DDMathParser: Parsing dynamic formula which contain $ to refer objects value within dictionary

Question

I am struggling a little bit to use DDMathParser framework for expression requirement I have. I have JSON of fields & based on expressions certain fields can be set required, hidden or set the value of it.

Expressions in required tag in sample JSON are not fixed & so not getting how to achieve dynamic approach for expression.

[
  {
    "name": "firstName",
    "value": "Ameer",
    **"required": true**
  },
  {
    "name": "lastName",
    "value": "Shaikh",
    **"required": "$firstName != ‘’"**
  },
  {
    "name": "designation",
    "value": "",
    **"required": "$firstName == ‘Ameer’ && $lastName == ‘Shaikh’"**
  },
  {
    "name": "location",
    "value": "",
    **"hidden": false**
  }
]

Actually, expression in JSON contains $ to represent one of the dictionary objects value from JSON. Wherein framework internally treats it as a local variable.

These expressions may have different combinations as well. There may be several expression apart from "required" parameters. I need to run all relevant expressions for any change in value in UI.

EDIT

let expression = "$firstName == ‘Ameer’ && $lastName == ‘Shaikh’"
    let dict = ["firstName": "Amir", "lastName": ""]
    let e = try! Expression(string: expression)
    let result = try! Evaluator.default.evaluate(e, substitutions: dict)

Though it should parse a correct value from JSON, I have hard coded substitutions string to at least get a breakthrough. Here, substitutions expect String & Double & give error as "Cannot convert a value of type [String: String] to expected arg type substitutions (Dcitionary). Is there any way to pass String: String substitutions?

Answer 1

DDMathParser is not built to do string evaluations. It's technically possible to make it work, but it's a bit beyond the scope of the framework.

For this situation, I think you'd be better off using NSPredicate, which does allow string comparisons and variable substitutions.