Implicit conversion with implicit parameter - scala

Question

I'm implementing a Java interface with a lot of methods with Object parameters, which in my case are really Strings containing user names:
public interface TwoFactorAuthProvider {
boolean requiresTwoFactorAuth(Object principal);
... //many more methods with the same kind of parameter
}
I'm trying to use implicit conversion to convert these to User objects in my implementation:
class TwoFactorAuthProviderImpl(userRepository: UserRepository)
extends TwoFactorAuthProvider {
def requiresTwoFactorAuth(user: User): Boolean = {
...
}
}
When I define the conversion in the companion object of my class, it is picked up just fine and my class compiles:
object TwoFactorAuthProviderImpl {
implicit def toUser(principal: Any): User = {
null //TODO: do something useful
}
}
However, to be able to do the conversion, I need access to the user repository, which the TwoFactorAuthProviderImpl instance has, but the companion object does not. I thought I could possibly use an implicit parameter to pass it:
implicit def toUser(principal: Any)(implicit repo: UserRepository): User = {
val email = principal.asInstanceOf[String]
repo.findByEmail(email)
}
But with the implicit parameter, the conversion is no longer picked up by the compiler (complaining that I'm not implementing the interface).
Is there a way to get the implicit conversion that I want, or is this outside the scope of what you can do with implicits?

Answer 1

This should work just fine - can you supply the exact compilation error? Not implementing what interface? It looks like you would have to declare as follows:
class TwoFactorAuthProviderImpl(implicit userRepository: UserRepository)
Here's an example for the REPL to show that implicits can have implicits; I'm using paste mode to ensure that module X is the companion object of the class X
scala> :paste
// Entering paste mode (ctrl-D to finish)
case class X(i: Int, s: String)
object X { implicit def Int_Is_X(i: Int)(implicit s: String) = X(i, s) }
// Exiting paste mode, now interpreting.
defined class X
defined module X
scala> val i: X = 4
<console>:9: error: value X is not a member of object $iw
val i: X = 4
^
But if we add an implicit string in scope
scala> implicit val s = "Foo"
s: java.lang.String = Foo
scala> val i: X = 4
i: X = X(4,Foo)
Implicits advice
Don't go overboard with implicit conversions - I think you are going too far in this sense - the principal is implicitly a mechanism by which you can discover a user, it is not implicitly a user itself. I'd be tempted to do something like this instead:
implicit def Principal_Is_UserDiscoverable(p: String) = new {
def findUser(implicit repo: UserRepository) = repo.findUser(p)
}
Then you can do "oxbow".findUser

Answer 2

Thanks to Oxbow's answer, I now have it working, this is only for reference.
First of all, a value that should be passed as an implicit must itself be marked implicit:
class TwoFactorAuthProviderImpl(implicit userRepository: UserRepository) ...
Second, implicit conversions are nice and all, but a method implementation signature must still match the signature of its declaration. So this does not compile, even though there is a conversion from Any to User:
def requiresTwoFactorAuth(principal: User): Boolean = { ... }
But leaving the parameter as Any, as in the declaration, and then using it as a user works just fine:
def requiresTwoFactorAuth(principal: Any): Boolean = {
principal.getSettings().getPhoneUsedForTwoFactorAuthentication() != null
}
Also, the conversion really doesn't have to be in the companion object in this case, so in the end, I left the implicit parameters out.
The full source code:
class TwoFactorAuthProviderImpl(userRepository: UserRepository)
extends TwoFactorAuthProvider {
private implicit def toUser(principal: Any): User = {
val email = principal.asInstanceOf[String]
userRepository.findByEmail(email)
}
def requiresTwoFactorAuth(principal: Any): Boolean = {
//using principal as a User
principal.getSettings().getPhoneUsedForTwoFactorAuthentication() != null
}
...
}