Structural types and parameter bounds

[Mikhail Vorozhtsov]

Hi, list.

I'm getting puzzled here. Why doesn't the following code typecheck?

$ cat O.scala 

trait In {

  type Out

}

def f[O, I <: In { type Out = O }](i: I) {}

def g[I <: In](i: I) = f[I#Out, I](i: I)

$ scala O.scala

/home/mikhail.vorozhtsov/O.scala:8: error: type arguments [I#Out,I] do not conform to method f's type parameter bounds [O,I <: this.In{type Out = O}]

def g[I <: In](i: I) = f[I#Out, I](i: I)

                        ^

one error found

especially when

def f[O, I <: In](i: I)(implicit w: I#Out =:= O) {}

does.

 

[Ruediger Keller]

Why do you try to specify the type parameter O separately for method
f? It is already available as I#Out, just as you do it for g.

You can do:

def f[I <: In](i: I) {}

Or something like:

def f[I <: In](i: I)(implicit ev: I#Out =:= Int) { }

Not sure what you try to achieve here.

Regards,
Rüdiger

2011/3/22 Mikhail Vorozhtsov <mikhail.v...@gmail.com>:

[Mikhail Vorozhtsov]

It's a reduced test case. =:= has very limited usage since it's not transitive or

even symmetric and doesn't aid inference the way that parameter bounds do.

Here is a more motivating example:

trait Cap

trait Op {

  type C <: Cap

}

trait Ops { list =>

  type C <: Cap

  def seq: Seq[Op { type C >: list.C <: Cap }]

}

final class Ops1[O <: Op](op: O) { list =>

  type C = O#C

  def seq: Seq[Op { type C >: list.C <: Cap }] = Seq[O](op)

}

error: type mismatch;

 found   : Seq[O]

 required: Seq[this.Op{type C >: Ops1.this.C <: this.Cap}]

Meaning that O is not a subtype of Op { type C >: O#C <: Cap }, which I find very disturbing.

And indeed:

scala> def f[O <: Op] = implicitly[O <:< Op { type C = O#C }]

<console>:9: error: could not find implicit value for parameter e: <:<[O,Op{type C = O#C}]