Why toList() creates a List<dynamic> in Dart?

Question

I have this method, which compiles with no problems in Dart 2. However at run-time I get the following error

type 'List<dynamic>' is not a subtype of type 'List<ExchangeRate>'

As you see in the code I create and return new ExchangeRate objects within .map() and then after that I return a rateEntries.toList() which I expect to be of type List<ExchangeRate>, however it seems to be inferred as type List<dynamic>!!!

@override
Future<List<ExchangeRate>> getExchangeRatesAt(DateTime time, Currency baseCurrency) async {
  final http.Client client = http.Client();
  final String uri = "some uri ...";
  return await client
    .get(uri)
    .then((response) {
      var jsonEntries = json.decode(response.body) as Map<String, dynamic>;
      var rateJsonEntries = jsonEntries["rates"].entries.toList();
      var rateEntries = rateJsonEntries.map((x) {
        return new ExchangeRate(x.value.toDouble());
      });
      return rateEntries.toList(); // WHY IS IT RETURNING A List<dynamic> here?
    })
    .catchError((e) => print(e))
    .whenComplete(() => client.close());
}

However if I cast it specifically to ExchangeRate it would be fine.

    return rateEntries.toList().cast<ExchangeRate>();

This casting at the end seems redundant to me, why should I need it?

Answer 1

Well, it seems that the cast is necessary to fully define the type.

But, you can avoid the cast if you add any of the following snippets:

1. Give the correct type to the rateJsonEntries variable

   List<dynamic> rateJsonEntries = jsonEntries["rates"].entries.toList();

   For whatever reason this works in my case.

2. Add the parameter type to the map() method

   var rateEntries = rateJsonEntries.map<ExchangeRate>((x) { return new ExchangeRate(x.value.toDouble()); });