scala error illegal variable in pattern alternative on pattern matching case

Question

I am new to scala. I was writing a pattern matching as below:

  val capitals = Map("France" -> "Paris", "Japan" -> "Tokyo")
  show(capitals.get("test"))
  def show(x: Option[String]) = x match {
    case Some(s) | None  => s
  }

I am getting error:

Error: illegal variable in pattern alternative
    case Some(s) | None  => s
              ^

I am trying to see how can I achieve or condition like I have in if statement in java

if (str == null || str.isEmpty())

Can you help rewrite the code or point out the mistake?

Question: How do I mention or condition in a case pattern matching?

slouc · Accepted Answer · 2016-04-20 16:57:36Z

7

This is how you pattern match on Options:

def show(x: Option[String]) = x match {
  case Some(s) => s
  case None => "N/A"
}

(by the way, you can also do something like this):

capitals.get("test").getOrElse("N/A")

Now, to add OR conditions to a pattern match case, you cannot use bound variables. This will work, however:

def show(x: Option[String]) = x match {
  case Some(_) | None => "a"
}

Note that the only difference is in Some(_) as opposed to your Some(s). Using Some(s) wouldn't make much sense because you can't reuse that s anyway (what if None comes, what would s be in that case?)

edited Apr 20, 2016 at 16:57

answered Apr 20, 2016 at 16:47

slouc

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4 Comments

happybayes Over a year ago

this is informative. what I am trying ti achieve with is? ignoring this scenario..is or statement is possible? I have three case saying case Some[], case None, case _ . Instead writing three cases, i wanted to find if I can give it as or. Like I wanted do an operation on a or condition.How do i achieve it instead writing two cases?

happybayes Over a year ago

thanks for the insightful answer. this is what exactly I am looking for.

ristohietal Over a year ago

Is it possible to reduce something like this: intOpt match { case Some(i) if (i < 2) => true case None => true case _ => false } into ... case None | Some(_) if (_ < 2) => true somehow?

slouc Over a year ago

@ristohietal I'm not sure, because "if" part doesn't make sense in case of None. But I also personally wouldn't like it even if it were possible. Your statement with three distinct cases makes perfect sense and is easier to read. Perhaps putting the None case before Some(i) if (i < 2) case makes it even easier to read and understand. Of course, this is all highly subjective.

Brian · Accepted Answer · 2016-04-21 16:44:37Z

2

I think this is what you are trying to achieve. If s has a value in the map return, s. If s has no value in the map, return a message indicating so.

val capitals = Map("France" -> "Paris", "Japan" -> "Tokyo")

def show(x: Option[String]) = x match {
  case Some(s)  => s
  case None     => "x has no value"
} 

show(capitals.get("mani"))

A similar and more concise version of show is capitals.get("mani").getOrElse("No value found") which returns No value found.

Further, you can use a guards to check various conditions on s such as if the first char is upper case. This first case will only match if s begins with an upper case character.

 def show(x: Option[String]) = x match {
     case Some(s) if(s.head.isUpper) => s
     case None     => "No value"
 }

edited Apr 21, 2016 at 16:44

answered Apr 20, 2016 at 16:52

Brian

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Comments

cchantep · Accepted Answer · 2016-04-20 16:47:03Z

1

Matching Some(s) | None doesn't make sense, as if (true || false) (just no-op as it doesn't dispatch cases).

x match {
  case Some(s) => println(s"Some($s)")
  case _ => println("_None")
}

answered Apr 20, 2016 at 16:47

cchantep

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Comments

GNRX · Accepted Answer · 2019-05-11 15:31:24Z

0

The most direct solution is using the method defined for maps, which takes the default as the second argument:

capitals.getOrElse("test","N/A")

answered May 11, 2019 at 15:31

GNRX

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