sclass JsonDiff {
  srecord DiffResult(O a, O b) {}
  sclass Removed > DiffResult { *(O *a, O *b) {} } // b missing
  sclass Added > DiffResult { *(O *a, O *b) {} } // a missing
  sclass DifferentType > DiffResult { *(O *a, O *b) {} }
  sclass Different > DiffResult { *(O *a, O *b) {} } // same type, generally different
  
  // returns null if identical
  sO diff(S json1, S json2) {
    O o1 = jsonDecode(json1), o2 = jsonDecode(json2);
    ret diff(json1, json2);
  }
  
  sO diff(O a, O b) {
    if (eq(a, b)) null;
    if (a == null) ret new Removed(a, b);
    if (b == null) ret new Added(a, b);
    if (a.getClass() != b.getClass()) ret new DifferentType(a, b);
    // from here on, we know that a and b have the same type
    if (a cast Map)
      ret mapKeyAndFunction_lhm(withoutNullValues(concatAsOrderedSet(keys(a), keys(b)),
        key -> diff(a.get(key), b/Map.get(key)));
    // TODO: lists
    ret new Different(a, b);
  }
}