Is there a way to execute a stand-alone expression?

[Arshia001]

The engine only accepts complete scripts. Also, the constructor for NodeList is internal, meaning you can't create new scripts in user code. Is there a way to evaluate expression nodes directly? This is useful for implementing e.g. simple one-line calculations in a system's configuration, while disallowing complex scripts, like so:

some-config:
  some-item:
    preprocess: x + 12 * 8 - 1

One would use JSParser.ParseExpression to parse that config entry as an expression, provide some value for x, and evaluate it.

[lahma]

I think your best option is to create an AstVisitor that validates the input to contain only allowed elements. Parse the script, validate the AST (Script instance) , if all OK, then let the string run trough engine.Evaluate("x + 12 * 8 - 1").

[Arshia001]

So I guess it's currently not possible then. I don't mind using a validated script, but I'm pretty sure the ability to evaluate individual statements and expressions would prove useful sometime, somewhere. If it's acceptable in theory, I can look into adding the feature.

Also, why run it as a string? You already have a validated and parsed AST. Wouldn't it be better (performance-wise) to run the AST directly?

[lahma]

Yes, run the script instance directly so no re-parsing overhead.

I don't think that running only an expression is an easy feat as JS initialization etc is all built based on analyzing script structure. Validated script is basically running only expressions, without changes to engine logic.

[Arshia001]

Actually, I think it would (in its most basic form) be as simple as:

Execute(Expression exp) => Execute(new Script(new NodeList(new[] { new ExpressionStatement(exp) })));

Execute(Statement stmt) => Execute(new Script(new NodeList(new[] { stmt })));

[lahma]

Then one could ask for ability to introduce variables too, then we're supporting statement list.. I really think that validating the script instance is easiest for such specific use cases 🙂