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.

Step 1: Start a new project called calculator

Step 2: Add the vbsp.bas file to the project

Step 3: Create a form window called calculator

Edit the calculator form window, 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:

Step 4: Write the Prolog code

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.

Step 5: Write the Visual Basic code with the Prolog calls

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 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