Implicit conversion with generic implicit parameter

Question

I seems that combining implicit conversions with generic implicit parameters does not work, as in example 2b below:

object Test { case class Foo(i: Int) case class Bar(i: Int) case class Zip(i: Int) object Foo { // 1) implicit def toBar(foo: Foo)(implicit g: Int => Bar): Bar = g(foo.i) // 2) implicit def toT[T](foo: Foo)(implicit g: Int => T): T = g(foo.i) } // 1) implicit val b = (i: Int) => Bar(i) val bar: Bar = Foo(3) // 2a) implicit val z = (i: Int) => Zip(i) val zip: Zip = Foo.toT(Foo(3)) // 2b) val zip2: Zip = Foo(3) // <- compiler error, implicit conversion not applied }

Is there any theoretical reason why this doesn't work, or is it a limitation of the implementation?

Malte Schwerhoff · Accepted Answer · 2012-11-01 13:30:13Z

For what it's worth, if you run the following reduced version of your code

case class Foo(i: Int) case class Zip(i: Int) implicit def toT[T](foo: Foo)(implicit g: Int => T): T = g(foo.i) implicit val z = (i: Int) => Zip(i) val zip2: Zip = Foo(3) // <- compiler error, implicit conversion not applied

with -Yinfer-debug, you'll get lots of debug information (Scala 2.9.2) about what is going on behind the scenes. I am not familiar with Scala compiler internals, but the following two output snippets might hint at the problem. The first one is (line 51ff of the gist)

[inferImplicit view] pt = this.Foo => this.Zip Implicit search in Context(Main.$anon.zip2@ValDef scope=1100551785) { search this.Foo => this.Zip target $anon.this.Foo.apply(3) isView true eligible toT: [T](foo: this.Foo)(implicit g: Int => T)T }

and I interpret it as "we are looking for an implicit this.Foo => this.Zip and a candidate worth looking at is toT: [T](foo: this.Foo)(implicit g: Int => T)T. This snippet is followed by output that suggests that Scala then tries to instantiate T, but line 81 finally says

inferMethodInstance, still undetermined: List(type T)

My interpretation is, that Scala somehow fails to instantiate T with Zip, which is why the candidate is finally discarded.

That said, I don't see a theoretical problem with your code, and I assume that it is only a shortcoming of the compiler.

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