Matching on class members "error: not found: value &&"

Question

I'm trying to get going in Scala from the Twitter Scala School but am stumbling over syntax errors. When I run the Pattern Matching code from the "Basics continued" tutorial http://twitter.github.io/scala_school/basics2.html#match through my sbt console the compiler turns me back with "error: not found: value &&". Has something changed in Scala to take what probably worked when the tutorial was written but doesn't work now? The classes involved are

class Calculator(pBrand: String, pModel: String) {
  /**
   * A constructor
   */
  val brand: String = pBrand
  val model: String = pModel
  val color: String = if (brand.toUpperCase == "TI") {
    "blue"
  } else if (brand.toUpperCase == "HP") {
    "black"
  } else {
    "white"
  }

  // An instance method
  def add(m: Int, n: Int): Int = m + n
}

class ScientificCalculator(pBrand: String, pModel: String) extends Calculator(pBrand: String, pModel: String) {
  def log(m: Double, base: Double) = math.log(m) / math.log(base)
}

class EvenMoreScientificCalculator(pBrand: String, pModel: String) extends ScientificCalculator(pBrand: String, pModel: String) {
  def log(m: Int): Double = log(m, math.exp(1))
}

My repl looks something like this...

bobk-mbp:Scala_School bobk$ sbt console
[info] Set current project to default-b805b6 (in build file:/Users/bobk/work/_workspace/Scala_School/)
[info] Starting scala interpreter...
[info] 
Welcome to Scala version 2.9.2 (Java HotSpot(TM) 64-Bit Server VM, Java 1.7.0_17).
Type in expressions to have them evaluated.
Type :help for more information.
...
scala> def calcType(calc: Calculator) = calc match {
     |   case calc.brand == "hp" && calc.model == "20B" => "financial"
     |   case calc.brand == "hp" && calc.model == "48G" => "scientific"
     |   case calc.brand == "hp" && calc.model == "30B" => "business"
     |   case _ => "unknown"
     | }
<console>:9: error: not found: value &&
         case calc.brand == "hp" && calc.model == "20B" => "financial"
                                 ^
<console>:10: error: not found: value &&
         case calc.brand == "hp" && calc.model == "48G" => "scientific"
                                 ^
<console>:11: error: not found: value &&
         case calc.brand == "hp" && calc.model == "30B" => "business"
                                 ^
scala>

How to I get the use case of AND on my cases when I'm doing Matching on Class Members?

Thanks in advance. I'm new to this.

Btw, you use an outdated version of Scala. Current version is `2.10.2`. — kiritsuku, Jun 11 '13 at 00:02
The results are the same when I force SBT console to use a more recent version of Scala. — Bob Kuhar, Jun 11 '13 at 01:27
Yea, of course, but an outdated version is an outdated version... — kiritsuku, Jun 11 '13 at 02:19

score 3 · Accepted Answer · answered Jun 11 '13 at 09:57

3

If you're matching by value, as in your case, you can not only use guards, but stick to plain pattern matching:

def calcType(calc: Calculator) = (calc.brand, calc.model)  match {
     case ("hp", "20B") => "financial"
     case ("hp", "48G") => "scientific"
     case ("hp", "30B") => "business"
     case _             => "unknown"
}

I find this one easier to parse.

answered Jun 11 '13 at 09:57

om-nom-nom

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That's what I'm talking about! Too bad I can't undo the Right answer check as I think this version captures, exactly, the spirit of the case. What, exactly, happen here? Your function turn the brand and model values from the Calculator into a tuple and sends that kid through the match? – Bob Kuhar Jun 11 '13 at 15:09
Yep, what is going on here is matching against tuple. Moreover, if Calculator was a case class, you could write something like `calc match { case Calculator("hp", "20B") => "financial" .... `. *(Shameless plug -- actually you can undo)* – om-nom-nom Jun 11 '13 at 15:12
@BobKuhar: It is possible to undo the right answer. Just mark a different one as the right one... – kiritsuku Jun 12 '13 at 10:53

score 2 · Answer 2 · edited May 23 '17 at 12:18

2

When you want to test a condition with a pattern, then you need to use a guard:

calc match {
  case _ if calc.brand == "hp" && calc.model == "20B" => "financial"
  ...
}

With the _ you denote that you don't bother of the concrete value calc has, but of some other condition mentioned in the guard.

Btw, it is possible to write a conjunction extractor:

object && {
  def unapply[A](a: A) = Some((a, a))
}

But it will not work in your concrete case.

edited May 23 '17 at 12:18

Community

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answered Jun 11 '13 at 00:01

kiritsuku

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Hmmm. So the twitter doc stuff can't work as written? They cover "guards", very briefly, in the section preceding the one I lifted my code from. It looks like this is a known issue: https://github.com/twitter/scala_school/pull/89 – Bob Kuhar Jun 11 '13 at 01:28
Thats it! But funky. The need to "capture" the _ seems redundant to me. Regardless. It compiles and my understanding of "the new shiny" has improved. – Bob Kuhar Jun 11 '13 at 01:46

Matching on class members "error: not found: value &&"

2 Answers2