compute function
Implementation
num compute(String exStr) {
List<String> acc = [];
exStr.split(' ').forEach((cur) {
if (calc.containsKey(cur)) acc.add(Function.apply(calc[cur], [num.parse(acc.removeLast()), num.parse(acc.removeLast())].reversed.toList()).toString());
if (isNumber.hasMatch(cur)) acc.add(cur);
if (consNumber.containsKey(cur)) acc.add(consNumber[cur].toString());
});
return num.parse(acc.removeAt(0));
}