Question

The application I am working on allows embedding script sinppets into a document. For example:

SomeText
<* PrintLn("This line is generated by a script"); *>
Some other text
<* PrintLn("This line is generated by a script, too"); *>
Some more lines

Results

SomeText
This line is generated by a script
Some other text
This line is generated by a script, too
Some more lines

I am using DWScript. Internally the first script snippet is compiled & executed. Than the next is RecompiledInContext and executed and so on. A function/variable/etc declared in a snippet becames available in all later snippets.
However the variable values are lost between snippets. For example:

SomeText
<* var x: Integer = 5; *>
Some other text
<* PrintLn(x); *>
Some more lines

After generating the document:

SomeText
Some other text
0  <-- I would like this to be 5
Some more lines

Here is a sample application which illustrates the problem:

program POC.Variable;

{$APPTYPE CONSOLE}

{$R *.res}

uses
  dwsExprs,
  dwsComp,
  dwsCompiler; 

var
  FDelphiWebScript: TDelphiWebScript;
  FProgram: IdwsProgram;
  FExecutionResult: IdwsProgramExecution;

begin
  FDelphiWebScript := TDelphiWebScript.Create(nil);
  try
    FProgram := FDelphiWebScript.Compile('var x: Integer = 2;');
    FProgram.Execute;

    FDelphiWebScript.RecompileInContext(FProgram, 'PrintLn(x);');

    FExecutionResult := FProgram.Execute;
    // The next line fails, Result[1] is '0'
    Assert(FExecutionResult.Result.ToString[1] = '2');
  finally
    FDelphiWebScript.Free;
  end
end.

Is there a way to “transfer” or “keep” the variable values between executions?

Here is an updated code of Andrew’s answer which does not work:

begin
  FDelphiWebScript := TDelphiWebScript.Create(nil);
  try
    FProgram := FDelphiWebScript.Compile('PrintLn("Hello");');

    FExecution:= FProgram.BeginNewExecution();

    FDelphiWebScript.RecompileInContext(FProgram, 'var x: Integer;');
    FExecution.RunProgram(0);
    WriteLn('Compile Result:');
    WriteLn(FExecution.Result.ToString);

    FDelphiWebScript.RecompileInContext(FProgram, 'x := 2; PrintLn(x);');
    FExecution.RunProgram(0); // <-- Access violation
    WriteLn('Compile Result:');
    WriteLn(FExecution.Result.ToString);

    FExecution.EndProgram();
    ReadLn;
  finally
    FDelphiWebScript.Free;
  end
end;

Answer 1

The issue is that when RecompileInContext() adds new global variables, they don’t have space allocated, as space allocation is performed by BeginNewExecution, but it should work if the variables pre-exist, or if new variables are added within a function so are local vars, rather than global ones).

So if you change the “updated code” to something like this, it will work

FProgram := DelphiWebScript1.Compile( 'PrintLn("Hello");'
                                     +'var x: Integer;');

FExecution:= FProgram.BeginNewExecution();

FExecution.RunProgram(0);
SynEdit1.Lines.Add('Compile Result:');
SynEdit1.Lines.Add(FExecution.Result.ToString);

DelphiWebScript1.RecompileInContext(FProgram, 'x := 2; PrintLn(x);');
FExecution.RunProgram(0); // <-- Access violation
SynEdit1.Lines.Add('Compile Result:');
SynEdit1.Lines.Add(FExecution.Result.ToString);

FExecution.EndProgram();

Edit: this as now been fixed by r1513 in the DWScript SVN.