Opening and closing a query

Prolog queries are represented in Visual Basic in textual form, i.e. as a string containing the query, but not followed by a full stop.

For example, the following Visual Basic code fragments create valid Prolog queries:

        'Q1 is a query finding the first "good" element of the list [1,2,3]
        Q1 = "member(X,[1,2,3]), good(X)"
     
        'create a Q2 query finding the first "good" element of the list
        '[1,2,...,N]:
        Q2 = "member(X,["
        For i = 1 To N-1
           Q2 = Q2 & i & ","
        Next
        Q2 = Q2 & N & "]), good(X)"
     

Before executing a query, it has to be explicitly opened, via the PrologOpenQuery function, which will return a query identifier that can be used for successive retrieval of solutions.

The PrologCloseQuery procedure will close the query represented by a query identifier. The use of an invalid query identifier will result in undefined behavior.

Example:

        Dim qid As Long
     
        Q1 = "member(X,[1,2,3]), good(X)"
        qid = PrologOpenQuery(Q1)
     
           ... <execution of the query> ...
     
        PrologCloseQuery(qid)