noodling towards a functional brain

Wednesday, February 03, 2010

On Extraction

Scala case classes are great; they give you a tremendous leg up when it comes to implementing pattern matching. However, there's a drawback: the unapply method of case class companion objects take arguments of the type of the case class, and return Option[TupleN[...]] where N is the number of variables to the case class constructor. What's more, it's not possible to overload or override the unapply method to deconstruct other types of by creating your own companion object on the side, as shown in this REPL session:
scala> case class  Foo(val i: Int, val s: String)
defined class Foo

scala> object Foo {
     |   def unapply(s: String): Option[Foo] = Some(new Foo(s.substring(0, 2).toInt, s.substring(2)))
     | }
defined module Foo

scala> "12abcd" match {
     |   case Foo(i, s) => println(s, i)
     | }
<console>:10: error: wrong number of arguments for object Foo
         case Foo(i, s) => println(s, i)
              ^
<console>:10: error: wrong number of arguments for method println: (Any)Unit
         case Foo(i, s) => println(s, i)
Sadly, it doesn't even seem to be possible to move this extractor to another class and get the desired result:
scala> object SFoo {                                                                                 
     |   def unapply(s: String): Option[Foo] = Some(new Foo(s.substring(0, 2).toInt, s.substring(2)))
     | }
defined module SFoo

scala> "12abcd" match {                 
     |   case SFoo(i, s) => println(s, i)
     | }
<console>:9: error: wrong number of arguments for object SFoo
         case SFoo(i, s) => println(s, i)
              ^
<console>:9: error: wrong number of arguments for method println: (Any)Unit
         case SFoo(i, s) => println(s, i)

Fortunately, there is a way around this issue: simply define your own class that extends ProductN! Then, you can define as many extractors as you want in different pseudo-companion objects and use them idiomatically:
scala> class Foo(val i: Int, val s: String) extends Product2[Int, String] {
     |   val (_1, _2) = (i, s)
     | }
defined class Foo

scala> object Foo {
     |   def unapply(f: Foo): Option[(Int, String)] = Some((f.i, f.s))
     | }
defined module Foo

scala> object SFoo {
     |   def unapply(s: String): Option[Foo] = Some(new Foo(s.substring(0, 2).toInt, s.substring(2)))
     | }
defined module SFoo

scala> "12abcd" match {
     |   case SFoo(i, s) => println(s, i)
     | }
(abcd,12)

scala> new Foo(1, "hi") match {
     |   case Foo(i, s) => println(s, i)
     | }
(hi,1) 

Also, naturally, the trivial extractor (Foo => Foo) can supplant the Foo => Tuple2 extractor above:
scala> object Foo {
     |   def unapply(f: Foo): Option[Foo] = Some(f)
     | }

scala> new Foo(1, "hi") match {                  
     |   case Foo(i, s) => println(s, i)         
     | }                                
(hi,1)

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