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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)