Node:Opening and Closing a Query, Next:, Up:How to Call Prolog from Visual Basic

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
Q2 = "member(X,["
For i = 1 To N-1
   Q2 = Q2 & i & ","
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. For example:

Dim qid As Long

Q1 = "member(X,[1,2,3]), good(X)"
qid = PrologOpenQuery(Q1)

   ... <execution of the query> ...