Scala extractors: a cumbersome example

Question

Everything started from a couple of considerations:

Extractors are Scala objects that implements some unapply methods with certain peculiarities (directly from «Programming in Scala 2nd edition», I've checked)
Objects are singletons lazy initialised on the static scope

I've tried to implement a sort of «parametric extractors» under the form of case classes to try to have an elegant pattern for SHA1 checking.

I'd like to check a list of SHA1s against a buffer to match which of them apply. I'd like to write something like this:

val sha1: Array[Byte] = ...
val sha2: Array[Byte] = ...

buffer match {
  case SHA1(sha1) => ...
  case SHA1(sha2) => ...
  ...
}

Ok, it looks weird, but don't bother now.

I've tried to solve the problem by simply implementing a case class like this

case class SHA1(sha1: Array[Byte]) {
  def unapply(buffer: Array[Byte]): Boolean = ...
}

and use it like

case SHA1(sha1)() =>

and even

case (SHA1(sha1)) =>

but it doesn't work: compiler fails.

Then I've a little changed the code in:

val sha1 = SHA1(sha1)
val sha2 = SHA1(sha2)
buffer match {
  case sha1() => println("sha1 Match")
  case sha2() => println("sha2 Match")
  ...
}

and it works without any issue.

Questions are:

Q1: There are any subtle implications in using such a kind of «extractors»

Q2: Provided the last example works, which syntax was I supposed to use to avoid to define temporary vals? (if any provided compiler's job with match…case expressions)

EDIT The solution proposed by Aaron doesn't work either. A snippet:

case class SHA1(sha1: Array[Byte]) { 
  def unapply(buffer: Array[Byte]) = buffer.length % 2 == 0
}

object Sha1Sample {
  def main(args: Array[String]) {
    println("Sha1 Sample")

    val b1: Array[Byte] = Array(0, 1, 2)
    val b2: Array[Byte] = Array(0, 1, 2, 3)
    val sha1 = SHA1(b1)
    List(b1, b2) map { b =>
      b match {
        case sha1() => println("Match") // works
        case `sha1` => println("Match") // compile but it is semantically incorrect
        case SHA1(`b1`) => println("SOLVED")  // won't compile
        case _ => println("Doesn't Match")
      }
    }
  }
}

Effectively I should consider to delete this post. A1 should be «NO» provided RegularExpression work in the same manner and Q2 should be related to some constraints imposed by the compiler… — Lord of the Goo, Nov 20 '13 at 02:32

score 0 · Answer 1 · edited May 23 '17 at 12:28

0

Short answer: you need to put backticks around lowercase identifiers if you don't want them to be interpreted as pattern variables.

case Sha1(`sha1`) => // ...

See this question.

edited May 23 '17 at 12:28

Community

1
1

answered Nov 20 '13 at 23:28

Aaron Novstrup

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It seems it doesn't work either. Please review my edited post. Thank you. – Lord of the Goo Nov 21 '13 at 09:44
Ah, I see. I misunderstood your use case. The compiler is expecting a pattern extractor where your `SHA1` appears, so yes you'll need to apply `SHA1` to a specific hash outside of the match expression. – Aaron Novstrup Nov 21 '13 at 21:30

Scala extractors: a cumbersome example

1 Answers1