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PROLOG Digest            Friday, 13 Mar 1987       Volume 5 : Issue 15

Today's Topics:
     Implementation - Logical Variable & Turbo & Lint & Confusion
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Date: Thu, 12 Mar 87 11:08:45 PST
From: Fernando Pereira <pereira@sri-candide.ARPA> 
Subject: Logical variable in Turbo Prolog

Mike Newton's original message/review, and the observations for which
I got thanks :-), were for an early version of Turbo Prolog. I've not
experimented with Turbo since then, but from looking at a manual it
appears that it is possible to declare parameters or structure members
as ``reference'', allowing that example to run as Steve Hardy pointed out.

However, this does not mean one has the full power of the logical variable,
because the ``reference'' objects must be predeclared. Will the following
work in Turbo?

        ?- eq(f(X), FX), eq(FY, f(Y)), eq(FX, FY), eq(FX, f(a)).

with eq defined by

        eq(X, X).

-- F

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Date: 10 Mar 87 13:29:31 GMT
From: Chris Moss <mcvax!ukc!icdoc!cdsm@seismo.css.gov>  
Subject: How do you like Turbo-Prolog?

> /***** iaoobelix:comp.lang.pro / thumper!steve /  5:30 pm  Feb 28, 1987*/
> > I'm just starting with prolog, and for $99 Turbo-Prolog looks
> > attractive.  I'd like to know what people think of it.
> >             -Steve Miller    ihnp4!bellcore!thumper!steve

This is SILLY. It would be better to say Prolog with Pascal type
structure.  Clearly its biggest failing is that it doesn't have a
polymorphic type checker (a la Mycroft/O'Keefe paper for instance).
But its type system is better than theirs in some respects (one can
declare subdomains which do get some checking).

Personally I doubt that I'd use Turbo much, but I'm real interested to
see how much people like the addition of typechecking. If you think you
can't write REAL programs in Turbo, look at their program disk. There's
a neat natural language system there that is efficient and extensible.
-- In case you want to know, they get round the lack of metacall easily
by effectively defining "call" for all the queries (in the internal
query language) they want to do.

> Also, and most seriously (thanks to Fernando Pereira) there  are two more
> *MAJOR* problems:  the logical variable doesn't really
> exist: it is a runtime error to do variable-to-variable unifications
> as in
>               p(X,X).
>
>               ?- p(X,Y), p(X,a).
>

This is plain WRONG! I have just typed and executed the following
program:

        domains
                rsymbol = reference symbol
        predicates
                eqn(rsymbol, rsymbol)
        clauses
                eqn(X,X).

and executed the following queries with the results (more or less
verbatim from the screen):

Goal :  eqn(X,foo)
        X=foo
        1 solution

Goal :  eqn(X,X)
        X=_
        1 solution

Goal :  eqn(X,Y)
        X=_,  Y=_

Goal :  eqn(X,Y), eqn(Y,foo)
        X=foo, Y=foo
        1 solution

I think this shows the pluses and minuses of TurboProlog.

  a. eqn only works on symbols (atoms) not other things (as it stands).
  It's impossible to write a full "eq".
  b. You must inform it (by "reference") if you want full unification -
  i.e. unbound variables in the result.
  c. Leaving out the reference in the domain gives a run-time error for
  the second and after goals.
  d. It's got MASSES of error messages - far more, and I dare say, far
  better than most other Prolog systems.
  e. The domain and predicate declarations are good documentation but
  pretty onerous. In particular it's wrong to declare but not finish a
  definition - Not good for top-down program development.

> The article forwarded by Mike Newton seem to ascribe some of these
> modifications to  Fernando Pereira. If he was reponsible for these, I
> must admit to some surprise since he was involved in the early
> implementations of DEC-10 Prolog, and really should have known better....
>       Rick Innis (csrdi@its63b.uucp || rick@uk.ac.edinburgh)

Before this gets too much circulation, Fernando was CRITICIZING not
IMPLEMENTING Turbo (and whether he was cited correctly I don't know.
Were you, Fernando?) It was actually implemented by some guy in Denmark
whose name I forget right now.

To sum up, I'm NOT advocating Turbo, but if you want to criticize
get your facts right!

-- Chris Moss

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Date: Thu, 12 Mar 87 05:46:25 pst
From: Randy Reed <tektronix!reed!iscuva.ISCS.COM!randyg@cad.Berkeley.EDU> 
Subject: Lint       

Alint(Arity lint) is a program that does "Lint" style checking for
Arity prolog. That means it checks for suspicious code, Variables only
referenced once(A sure sign you mispelled something, typo's being the
main source of errors in Prolog), predicates not referenced elsewhere,
type mismatches, and other errors. The reason Lint is not usually part
of the compiler is that a good lint will be somewhat overenthusiastic
in finding errors (you never get a completely clean program). Borland
included it in their compiler, and forces you to press a key to ignore
a found error. Kludging around a bug in turbo, such as the set
predicate findall in 1.0, can be a nightmare.

-- Randy Gordon

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Date: 11 Mar 87 11:26:16 GMT
From: Drew Adam <mscvax!inria!crcge1!adams@seismo.css.gov>
Subject: How do you like Turbo-Prolog?

Rick Innis writes:

>Maybe I'm just spoiled rotten from living where Prolog was first
>implemented, but having used three generations of Prolog interpreters I
...
>       Rick Innis (csrdi@its63b.uucp || rick@uk.ac.edinburgh)

I'm confused... you live in Marseille but your e-mail address is Edinburgh?
(Or does "from living" mean "from having lived [in Marseille]"?)

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End of PROLOG Digest
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