Selector pattern on the dart

Selector.dart

void main() {
  final selector = Selector();
  selector
    .when((value) => value == 3).then((value) => value * 2)
    .whenIsEqual((value) => value == 'QQQ').then((value) { print(value); return null; })
    .fallback((value) => { 'x': 42 });
   
  final selector2 = Selector<int, int>();
  selector2
    .when((value) => value == 3).then((value) => value * 2)
    .whenIsEqual('QQQ').then((value) { print(value); return null; })
    .fallback((value) => { 'x': 42 });
  
  print(selector(15));
  print(selector('QQQ'));
  print(selector('asd'));
  print(selector(3));
}

class Selector<TArgument, TReturn> {
  final Map<bool Function(TArgument value), TReturn Function(TArgument value)> _cases = {};

  TReturn Function(TArgument value)? _fallback;

  bool Function(TArgument value)? _currentCondition;

  Selector();

  TReturn call(TArgument value) {
    for (final someCase in _cases.entries) {
      if (someCase.key(value)) return someCase.value(value);
    }
 
    if (_fallback != null) return _fallback!(value);

    throw Exception('fallback function is not defined');
  }
  
  Selector<TArgument, TReturn> when(bool Function(TArgument value) conditionFn) {
    _currentCondition = conditionFn;
    return this;
  }

  Selector<TArgument, TReturn> whenIsEqual(TArgument checkedValue) {
    _currentCondition = (value) => value == checkedValue;
    return this;
  }

  Selector<TArgument, TReturn> then(TReturn Function(TArgument value) runnerFn) {
    if (_currentCondition == null) throw Exception('You couldn\'t use "then" without "when*"');
    _cases[_currentCondition!] = runnerFn;
    return this;
  }

  Selector<TArgument, TReturn> fallback(TReturn Function(TArgument value) runnerFn) {
    _fallback = runnerFn;
    return this;
  }
}