46.4.1 Example 1 - Calculator

This example contains a simple program that allows you to enter an arithmetic expression (conforming to Prolog syntax) as a string and displays the value of the given expression, as shown in the following figure:

The calculation itself will be done in Prolog.

We now we will go through the steps of developing this program.

1. Start a new project called calculator.
2. Add the vbsp.bas file to the project.
3. Create a form window called calculator. Edit it, adding two textboxes txtExpr and txtValue, and two command buttons, cmdCalc and cmdQuit:
   

   Save the form window to the calculator.frm file. Then the project will contain the following two files:

   
4. Write the Prolog code in the file calc.pl, evaluating the given expression with the is/2 predicate, and providing a minimal level of exception handling:

             prolog_calculate(Expr, Value) :-
                on_exception(Exc, Value is Expr, handler(Exc,Value)).
             
             handler(domain_error(_,_,_,_),'Incorrect expression').
             handler(Exc,Exc).
        

   Note that this example focuses on a minimal implementation of the problem, more elaborate exception handling will be illustrated in the Train example (see Example 2 - Train).

   Compile this file, and deposit the file calc in the directory where the calculator.vbp project is contained.

5. Now you have to write the Visual Basic code in which SICStus Prolog will be called at two points:
   • Initialize Prolog in the Form_Load procedure executed when the calc form is loaded, calling the PrologInit() function and loading the calc file with the help of the PrologQueryCutFail(..)) function:

                    Private Sub Form_Load()
                        If PrologInit() <> 1 Then GoTo Err
                        If PrologQueryCutFail("ensure_loaded(app(calc))") <> 1 Then GoTo Err
                        Exit Sub
                    
                    Err:
                        MsgBox "Prolog initialization failed", 48, "Error"
                        Unload Me
                    End Sub
               

   • Do the expression evaluation in the calculate procedure activated by the cmdRun command button. This procedure will execute the prolog_calculate(X,Y) procedure defined in the calc.pl Prolog file:

                    Public Function calculate(ByVal Expr As String) As String
                        Dim qid As Long
                        Dim result As String
                        Dim ret As Long
                        Dim Q As String
                    
                        Q = "prolog_calculate(" & Expr & ",Value)"
                        qid = PrologOpenQuery(Q)
                        If qid = -1 Then GoTo Err ' e.g. syntax error
                    
                        ret = PrologNextSolution(qid)
                        If ret <> 1 Then GoTo Err ' failed or error
                    
                        ret = PrologGetString(qid, "Value", result)
                        If ret <> 1 Then GoTo Err
                        calculate = result
                        Call PrologCloseQuery(qid)
                    
                        Exit Function
                    
                    Err:
                        MsgBox "Bad expression", 48, "Error!"
                        calculate = ""
                    End Function
               

   • Deinitialize Prolog in the Form_Unload procedure executed when the calc form is unloaded, e.g. when the application exits.

                    Private Sub Form_Unload(Cancel As Integer)
                        PrologDeInit
                    End Sub