Node:Example 1 - Calculator, Next:Example 2 - Train, Up:Examples

The code for the following examples are available in the directory library\vbsp\examples in the SICStus installation directory.

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