Dart 2.X List.cast() does not compose

Question

The upcoming Dart 2.X release requires strong typing. When working with JSON data we must now cast dynamic types to an appropriate Dart type (not a problem). A

Answer 1

I wrote Ignoring cast fail from JSArray to List<String>, so let me try and help here too!

So I add the .cast<String>() as the documentation and related link suggest and still receive the warning:

List<String> docFixBroken = json["data"].cast<String>().map( (s) => s.toUpperCase() ).toList();
List<String> alsoBroken = List.from( (json["data"] as List).cast<String>() ).map( (s) => s.toUpperCase() ).toList();

Unfortunately, List.from does not persist type information, due to the lack of generic types for factory constructors (https://github.com/dart-lang/sdk/issues/26391). Until then, you should/could use .toList() instead:

(json['data'] as List).toList()

So, rewriting your examples:

List<String> docFixBroken = json["data"].cast<String>().map( (s) => s.toUpperCase() ).toList();
List<String> alsoBroken = List.from( (json["data"] as List).cast<String>() ).map( (s) => s.toUpperCase() ).toList();

Can be written as:

List<String> notBroken = (json['data'] as List).cast<String>((s) => s.toUpperCase()).toList();

Hope that helps!