground)) <= unique.

Indeed.

> test.m:027: In clause for `test_pred(di, uo, di, uo)':
> test.m:027:   in argument 2 of call to predicate `test.write/3':
> test.m:027:   mode error: variable `STATE_VARIABLE_Socket_3' has 
> instantiatedness
> test.m:027:   `mostly_unique',
> test.m:027:   expected instantiatedness was `unique'.
> 
> Yes, but I've ensured that you can't backtrack across the call to read, 
> so clearly the object is actually unique. Is there any way to convince 
> the compiler of this?

No, I think you've scored a hat trick of bugs.

> What I'm actually trying to do here is to parse input from a socket 
> using a DCG. You can backtrack over calls to 'read' since I'm keeping a 
> buffer of everything that has been read. You can't backtrack over 
> 'write' since you can't unsend things you have already sent. I could 
> make 'read' take a unique object, but this would mean you couldn't 
> backtrack over it, and this would remove half the point of using a DCG 
> since the grammar then has to be effectively LL(1). Any suggestions as 
> to how I can deal with this?

I prefer to read_file_as_string and process that in one go, especially
when using DCGs.

There are two other solutions: (1) dive into the compiler and fix the
bug yourself (this is open source, after all); (2) buy us much beer that
we may be inspired to fix the bug sooner :-)

-- Ralph
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