Safely chaining implicit conversions - scala

Question

You can do this to get implicit conversions to chain:
package language
object chainedImplicits {
implicit def chainImplicits[A, B, C](a: A)(implicit conv1: A => B, conv2: B => C): C = conv2(conv1(a))
}
but this is obviously not safe.
I can't see anything wrong with this version, though:
package language
package chainedImplicits {
final class Chained[A, B] private[chainedImplicits] (val f: A => B)
trait Low { this: `package`.type =>
implicit def startChaining(implicit conv: A => B): Chained[A, B] = new Chained[A, B](conv)
implicit def chainImplicits[A, B, C](implicit conv1: Chained[A, B], conv2: B => C): Chained[B, C] = new Chained(conv1.f andThen conv2)
}
}
package object chainedImplicits extends Low {
implicit def endChain[A, B](a: A)(implicit conv: Chained[A, B]): B = conv.f(a)
}
Is there a catch here?

Answer 1

First, I can't get your first "obviously not safe" example to do anything:
import language.implicitConversions
object Test {
implicit def chain[A, B, C](a: A)(implicit ab: A => B, bc: B => C): C = bc(ab(a))
case class X()
case class Y()
case class Z()
implicit def xy(x: X) = Y()
implicit def yz(y: Y) = Z()
val z: Z = X()
}
...
type mismatch;
found : Test.X
required: Test.Z
val z: Z = X()
^
Next, the obvious "catch" in your second example is that it doesn't compile. Did you actually try it? Regardless, after "fixing" it, it still doesn't do what you want:
import language.implicitConversions
class Convert[A, B](val convert: A => B) extends AnyVal
trait Convert0 {
implicit def convert[A, B](implicit ab: A => B): Convert[A, B] = new Convert(ab)
implicit def convert[A, B, C](implicit ab: Convert[A, B], bc: B => C): Convert[A, C] = new Convert(ab.convert andThen bc)
}
object Convert extends Convert0 {
implicit def convert[A, B](a: A)(implicit convert: Convert[A, B]): B = convert.convert(a)
}
object Test {
case class X()
case class Y()
case class Z()
implicit def xy(x: X) = Y()
implicit def yz(y: Y) = Z()
val z: Z = X()
}
This gives the same type mismatch error. To my knowledge, if you want implicit conversions to chain, you have to be explicit about it:
import language.implicitConversions
object test {
case class X()
case class Y()
case class Z()
// anything convertible to X is convertible to Y
implicit def xy[A <% X](x: A) = Y()
// anything convertible to Y is convertible to Z
implicit def yz[B <% Y](y: B) = Z()
val z: Z = X()
}