@nex3 I'm working on the changes to Options.functions, and from my understanding we need to simplify return values as explained here:

Return a copy of result.internal with all calculations it transitively contains (including the return value itself if it's a calculation) replaced with the result of simplifying those calculations.

It sounds like we need to modify _parseFunctions to simplify the result if it's a SassCalculation instance, something like this:

if (result is Value) {
  if (result is SassCalculation)
    return SassCalculation._simplify(result);
  return result;
}

But _simplify is a private static method. Should we make it public? Am I approaching this correctly?

@jerivas Since we don't want _simplify to be used by users outside the Sass package (since they can never get ahold of an unsimplified calculation anyway), it may be simpler to just call SassCalculation.calc() instead, which will in practice simplify a calculation.