@Flag annotation doesn't work with Seq/List

[amartinsn]

Getting an error when defining a flag that returns a Seq and try to inject that in a Controller implementation using the @Flag annotation.

Expected behavior

Given this flag defined within a TwitterModule:

flag("things", Seq[String]("foo", "bar"), "A list of things")

When I inject that flag in a Controller:

class MyController @Inject()(
  @Flag("things") things: Seq[String]) extends Controller {
   ...
}

Then things val should be Seq("foo", "bar")

Actual behavior

Currently I'm getting the following error when bootstrapping the server:

com.google.inject.ConfigurationException: Guice configuration errors:

1) No implementation for scala.collection.Seq<java.lang.String> annotated with @com.twitter.inject.annotations.Flag(value=enabled) was bound.
  while locating scala.collection.Seq<java.lang.String> annotated with @com.twitter.inject.annotations.Flag(value=enabled)
    for parameter 0 at com.example.MyController.<init>(MyController.scala:11)
  while locating com.example.MyController

Steps to reproduce the behavior

The steps are pretty much defined in the Expected behavior section. Please let me know if you need more details.

[amartinsn]

It might have something to do with the Flag type information being hardcoded as String.

https://github.com/twitter/finatra/blob/develop/inject/inject-app/src/main/scala/com/twitter/inject/app/internal/FlagsModule.scala#L18

[cacoco]

Just an update here. Flags don't currently carry enough type information to allow for correctly binding a parameterized type (like Seq or List). We could do better for binding non-parameterized types instead of always a String but without some re-working of Flags this is pretty challenging. And even then it's non-trivial to parse the parameterized types correctly via reflection to provide the correct type key for Guice binding.

[cacoco]

Good news! In 92a4706, @vkostyukov added TypeConversions which support converting Seq and List of Flaggable types meaning that you can now define a Flag flag("things", Seq[String]("foo", "bar"), "A list of things") and inject it via:

class MyController @Inject()(
  @Flag("things") things: Seq[String]) extends Controller {
   ...
}

as we now have registered Guice TypeConverters to translate the bound String type into a Seq/List of the Flaggable type. This should work for any Flag defined as a Seq/List of a Flaggable type.