[PPL-devel] [Fwd: Re: Any Prolog language lawyer out there?]

[Roberto Bagnara]

-------- Original Message --------
Subject: Re: Any Prolog language lawyer out there?
Date: 19 Oct 2004 14:59:26 GMT
From: Jan Wielemaker <jan at ct.xs4all.nl>
Reply-To: jan at nospam.swi.psy.uva.nl
Organization: SWI, University of Amsterdam
Newsgroups: comp.lang.prolog
References: <41750912.8040102 at cs.unipr.it> <1098196164.417498 at seven.kulnet.kuleuven.ac.be>

In article <1098196164.417498 at seven.kulnet.kuleuven.ac.be>, Bart Demoen wrote:
> Roberto,
> 
> 
>> Am I correct if I say that all the 6 tested systems are /not/ violating
>> the ISO Prolog standard because unary plus is not mentioned in its Table 7
>> (p. 19) and thus its presence and priority are implementation-defined?
>> All the best,
>> 
>>     Roberto
>> 
>> P.S.  Standard or not standard, I find all this quite striking.
>> 
> 
> 
> The + not figuring in ISO as fy, makes the situation murky. But - is in ISO as 200 fy.
> And one finds differences like:
> 
> In SICStus:
> 
>| ?- prolog_flag(language,_,iso).
> yes
>| ?- current_op(200,fy,-).
> yes
>| ?- functor(- 4,X,Y).
> X = -4,
> Y = 0 ?

I think this is a mistake.  Reading "Prolog, the standard", I find
(p. 230) that an integer is an integer token, which is defined as a
sequence of digits (besides binary, octal, etc).  If this is really
true, than negative numbers cannot be represented and only get their
property when passed to one of the arithmetic predicates.

I think this is undesirable.  For one thing, on engines using fixed
with 2-complements representation (allowed!) one must write the
smallest integer as (-2147483647-1) !?  Also, one always needs is/2 to
represent a negative integer.  So, the tokeniser should deal with
'-'{digit}+. The situation for + is a bit less clear as it is not
_needed_ to represent positive integer terms.

Indeed a bit depressing ...

	Cheers --- Jan

-- 
Prof. Roberto Bagnara
Computer Science Group
Department of Mathematics, University of Parma, Italy
http://www.cs.unipr.it/~bagnara/
mailto:bagnara at cs.unipr.it