scala-unstable-identifiers

scala-unstable-identifiers.scala

//paste the contents of this file on a scala console and inspect the results

abstract class Token
case class TNum(n: Double) extends Token
case class TOpenP() extends Token
case class TCloseP() extends Token
case class TSum() extends Token
case class TDiff() extends Token
case class TMult() extends Token
case class TDiv() extends Token
case class TNeg() extends Token


val priorities = List(
    (0, TSum()),
    (0, TDiff()),
    (1, TMult()),
    (1, TDiv()),
    (1, TNeg()),
    (3, TOpenP())
)

//prints one
priorities.collectFirst({ case (x, TMult()) => x }).get

val op: Token = TMult()

//prints zero... waat!!
priorities.collectFirst({ case (x, op) => x }).get

//this solves the issue, but it is not idiomatic enough, it looks kind of ugly
priorities.collectFirst({ case (x, y) if y == op => x }).get

//and this too prints one, this is the idiomatic solution, it is called stable identifier
priorities.collectFirst({ case (x, `op`) => x }).get

//this also prints one, this is another way for referencing a stable identifier
val Op: Token = TMult()
priorities.collectFirst({ case (x, Op) => x }).get