Pattern matching on non-literal values

Question

I feel like this is a silly question, but I'll ask anyway... I'm trying to do something like this:

def func(x: Int, y: Int) = {
  val value: Int = 0 //from config
  (x, y) match {
    case (value, value) => "foo"
    case _ => "bar"
  }
}

But both the repl and intelliJ shower me with warnings. (e.g. "patterns after a variable pattern cannot match"; "suspicious shadowing by a variable pattern"; etc.). Is there a correct way to match on non-literal values?

score 14 · Accepted Answer · answered Aug 16 '14 at 22:32

14

Yes! There are two ways to get what you want. The first is to capitalize the names of the variables you wish to match against:

def func(x: Int, y: Int) = {
  val Value: Int = 0 // from config
  (x, y) match {
    case (Value, Value) => "foo"
    case _ => "bar"
  }
}

If you don't want to go that route (because it's not idiomatic to capitalize variable names, etc.), you can backtick them in the match:

def func(x: Int, y: Int) = {
  val value: Int = 0 // from config
  (x, y) match {
    case (`value`, `value`) => "foo"
    case _ => "bar"
  }
}

I'd suggest using backticks in most situations.

answered Aug 16 '14 at 22:32

Travis Brown

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1

it might be worth adding that the reason is that pattern matching requires a _stable identifier pattern_, which you can obtain using literals, backticks or capitalized names. – Gabriele Petronella Aug 16 '14 at 23:28
1

I love it when Travis Brown goes downscale. Is this the only TB answer without a type param or a macro in it? – som-snytt Aug 18 '14 at 15:03
@som-snytt: I like to kick back on Sundays. – Travis Brown Aug 18 '14 at 15:09

Pattern matching on non-literal values

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