Scala: How to use case keyword without match keyword?

Question

I wrote a section code for testing:

class A extends JavaTokenParsers {
    def str: Parser[Any] = stringLiteral ~ ":" ~ stringLiteral ^^ 
                    { case x ~ ":" ~ y => (x, y) }    //how to use case keyword like this?
}

object B extends A with App{
    val s = """
            "name": "John"
            """
    println(parseAll(str, s))
}

I read "Chapter 15: Case Classes and Pattern Matching" of Programming in Scala Second Edition , but I never saw case used like this:

... ^^ { case x ~ ":" ~ y => (x, y) }

It's not match keyword, but ^^ looks like match. I know partial functions and I can use case by this way:

object C extends App {
    def a(f: Int => Int) = {
        f(3)
    }
    a(x => x + 1)
    a { case x => x + 1 }
}

But they are all different:

^^ function like match, which can use content before ^^ matching content behind case
case in ^^ function can use function(like ~)

How to write custom function like ^^? Can you write a concrete example? Thanks a lot!

Gabriele Petronella · Accepted Answer · 2016-01-05 11:16:57Z

10

It's just syntactic sugar. In scala, you can use any method that takes a single parameter as a binary operator.

Example:

class Foo(x: String) {
  def ^^(pf: PartialFunction[String, Int]): Option[Int] =
    if (pf.isDefinedAt(x)) Some(pf(x)) else None
}

val foo = new Foo("bar")
foo ^^ {
  case "baz" => 41
  case "bar" => 42
}

// result: Some(42)

answered Jan 5, 2016 at 11:16

Gabriele Petronella

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2 Comments

Guo Over a year ago

Thank you for your answer! But it looks very simple. I want case expression can contain variables(like x,y above) and function(like ~ above). Thanks again!

Gabriele Petronella Over a year ago

that's an entirely different matter. You can define a custom unapply and define the behavior of the extractor. Look up "extractors" and you'll find a lot of information about it.

PhilBa · Accepted Answer · 2016-01-05 12:39:08Z

4

As you mentioned, you can use a block with the case keyword to create a partial function.

val doubleIntGtThree: PartialFunction[Int, Int] = {
  case x: Int if x > 3 => x * 2
}
doubleIntGtThree(4) // return 8
doubleIntGtThree(2) //throws a matchError

edited Jan 5, 2016 at 12:39

answered Jan 5, 2016 at 12:25

PhilBa

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2 Comments

Guo Over a year ago

Thank you for your answer! But Partial function is not what I need for my question. They are different.

PhilBa Over a year ago

Ok, sorry I didnt fully understand what you wanted to achieve.

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