Safely chaining implicit conversions

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?

Why is the first code block of code obviously not safe? Could Scala create an infinite chain of implicits by instantiating ``A`` and ``C`` with the same concrete type? — Malte Schwerhoff, Jul 14 '12 at 09:14
@mhs No, because Scala will only invoke an implicit conversion if the code would not type-check otherwise. The danger with the first one is that chains of implicits might not be unique modulo final signature. — Ptharien's Flame, Jul 14 '12 at 10:46

score 0 · Answer 1 · answered Apr 19 '13 at 21:54

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()
}

Safely chaining implicit conversions

1 Answers1

Linked