Re: [eclipse-clp-users] ECLiPSe stream usage

Hi Giuseppe,

Giuseppe Di Guglielmo wrote:
> Dear all,
> I attached a small test. In the code, I'm trying to retrieve the result for
> the submitted constraint. Why does the 'ec_queue_read' function not collect
> any data from the standard output (or error)?
> 
...
> #include "eclipseclass.h"
> 
> #include <iostream>
> using namespace std;
> 
> main()
> {
>     char buf[1024];
>     int n;
> 
>     ec_set_option_int(EC_OPTION_IO, MEMORY_IO);
>     ec_init();
>     
>     ec_post_string("compile(clp_utils)");
> 
>     ec_post_string("integer_t(U, -128, 127), (U #= 11), search_int_func( U
> )");
> 

I am not sure if I understood you correctly -- are you expecting 
ec_queue_read in your following code to return the results from the 
goals you have just posted?

Your previous message about `permanent variables' suggest you think 
ec_resume() is the same as executing a query at the toplevel in an
interactive ECLiPSe -- this is not correct. They are different:

1. The ECLiPSe toplevel prints the bindings to any variables in the
    query; no such printing is done when you execute posted goals
    during ec_resume(). You need to explicitly write any outputs in
    the goals that are executed. Since your second posted goal does
    not do any outputs, there is nothing for ec_queue_read() to
    read.

2. Interactive ECLiPSe backtrack to the toplevel after executing
    each query at the toplevel. No automatic backtracking is done
    when you return from ec_resume() (unless the posted goal(s) failed).
    If the posted goals succeeded, any further posted goals will be
    executed at the next ec_resume without the backtracking of the
    However, the parsing of the goals (when you post the goals as
    string) is done independently at each ec_resume(), so you cannot
    link the variables from the posted goals to each other.

A more general comment about what you are doing: it is probably not
a good idea to try to post goals as strings and read the outputs
back as strings, unless your posted goals and outputs are simple.
This is particularly the case with posting multiple goals and
multiple ec_resume(). There are several reason for this:

a) It is easy to make errors in your post goals, e.g. in the syntax,
because of various differences, such as the need for quoting. It is
particularly easy to make errors if you are constructing the goals in 
your program. There is no easy way to debug such errors. For example,
did you really intend to post the goal write(Hello) (which writes a 
variable with the name Hello) rather than write("Hello") (write the
string Hello) in:

ec_post_string("write(Hello)");

The goal also happens to write Hello to the output because ECLiPSe
writes the source name of the variable when it is not instantiated.
To post write("Hello"), you will need to correctly quote the inner 
quotation marks.

This is one reason why you should keep the goal simple, or you
should use exdr format, as Andrea and Joachim have suggested.

b) You cannot link the variables in different posted goals to each
other if you are post the goals as strings (or exdr format). You can
link the variables if you use the lower level representation of ECLiPSe
terms(EC_refs etc.), but it is tedious (and thus easy to get wrong) to
build complex goals.

You may want to look at the FAQ `How do I create and pass constraints to
ECLiPSe from an external language?' at

http://eclipse-clp.wiki.sourceforge.net/FAQ

for more disccusion on this issue.

Cheers.

Kish



>     ec_resume(); 
> 
>     while( (n = ec_queue_read(1, buf, 1024)) > 0 ) // DOES NOT WORK!
>     {
>       buf[n] = 0;
>       cout << "eclipse: " << buf << "\n";
>     }
> 
> #if 0
>     n = ec_queue_read(2, buf, 1024);
>     buf[n] = 0;
>     cout << "eclipse error returned: " << buf << ".\n";
> #endif
> 
>     ec_cleanup();
>     exit(0);
> }
> ---
> 
> Giuseppe
> 
> 
> 
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