Re: [eclipse-clp-users] Accessing Eclipse predicates

Thanks everyone.



On Fri, Aug 16, 2013 at 1:41 PM, Kish Shen <kisshen_at_cisco.com> wrote:

> On 16/08/2013 01:47, Volkan Unsal wrote:
>
>> Being new to Eclipse, I am trying to understand some basic concepts. I
>> don't understand why are some Eclipse predicates not available from inside
>> loops. For example:
>>
>> # test.ecl
>> main :-
>>    (foreach(A,[1,2]) do
>>        Z is string(A),
>>        writeln( Z )
>>      ).
>>
>>
> If you have Z is string(A) outside a loop, you would get the same error.
>
> The problem you are seeing is because is/2 expects an arithmetic
> expression on the right-hand side, and if you look at the documentation for
> is/2 (which you can get by "help is/2" at the top-level):
>
> >   This evaluation mechanism outlined above is not restricted to the
> >   predefined arithmetic functors shown in the table.  In fact it works
> for
> >   all atoms and compound terms.  It is therefore possible to define a new
> >   arithmetic operation by just defining an evaluation predicate.
> >   Similarly, many ECLiPSe built-ins return numbers in the last argument
> >   and can thus be used as evaluation predicates (e.g.cputime/1, random/1,
> >   string_length/2, ...).  Note that recursive evaluation of arguments is
> >   only done for the predefined arithmetic functions, for the others the
> >   arguments are simply passed to the evaluation predicate.
>
> So string(A) is treated as a call to a predicate string/2, which is not
> defined, as reported by the error message you saw:
>
>
>  $ eclipse -b test.ecl -e main
>> calling an undefined procedure string(1, _105343) in module eclipse
>> abort
>>
>>
>
>  These are available from the command line when I query the data, but not
>> from within the program. How do I do type checking on the variables if I
>> can't use these...?
>>
>
> is/2 is intended (as in other Prologs) for evaluating an arithmetic
> expression. If you want to do type checking, you should not put the type
> check predicate on the rhs of is/2. You should just call it, e.g.
>
>    (foreach(A, As) do string(A))
>
>
>  Almost exactly the same problem with comparison operators:
>>
>> fn1 :-
>>    (foreach(A,[1,2]) do
>>        Z is compare("<", A, 1),
>>        writeln( Z )
>>      ).
>>    %% calling an undefined procedure compare("<", 1, 1, _103944) in module
>> eclipse
>>    %% abort
>>
>>
> Again, you should just have compare("<", A, 1) on its own, although here
> you can write:
>
> 1 is compare("<", A)
>
> because the last argument does return a number.
>
> Cheers,
>
> Kish
>
>