Scala and typecheck for dependent types

Question

Given java classes "abstract Credentials" and deriving "EmailPassword"/"OAuth" classes.

and interface CredentialsStorage, with appropriate implementations EmailPasswordStorage and OAuthStorage

I need to create some sort of an array with triples: storage, valid credentials, invalid credentials)

So I started with this:

type T <: Credentials

type S <: CredentialsStorage[T]

private var testData: Array[(S, T, T)] = Array(
  (emailStorage, validEmailPasswd, new EmailPasswordCredentials("1", "2")),
  (oAuthStorage, validAuthToken, new OAuthCredentials("invalid auth token", OAuthService.FACEBOOK))
)

however this doesn't compile

error: type mismatch;
found   : storage.EmailPasswordStorage[credentials.EmailPassword]
required: CredentialsStorageTest.this.S
(emailStorage, validEmailPasswd, new EmailPasswordCredentials("1", "2")),

how do I fix this problem and what is the correct definition of dependent/existential types here?

UPD I solved the problem with definition of tuple type itself:

type T[A] = (CredentialsStorage[A],A,A)

val testData : Array[T[_ <: Credentials]] = ...

Answer 1

type T <: Credentials defines an abstract type. You haven't defined what T is in this instance. Try including type T = credentials.EmailPassword and type S = storage.EmailPasswordStorage[T] .