Scala view application puzzler

Question

Say we have the following two traits:

trait Foo[A] { def howMany(xs: List[A]) = xs.size }
trait Bar

And an implicit conversion from the second

Answer 1

Your implicit conversion seems to be doing exactly what you told it to do.

implicit def bar2foo[A](bar: Bar) = new Foo[A] {}

Converts a Bar to a new Foo[A] object. So in turn

scala> bar howMany stuff
:13: error: type mismatch;
 found   : List[Int]
 required: List[A]
              bar howMany stuff

It looks for an 'A' type.

In order to make this work the way you want it to you (I think), instead of defining the view on the trait you can do it on the function.

trait Foo { def howMany[A](xs: List[A]) = xs.size }
trait Bar
implicit def bar2foo[A](bar: Bar) = new Foo{}
val bar = new Bar {}
val stuff = List(1, 2, 3)

then it should give you the result you desire.

scala> bar howMany stuff
res0: Int = 3

or you can define the view on the implicit function

trait Foo[A] { def howMany(xs: List[A]) = xs.size }
trait Bar

implicit def bar2foo[A](bar: Bar) = new Foo[Int] {}

val bar = new Bar {}
val stuff = List(1, 2, 3)

Personally I think defining it on the function is cleaner.