Scala pattern matching against something of multiple types

Question

Suppose c can be Updatable or Drawable or both. If it's both, I want to handle Updatable first.

Given this code:

case c: Updatable => c.update()
case c: Drawable => c.draw()

Has one problem: it only evaluates one of the choices. Sometimes, c can be both so I need to run both of these.

I know there's | mechanism that looks like this:

case c @ (_: Updatable | _: Drawable) => c.update(); c.draw()

The problem here is that I can't call both update and draw because it's an |.

I think I'm looking for something like this, but doesn't compile:

case c @ (_: Updatable & _: Drawable) => c.update(); c.draw()
case c: Updatable => c.update()
case c: Drawable => c.draw()

Is there such a thing? I know I could open it up and write isInstacenOf, but I'd prefer a pattern match if it's even possible.

Landei · Accepted Answer · 2014-04-11 13:14:50Z

6

def f(c:Any) = c match {
  case d:Drawable with Updatable => println("both")
  case d:Drawable => println("drawable")
  case u:Updatable => println("updatable")
}

answered Apr 11, 2014 at 13:14

Landei

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2 Comments

Kai Sellgren Over a year ago

This is a fantastic idea, why did I not think of this? ;)

Randall Schulz Over a year ago

The with keyword used in this context is conjunction (obviously). If you're interested in disjunctive / union types, check out this fantastic blog: chuusai.com/2011/06/09/scala-union-types-curry-howard

om-nom-nom · Accepted Answer · 2014-04-11 16:03:18Z

5

How about this?

trait Foo
trait Bar
class FooBarImpl extends Foo with Bar
type FooBar = Foo with Bar


def matching(x: Any) = x match {
    case _: FooBar  => "i'm both of them"
    case _: Foo    => "i'm foo"
    case _: Bar    => "i'm bar"
}

val x = new FooBarImpl

matching(x) 
// res0: String = i'm both of them
matching(new Foo{})
// res1: String = i'm foo

Although I'm not quite sure whether reflection kicks in or not.

edited Apr 11, 2014 at 16:03

answered Apr 11, 2014 at 13:15

om-nom-nom

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4 Comments

Kai Sellgren Over a year ago

I think defining the union type is nice if you need to pattern match like this often, but for 1 single case I'm going with what Landei showed.

om-nom-nom Over a year ago

@KaiSellgren nothing stops you from writing case _: Foo with Bar without defining any external type alias ;-)

Kai Sellgren Over a year ago

That is correct. Your answer is basically the same as what Landei did there. I happened to accept his answer due to the angle of the moon.

Randall Schulz Over a year ago

"Union" is a very poor choice for this construct. Union is typically interpreted as disjunctive while with is conjunctive.

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