Configurable Exception Handling for custom context variables

[gethrok]

Hi,
first of all thanks for the great recent upgrades to the date handling and other improvements!

I am now dealing with specific scenario, where we are using expression for data transformation from incoming JSONs through endpoints. The issue is following:

Imagine we have following JSON:

[{
          "productId": "p-1",
          "name": "Laptop",
          "quantity": 1,
          "price": 1200.50,
          "discount": null,
          "non_existing" : "test"
        },
        {
          "productId": "p-2",
          "name": "Mouse",
          "quantity": 2,
          "price": 25.00,
          "discount": 5.00
        } ] 

For each record we provide the JSON as custom context, but in the second record the "non_existing" parameter is missing, which is expected behaviour, however the Evaluator will throw exception in case the custom context variable is missing.

So far I did very ugly fix, essentially disabling context variable validation

public Object visit(Expr.Variable variable) {
        if (env == null) {
            throw new Exceptions.RuntimeException(
                variable.name,
                'Error executing variable expression: no context was provided.'
            );
        }

        if (env.contains(variable.name.lexeme)) {
            return env.get(variable.name.lexeme);
        } else {
            return '';
        }
    }

Instead of throwing the exception. I have tried to explore the option to include new Configuration parameter, however the Interpreter class has no access to Configuration class and it's a lot of refactoring to propagate it all the way to the Interpreter plus I don't think it's a good pattern. I was thinking about having some flag in the Environment instead, but that also feels a bit dirty (like a custom Global Context variable for example).

Do you have some suggestion?
Thank you!

[cesarParra]

Thanks @gethrok

I'm thinking that for something like this Environment is definitely the place where it makes the most sense to put it, since Interpreter already has access to it, and also the Environment is built using data from the Configuration.

One potential approach is adding a new config that represents a "missing variable default", for example something like this:

  // Configuration.cls
  private Object missingContextVariableDefault;
  global Configuration withMissingContextVariableDefault(Object defaultValue) {
      this.missingContextVariableDefault = defaultValue;
      return this;
  }

then pass that information along to the Environment and use that if the variable is not found.

But keeping in mind that this solution doesn't scale in cases where there might be more than one missing variable and they are of different data types. In your example non_existing is a String, so the missing variable default could be an empty string, so this wouldn't work for cases where there might be other non existing properties of other types, e.g. non_existing_number