ObjectMirror.invoke ignores typeArguments

[thosakwe]

• Dart SDK Version: 2.0.0-dev.64.1
• Mac OSX Sierra
• Browser: N/A

---

I'm updating the package json_god to work with Dart 2; it's a dart:mirrors-based reflection solution. The actual functionality works, but for the fact that Dart 2's generics are reified, and so to work with typed collections, I need to somehow attach type information to objects.

The solution I am attempting is to reflectively call List.cast with the necessary type argument; however, the ObjectMirror.delegate method forwards to ObjectMirror.invoke, which doesn't use the type arguments. Thus, the type arguments are ignored, and items like List<String> remain List<dynamic>.

---

My code:

logger.info('Casting list elements to ${typeArguments[0].reflectedType}');
var inv = new Invocation.genericMethod(#cast,
   [typeArguments[0].reflectedType], []);
logger.info('INVOCATION OF ${inv.memberName} with type args: ${inv
    .typeArguments}');
var output = reflect(it.toList()).delegate(inv);
logger.info('Casted list type: ${output.runtimeType}');

The current output:

[INFO] json_god: Casting list elements to String
[INFO] json_god: INVOCATION OF Symbol("cast") with type args: [String]
[INFO] json_god: About to deserialize es2015 to a String
[INFO] json_god: Value es2015 is a primitive
[INFO] json_god: About to deserialize stage-0 to a String
[INFO] json_god: Value stage-0 is a primitive
[INFO] json_god: Casted list type: CastList<dynamic, dynamic>

It's evident that that the type arguments are not actually being applied at all. I can't think of any possible workaround for this, unfortunately.

[thosakwe]

Is there anything else I can do about this? This bug means that anything that uses dart:mirrors for serialization, etc., will not be able to perform casts, and always produce runtime errors.

[devoncarew]

cc @rmacnak-google

[matanlurey]

FWIW, you really won't want to use .cast as part of a normal workflow. It creates a forwarding implementation of the underlying collection, and is mostly meant to be a temporary stop-gap. For more data sets, a defensive copy is probably better:

https://www.dartlang.org/guides/language/effective-dart/usage#avoid-using-cast

(Still doesn't help you with mirrors, of course)

[sestegra]

Did you try List<T>.from ?

[thosakwe]

Thanks for the tip - I was able to resolve it by using List.from, which works perfectly.

[mdempsky]

I'd like to work on this, if no one else is planning on looking into it soon.

The solution I'm thinking of:

1. Add an invokeGeneric method to ObjectMirror with signature:

   InstanceMirror invokeGeneric(
       Symbol memberName,
       List<Type> typeArguments,
       List positionalArguments,
       [Map<Symbol, dynamic> namedArguments])
   

   (Analogous to Invocation having method and genericMethod factory methods.)

   Q: Should List<Type> be List<TypeMirror>? Is there a reason List and Map<Symbol, dynamic> aren't List<InstanceMirror> and Map<Symbol, InstanceMirror>, respectively?

2. Change invoke and delegate to use this method instead.

3. Plumb the extra parameter down through runtime/lib/mirrors.cc and into runtime/vm/object.cc. I see some TODOs by @crelier about "Support invocation of generic functions with type arguments," so I assume once I get that far down, then the rest of the code below is in place.

Does that sound like a reasonable plan? Or are there other aspects of dart:mirrors that should be updated for generics at the same time?