@Override
    public ConfigExpression<I, O> simplifyOnce() {
      var key = arg.simplifyOnce();
      if (key instanceof ConfigExpression.Const<I, String> constKey) {
        var rawResult = signature.in.root().argument(constKey.value);
        return constOf(TypeConversion.convert(rawResult, signature.out));
      } else {
        return new GetArg<>(signature, key);
      }
    }
An expression that returns the value associated a given argument at runtime.
planetiler-custommap/src/main/java/com/onthegomap/planetiler/custommap/expression/ConfigExpression.java