Re: More extensive (variable) style checking

Hi Jan,

On 02/08/2013, at 16:41, Jan Wielemaker <[email protected]> wrote:

> Hi,
> 
> The traditional singleton checking is extremely useful, but also
> a bit limited.  I've been asked to enhance that, and messages from
> the compiler in general.  So, I've established a -still quite
> experimental- infrastructure to provide the compiler with
> variable name information and term layout and infrastructure to
> print messages from the compiler.
> 
> I first applied that to compiled BIPs, now =/2, ==/2, \==/2,
> var/1, nonvar/1 (more will follow).  So, if you do
> 
> 	t :- var(X), writeln(X).
> 
> It will tell you that var(X) is always true. Similar, it tells you X==Y
> is always false in this:
> 
> 	t(X) :- X == Y, writeln(Y).
> 
> So far, so good. The next one is probably also meaningful because it is
> also used by ECLiPSe: singleton detection in disjunctive branches. So
> 
> 	t :-
> 	    (   hello(X)
> 	    ;   world(X)
> 	    ).
> 
> will tell you that X is a singleton. Unlike ECLiPSe (AFAIK), this is not
> checked at the syntactical level by read/1, but by the compiler which
> discovers that X is not a real singleton, but semantically it a named
> variable that plays no role in communication. At that moment it will
> issue a warning.
> 
> The next one is less clear. A user complained to often initialise
> variables in one branch, forgetting to do so in another one. The check
> verifies that variables introduced in a disjunction and used after it
> are introduced in both branches. Thus,
> 
> 	t :-
> 	    (   hello(X)
> 	    ;   world
> 	    ),
> 	    writeln(X).
> 
> is considered something to warn about. This gives some warnings on the
> SWI-Prolog libraries. Most of them concern indeed ugly code, but not
> errornous. The question is whether this a meaningful warning? And, if
> so, how one can avoid the warning after validating that the code is
> correct? One option for the latter is
> 
> 	t :-
> 	    (   hello(X)
> 	    ;   world,
> 		X=_
> 	    ),
> 	    writeln(X).
> 
> Now, only the compiler will complain that X=_ is pointless, which in
> itself is good because it will replace it with the VM instruction TRUE.
> The open question is how to silence this though ...
> 
> Opinions? Other useful checks we can do at this level without a complete
> mode system?


Excellent news and I look forward to try it. SWI-Prolog is getting better and better :-) I find from time to time code issues, such as singleton variables, that only one Prolog compiler detects. Rotating between compilers helps but, of course, it's not always possible.

Cheers,

Paulo


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Paulo Moura
Logtalk developer

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