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selectDynamic/applyDynamic logic fails too eagerly #7059

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scabug opened this issue Feb 3, 2013 · 6 comments
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selectDynamic/applyDynamic logic fails too eagerly #7059

scabug opened this issue Feb 3, 2013 · 6 comments
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2.11.0-M8

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@scabug
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scabug commented Feb 3, 2013

This should work.

import scala.language.dynamics

class A extends Dynamic {
  def selectDynamic(method: String): B = new B(method)
}
class B(method: String) {
  def apply(args: Any*) = println("Called " + method)
}

object Test {
  def main(args: Array[String]): Unit = {
    val x = new A
    println(x.bippy("arg"))
  }
}

Instead, the compile fails:

% scalac3 ./a.scala 
./a.scala:13: error: value applyDynamic is not a member of A
error after rewriting to x.<applyDynamic: error>("bippy")
possible cause: maybe a wrong Dynamic method signature?
    println(x.bippy("arg"))
            ^
one error found

It does compile like this:

  println(x.bippy.apply("arg"))

But that renders it unsuitable for most plausible uses of Dynamic.

This is especially important because if this worked it would offer a way to achieve a more general form of overload than is currently possible.
@scabug
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scabug commented Feb 3, 2013

Imported From: https://issues.scala-lang.org/browse/SI-7059?orig=1
Reporter: @paulp
See #6355

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scabug commented Jul 10, 2013

@adriaanm said:
Unassigning and rescheduling to M6 as previous deadline was missed.

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scabug commented Dec 27, 2013

@xeno-by said:
Interestingly enough, if one changes selectDynamic to applyDynamic, everything works fine.

@scabug
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scabug commented Dec 27, 2013 •
edited by retronym

@xeno-by said:
One might say that this behavior is warranted by SIP-17: http://docs.scala-lang.org/sips/completed/type-dynamic.html. Here's a relevant excerpt:

  • If qual.sel is followed by a type argument list [Ts] (optionally) and an argument list (arg1, …, argn), where none of the arguments argi are named, and argn is not a sequence argument, i.e., of the shape e: _* (the argument list is required, though it may be empty), as in: qual.sel[Ts](arg1, …, argn), rewrite the expression to: qual.applyDynamic[Ts](“sel”)(arg1, …, argn)

  • If qual.sel is not followed by arguments or an assignment operator: qual.selectDynamic[Ts](“sel”).

If we follow the letter of the SIP, then x.bippy in x.bippy("arg") is followed by an argument list, hence it should get desugared into applyDynamic, not selectDynamic. Do you agree?

@scabug
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scabug commented Dec 27, 2013

@paulp said:
I agree that's what the sip says, but as you know I do not generally find such documents particularly relevant. If there's a better way to do things then it's a bug in the sip. I see zero utility in intentionally generating a call to a method which can be seen not to exist and in so doing making it impossible to reach the method which does exist.

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scabug commented Dec 27, 2013

@xeno-by said:
scala/scala#3310

@scabug scabug closed this as completed Dec 30, 2013
@scabug scabug added the has PR label Apr 7, 2017
@scabug scabug added this to the 2.11.0-M8 milestone Apr 7, 2017
@scabug scabug mentioned this issue Apr 7, 2017
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