Unexpected By-Name Parameter Evaluation

00_Description

This code is a simplification (mostly the removal of type parameters) of an attempt, as an exercise,
to create a syntax that approximates the C- and Java-like conditional expression (“question-colon”).
The first attempt was eager, which is of course less than ideal. Next I tried to alter the code to
produce a form that evaluated only one of the value expressions. In doing so I found that the left-
hand (true) expression is always evaluated, even when the controlling Boolean is false. It appears
that the by-name parameter used to convey the left-hand value expression is being evaluated at the
point in the source where it appears (lexically). The right-hand (false case) expression is only
evaluated when the controlling Boolean is false and the stack trace indicates that the evalutation
is triggered where expected, in the body of TrueLeftFalseRight.right.

Can someone help me explain what’s going on? It seems like a bug, but I’m loathe to conclude that.

01_CondExprFail.scala

object CondExprLazyFail {
  // Note: References in comments to type parameters reflect original generic code


  /** The pair of left / true and right / false values
    *
    * @param  leftVal   The value to produce when the controlling boolean is '''true'''
    * @param  rightVal  The value to produce when the controlling boolen is '''false'''
    * @tparam T         The type of ''left'' and ''right''
    */
  class TrueLeftFalseRight(leftVal: => Int, rightVal: => Int) {
    def left:  Int = leftVal
    def right: Int = rightVal
  }

  /** Wrap a right-hand / false-case value of type ''Int'' for pairing with its left-hand / true-case counterpart
    *
    * @param  right   The value that will become the right-hand / false-case member of the condintional expression
    * @tparam T       The type of ''right''
    */
  implicit
  class   FalseRight(right: => Int)
  {
    /** Pair this right-hand / false-case value with its left-hand / true-case counterpart
      *
      * @param  left  The value that will become the left-hand / true-case member of the condintional expression
      * @tparam U     The type of ''left'' (a supertype of [[T]])
      */
    def ^: (left: => Int) =
      new TrueLeftFalseRight(left, right)
  }

  /** Association between the controlling [[Boolean]] and the pair of values from which to select, based on its value
    *
    * @param  cond    The [[Boolean]] whose value controls the selection of the left / true or the right / false value
    */
  implicit
  class   CondExprLeft(val cond: Boolean)
  extends AnyVal
  {
    /** Choose which of the left / true or right / false values to produce
      *
      * @param  rhs   The pair of left-right / true-false values from which to select
      * @tparam T     The type of the values held in ''rhs''
      */
    def |? (rhs: TrueLeftFalseRight): Int =
      if (cond) rhs.left
      else      rhs.right
  }
}


object CondExprFail {
  import CondExprLazyFail._

  def main(args: Array[String]): Unit = {
    Seq(false, true) foreach { b =>
      println(s"-==--==--==--==--==--==--==--==--==--==--==--==--==--==--==--==--==--==--==--==--==--==--==--==--==-\nb=$b:")
      val v =
        b |?
        trueV ^:              // <<-- IDEA signals by-name value here
        falseV                // <<-- but not here (for what it’s worth)
      println(s"v[$b]=$v\n")
    }
  }

  def trueV  = { println("\ntrue  => 123"); println(); new Exception("true").printStackTrace(); println(); 123 }
  def falseV = { println("\nfalse => 321"); println(); new Exception("false").printStackTrace(); println(); 321 }
}

02_cond-expr-fail-out

[info] Running CondExprFail
-==--==--==--==--==--==--==--==--==--==--==--==--==--==--==--==--==--==--==--==--==--==--==--==--==-
b=false:

true  => 123

java.lang.Exception: true
	at CondExprFail$.trueV(CondExprFail.scala:67)
	at CondExprFail$$anonfun$main$1.apply(CondExprFail.scala:61)
	at CondExprFail$$anonfun$main$1.apply(CondExprFail.scala:57)
	at scala.collection.immutable.List.foreach(List.scala:381)
	at CondExprFail$.main(CondExprFail.scala:57)
	at CondExprFail.main(CondExprFail.scala)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.lang.reflect.Method.invoke(Method.java:606)
	at sbt.Run.invokeMain(Run.scala:67)
	at sbt.Run.run0(Run.scala:61)
	at sbt.Run.sbt$Run$$execute$1(Run.scala:51)
	at sbt.Run$$anonfun$run$1.apply$mcV$sp(Run.scala:55)
	at sbt.Run$$anonfun$run$1.apply(Run.scala:55)
	at sbt.Run$$anonfun$run$1.apply(Run.scala:55)
	at sbt.Logger$$anon$4.apply(Logger.scala:85)
	at sbt.TrapExit$App.run(TrapExit.scala:248)
	at java.lang.Thread.run(Thread.java:745)


false => 321

java.lang.Exception: false
	at CondExprFail$.falseV(CondExprFail.scala:68)
	at CondExprFail$$anonfun$main$1$$anonfun$1.apply$mcI$sp(CondExprFail.scala:62)
	at CondExprLazyFail$TrueLeftFalseRight.right(CondExprFail.scala:13)
	at CondExprLazyFail$CondExprLeft$.$bar$qmark$extension(CondExprFail.scala:48)
	at CondExprFail$$anonfun$main$1.apply(CondExprFail.scala:61)
	at CondExprFail$$anonfun$main$1.apply(CondExprFail.scala:57)
	at scala.collection.immutable.List.foreach(List.scala:381)
	at CondExprFail$.main(CondExprFail.scala:57)
	at CondExprFail.main(CondExprFail.scala)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.lang.reflect.Method.invoke(Method.java:606)
	at sbt.Run.invokeMain(Run.scala:67)
	at sbt.Run.run0(Run.scala:61)
	at sbt.Run.sbt$Run$$execute$1(Run.scala:51)
	at sbt.Run$$anonfun$run$1.apply$mcV$sp(Run.scala:55)
	at sbt.Run$$anonfun$run$1.apply(Run.scala:55)
	at sbt.Run$$anonfun$run$1.apply(Run.scala:55)
	at sbt.Logger$$anon$4.apply(Logger.scala:85)
	at sbt.TrapExit$App.run(TrapExit.scala:248)
	at java.lang.Thread.run(Thread.java:745)

v[false]=321

-==--==--==--==--==--==--==--==--==--==--==--==--==--==--==--==--==--==--==--==--==--==--==--==--==-
b=true:

true  => 123

java.lang.Exception: true
	at CondExprFail$.trueV(CondExprFail.scala:67)
	at CondExprFail$$anonfun$main$1.apply(CondExprFail.scala:61)
	at CondExprFail$$anonfun$main$1.apply(CondExprFail.scala:57)
	at scala.collection.immutable.List.foreach(List.scala:381)
	at CondExprFail$.main(CondExprFail.scala:57)
	at CondExprFail.main(CondExprFail.scala)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.lang.reflect.Method.invoke(Method.java:606)
	at sbt.Run.invokeMain(Run.scala:67)
	at sbt.Run.run0(Run.scala:61)
	at sbt.Run.sbt$Run$$execute$1(Run.scala:51)
	at sbt.Run$$anonfun$run$1.apply$mcV$sp(Run.scala:55)
	at sbt.Run$$anonfun$run$1.apply(Run.scala:55)
	at sbt.Run$$anonfun$run$1.apply(Run.scala:55)
	at sbt.Logger$$anon$4.apply(Logger.scala:85)
	at sbt.TrapExit$App.run(TrapExit.scala:248)
	at java.lang.Thread.run(Thread.java:745)

v[true]=123

[success] Total time: 3 s, completed Dec 31, 2015 9:14:20 AM
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