Rich Oliver Rich Oliver - 2 months ago 7
Scala Question

Scala: Triple Context Bounds, evidence parameters not found

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.

Answer

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[]??
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