CODEWITHSUNDEEP
scalaimplicit
pschorf
pschorf

Reputation: 529

Scala implicit conversion of implicit argument

I'm trying to write some code to convert an implicit argument, like the following:

case class A()
case class B()
object B {
    implicit def b2a(b: B): A = new A()
}

def foo(f: B => String) = f(new B())
def bar(implicit a: A) = "A"

foo { implicit b =>
  bar()
}

gives the error:

<console>:27: error: not enough arguments for method bar: (implicit a: A)String. 
Unspecified value parameter a.

Is there a way to use the implicit conversion without calling it manually?

Upvotes: 0

Views: 169

Answers (1)

Michael Zajac
Michael Zajac

Reputation: 55569

I believe this breaks the "one at a time" rule. There is no implicit A in the scope of your call to bar, only an implicit B. If the compiler tried to also implicitly convert the implicit B to an A, that would be two steps, instead of one.

However, you can work around this through implicit chaining. 

implicit def a(implicit b: B): A = new A()

If this method is in scope, the compiler will see an implicit A in scope, as long as it can fill the implicit B, which it can.

scala> foo { implicit b => bar }
res1: String = A

The specific error you were having was because you needed to remove the parenthesis, which I did here. That leads to the "implicit not found" error, described above.

Upvotes: 2

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