From Play 2.4.2 to 2.5.4: polymorphic expression cannot be instantiated to expected type

[Jon Kleiser]

Hi,

I'm just migrating a Play project frm 2.4.2 to 2.5.4. The only compiler error I get now, is this:

[error] /Volumes/P2/vib-2.5.4/app/login/Login.scala:139: polymorphic expression cannot be instantiated to expected type;

[error]  found   : [E]play.api.libs.iteratee.Iteratee[E,play.api.mvc.Result]

[error]  required: play.api.libs.streams.Accumulator[akka.util.ByteString,play.api.mvc.Result]

[error]         play.api.libs.iteratee.Done(redirToLogin(request), play.api.libs.iteratee.Input.Empty)

and line 139 is the one with "play.api.libs.iteratee.Done", here:

  def Authenticated[A]()(action: User => EssentialAction): EssentialAction = {

    EssentialAction { request =>

      getLoggedInUser(request).map { userInfo =>

        action(userInfo)(request)

      }.getOrElse {

        play.api.libs.iteratee.Done(redirToLogin(request), play.api.libs.iteratee.Input.Empty)

      }

    }

  }

Can anyone see what needs to be changed?

[Will Sargent]

You need to provide some extra type information -- here, you're calling Done() with redirToLogin and Input.Empty, but the compiler is not finding the ByteString type from redirToLogin.

Try explicitly defining the type:

val bytes: akka.util.ByteString = redirToLogin(request)

Done(bytes, Input.Empty)

and that should lead you to an error message that gives you more info.

Will.

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[Will Sargent]

Okay, I was jumping the gun.

You're passing back an Iteratee.  You need to pass back an Accumulator.  Check out:

https://www.playframework.com/documentation/2.5.x/StreamsMigration25#Migrating-Iteratees-to-Sinks-and-Accumulators

and that should help you.

[Jon Kleiser]

Hi Will,

Thanks for mentioning Accumulator. I changed the problematic line, and it now compiles. Looks good.

  def Authenticated[A]()(action: User => EssentialAction): EssentialAction = {

    EssentialAction { request =>

      getLoggedInUser(request).map { userInfo =>

        action(userInfo)(request)

      }.getOrElse {

        play.api.libs.streams.Accumulator.done(redirToLogin(request))