Skip to content
New issue

Have a question about this project? Sign up for a free GitHub account to open an issue and contact its maintainers and the community.

By clicking “Sign up for GitHub”, you agree to our terms of service and privacy statement. We’ll occasionally send you account related emails.

Already on GitHub? Sign in to your account

forSome scope introduction conflates distinct types #9410

Open
scabug opened this issue Jul 19, 2015 · 1 comment

Comments

Projects
None yet
1 participant
@scabug
Copy link

commented Jul 19, 2015

The following should not compile; outerd cannot prove that there exists one E that unifies the left.S and right.S, but introduces one anyway. We use that to throw a ClassCastException.

package forsomescope

trait Magic {
  type S
  def init: S
  def step(s: S): String
}

case class Pair[A](left: A, right: A)

object Main extends App {
  type Aux[A] = Magic {type S = A}

  // These method types are equivalent in meaning; they both mean the
  // left and right of the pair have the same 'S'.
  def a[A](p: Pair[Aux[A]]): String = b(p)
  def b(p: Pair[Aux[E]] forSome {type E}): String = a(p)

  // I can't call this one from outerd,
  def outertp[A](p: Pair[Aux[A]]) =
    p.right.step(p.left.init)

  // but I can call this one.
  def outere(p: Pair[Aux[E]] forSome {type E}) =
    outertp(p)

  // This one means the left and the right may have *different* S.  I
  // shouldn't be able to call outere with p.
  def outerd(p: Pair[Magic]) =
    outere(p)

  def boom =
    outerd(Pair(new Magic {
                  type S = String
                  def init = "hi"
                  def step(s: S) = s.reverse
                },
                new Magic {
                  type S = Int
                  def init = 42
                  def step(s: S) = (s - 3).toString
                }))

  boom
}

(scastie here) throws

java.lang.ClassCastException: java.lang.String cannot be cast to java.lang.Integer
	at scala.runtime.BoxesRunTime.unboxToInt(BoxesRunTime.java:101)
	at forsomescope.Main$$anon$2.step(test.scala:42)
	at forsomescope.Main$.outertp(test.scala:25)
	at forsomescope.Main$.outere(test.scala:29)
	at forsomescope.Main$.outerd(test.scala:34)
	at forsomescope.Main$.boom(test.scala:37)
	at forsomescope.Main$.delayedEndpoint$forsomescope$Main$1(test.scala:48)

On the other hand, if outerd directly calls the morally equivalent outertp (scastie here), we correctly get an error:

/tmp/rendererWlqSMa9Oi9/src/main/scala/test.scala:28: type mismatch;
 found   : forsomescope.Pair[forsomescope.Magic]
 required: forsomescope.Pair[forsomescope.Main.Aux[this.S]]
    (which expands to)  forsomescope.Pair[forsomescope.Magic{type S = this.S}]
Note: forsomescope.Magic >: forsomescope.Main.Aux[this.S], but class Pair is invariant in type A.
You may wish to define A as -A instead. (SLS 4.5)
    outertp(p)
            ^

Although, a better error would be more like when you do

import java.util.{ List => JList }
 
def doit[A](xs: JList[JList[A]]) = 42
 
def brokeit(xs: JList[JList[_]]) = doit(xs)

Which appropriately fails to compile (scastie here):

/tmp/rendererWlqSMa9Oi9/src/main/scala/test.scala:11: no type parameters for method doit: (xs: java.util.List[java.util.List[A]])Int exist so that it can be applied to arguments (java.util.List[java.util.List[_]])
 --- because ---
argument expression's type is not compatible with formal parameter type;
 found   : java.util.List[java.util.List[_]]
 required: java.util.List[java.util.List[?A]]
Note: java.util.List[_] >: java.util.List[?A], but Java-defined trait List is invariant in type E.
You may wish to investigate a wildcard type such as `_ >: java.util.List[?A]`. (SLS 3.2.10)
  def brokeit(xs: JList[JList[_]]) = doit(xs)
                                     ^

Original discovery context here.

@scabug

This comment has been minimized.

Copy link
Author

commented Jul 19, 2015

Imported From: https://issues.scala-lang.org/browse/SI-9410?orig=1
Reporter: Stephen Compall (s11001001)
Affected Versions: 2.10.5, 2.11.7, 2.12.0-M2

Sign up for free to join this conversation on GitHub. Already have an account? Sign in to comment
You can’t perform that action at this time.