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CHAPTER 5 PATTERN MATCHING
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Examining the Internals of Pattern Matching
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The next couple of paragraphs get into some gnarly parts of Scala. Let s write our own class that is nearly as syntactically pleasing as Scala s List. We will rely on a couple of syntactic features in Scala. The first is that Scala methods and classes can have operator characters as class names. The following are all valid method and class names: Foo, Foo_ , foo32, , :. The second is that methods that have a colon as their last character are evaluated right to left, rather than left to right. That means 3 :: Nil is desugared to Nil.::(3). Using these two features of Scala, let s define our own MList class that has the same patternmatching beauty as Scala s List class. The code in this section must be compiled because there are circular class references. You cannot type this code at the REPL. Here s the whole listing:
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class MList[+T] { def :[B >: T](x: B): MList[B] = new :(x, this) } case object MNil extends MList[Nothing] case class :[T](hd: T, tail: MList[T]) extends MList[T]
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First, let s define MList. It has a type of +T, which is part of Scala s type system, and it means that subclasses of MList have a covariant type.3 Covariant means that T in subclasses of MList can be the same class or a superclass of T.
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class MList[+T] { def :[B >: T](x: B): MList[B] = new :(x, this) } MList contains a single method, :, which is also the class name of the linked list cons cell. This allows chaining of constructing lists. Also, note that B, which is the type of the parameter, relates to T using the >: type relationship operator. This means that B must be the same class or a superclass of T. So, if you ve got an MList[String] and you add an Int cell, the MList s type becomes the class that s the superclass of both: Any. If you have an MList[Number] and you add an Int, the list is still MList[Number].
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Next, let s define the MNil singleton. This is an MList[Nothing]. Nothing is the subclass of every other class.4 Because MList is covariant, MList[Nothing] can serve as a member of every MList. If we ve got MNil and we call the : method with a String, because the superclass of the two is String, we add a String cell to MNil resulting in an MList[String].
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case object MNil extends MList[Nothing]
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3. Covariance and contravariance are scary terms and big concepts. We ll dive into them in 7. 4. It works best if you say it out loud.
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CHAPTER 5 PATTERN MATCHING
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Finally, let s define our cons cell. The cons cell holds the node and links to the tail of the list. The class name of the cons cell is :, which is the method name on MList that adds a new cell at the head of the list. The case class name : is the same as the method name : to unify the syntax of creating and pattern matching against the MList.
case class :[T](hd: T, tail: MList[T]) extends MList[T]
Finally, let s see how our new MList class looks in pattern matching:
def tryMList(in: MList[Any]) = in match { case 1 : MNil => "foo" case 1 : _ => "bar" case _ => "baz" }
So, this demonstrates that there s no internal magic to support Scala s List class in pattern matching. You can write your own classes that are as syntactically pleasing as Scala s libraries.
Pattern Matching As Functions
Scala patterns are syntactic elements of the language when used with the match operator. However, you can also pass pattern matching as a parameter to other methods. Scala compiles a pattern match down to a PartialFunction[A,B], which is a subclass of Function1[A,B]. So a pattern can be passed to any method that takes a single parameter function. This allows us to reduce
list.filter(a => a match { case s: String => true case _ => false })
list.filter { case s: String => true case _ => false }
Because patterns are functions and functions are instances, patterns are instances. In addition to passing them as parameters, they can also be stored for later use. In addition to Function1 s apply method, PartialFunction has an isDefinedAt method so that you can test to see whether a pattern matches a given value. If you try to apply a PartialFunction that s not defined for the value, a MatchError will be raised. How is this useful
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