[Peter Muraya]

Hi,
I see some inconsistency in the way Visual Prolog compiler traps and reports the following error: The interface 'winner{@Vacancy}' takes 1 argument(s). Consider the following code which I assumed to be valid; the compiler throws up this error at the the line marked /*1*/ , BUT NOT at the one marked /*2*/. To clear the error, I had to add an unbounded domain as illustrated by the commented statement -- which I did not like. Is there some logic to this behavior that I don't understand?

Code: Select all

interface winner{@Vacancy}
     domains
          method = all; best.
 
     properties
          vacancy:@Vacancy.
end interface
 
interface expression
     predicates
           trim:(winner::method) determ.   /*      1     */
           %trim:(winner{_}::method) determ.
end interface
 
class expression:expression
end class
 
implement expression
     clauses
           trim(winner::all).     /*    2     */
 
end implement
 
implement main
 
clauses
    run() :-
        succeed(). % place your own code here
 
end implement main
 
goal
    main::run().
 

[Martin Meyer]

Hi Peter,

I think, the compiler's behavior is not inconsistent there. The manual teaches:

This piece of code illustrates how to create an integer queue and insert an element in it:

Code: Select all

..., Q = queueClass{integer}::new(), Q:insert(17), ...

It is not necessary to apply the type explicitly, instead the compiler can infer it from the context:

Code: Select all

..., Q = queueClass::new(), Q:insert(17), ...

The compiler sees that Q must be an integer queue, because we insert 17 into it.

In your line /*1*/ the compiler cannot infer a value for type parameter @Vacancy from the context. However in line /*2*/ it can always determine the value of the type parameter from context (provided the context, i.e. line /*1*/, is stated syntactically correct). Thus, the type parameter cannot be ommitted in /*1*/, but can be ommitted in /*2*/.

The value for type parameter @Vacancy, which has to be supplied in /*1*/, does not need to be _. Usually the code would be something like:

Code: Select all

interface expression{@Vacancy}
     predicates
           trim : (winner{@Vacancy}::method) determ.   /*      1     */
end interface
 
class expression{@Vacancy} : expression{@Vacancy}
end class
 
implement expression{@Vacancy}
     clauses
           trim(winner::all).     /*    2     */
end implement

Many regards
Martin

[Martin Meyer]

Or, when you want, that expression does not have type parameter @Vacancy, code it in the way:

Code: Select all

interface winnerDomains
     domains
          method = all; best.
end interface
 
%---
 
interface winner{@Vacancy}
    open winnerDomains
    properties
        vacancy : @Vacancy.
end interface
 
%---
 
interface expression
    predicates
        trim : (winnerDomains::method) determ.   /*      1     */
end interface
 
class expression : expression
end class
 
implement expression
    clauses
        trim(winnerDomains::all).     /*    2     */
end implement

Regards
Martin

[Peter Muraya]

Thanks Martin.
Since this issue is being brought about by the parameter in the winner interface, I have had to move the method domain from where I thought would be its most logical home to one of my existing and non-parameterized interface -- similar to your winnerDomains in the second example.