Why toList() creates a List<dynamic> in Dart? - 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:
Give the correct type to the rateJsonEntries variable
List<dynamic> rateJsonEntries = jsonEntries["rates"].entries.toList();
For whatever reason this works in my case.
Add the parameter type to the map() method
var rateEntries = rateJsonEntries.map<ExchangeRate>((x) {
return new ExchangeRate(x.value.toDouble());
});