Scala: Triple Context Bounds, evidence parameters not found

Question

I have edited this to a simpler form of the question to which @Zhi Yuan Wang responded :

object ContBound { 
  def f2[A: Seq, B: Seq]: Unit = {
      val a1: Seq[A] = evidence$1
      val b2: Seq[B] = evidence$2 
     }

  def f3[A: Seq, B: Seq, C: Seq]: Unit = {
    val a1: Seq[A] = evidence$1
    val b2: Seq[B] = evidence$2
    val a3: Seq[C] = evidence$3      
  }   
}

I get the following errors:

not found value evidence$1
not found value evidence$2
type mismatch; found :Seq[A] required: Seq[C]

despite getting the following in the REPL:

 def f3[A: Seq, B: Seq, C: Seq]: Unit =
 |    {
 |       val a1: Seq[A] = evidence$1
 |       val b2: Seq[B] = evidence$2
 |       val a3: Seq[C] = evidence$3      
 |    }  
f3: [A, B, C](implicit evidence$1: Seq[A], implicit evidence$2: Seq[B], implicit evidence$3: Seq[C])Unit

Zhi's awnser is correct. The following compiles:

object ContBound { 
  def f2[A: Seq, B: Seq]: Unit = {
    val a1: Seq[A] = evidence$1
    val b2: Seq[B] = evidence$2 
  }

  def f3[A: Seq, B: Seq, C: Seq]: Unit = {
    val a1: Seq[A] = evidence$3
    val b2: Seq[B] = evidence$4
    val a3: Seq[C] = evidence$5      
  }   
}

However I still don't see this as correct behaviour, as these are parameters for two different methods and methods are normally allowed to reuse parameter names.

you might try looking at the output of `scalac -print`, or at the generated bytecode — Seth Tisue, Sep 28 '16 at 18:19

score 2 · Accepted Answer · answered Aug 25 '16 at 13:38

2

Have you tried

def comma3[A: RParse, B: RParse, C: RParse, D](f: (A, B, C) => D): D =
    expr match {
       case CommaExpr(Seq(e1, e2, e3)) =>
           f(evidence$3.get(e1), evidence$4.get(e2), evidence$5.get(e3))
}

since the the evidence$1 already used by

def comma3[]??

answered Aug 25 '16 at 13:38

Zhi Yuan Wang

36
3

Already used by `comma2`, you mean. And yes, this is it. And also shows why you should never, ever rely upon the compiler-generated numbering of hidden parameters. – Rex Kerr Sep 28 '16 at 19:43

Scala: Triple Context Bounds, evidence parameters not found

1 Answers1