Scala中的String和Int模式匹配

Question

Why does pattern matching in Scala work for String and AnyVal s such as Int ?

Usually we see things like Case classes or Extractors ...

Answer 1

Extractors and case classes are used just for two of 13 kinds of patterns in Scala , "Extractor patterns" and "Constructor patterns" respectively. You can't use Int or String in this kind of pattern ( case String(x) ). But you can use them in other kinds:

1. Typed patterns, as in case x: String . In that case there is nothing special about String , you can do the same with any class (but there is something special about Int and other primitives: case x: Int actually checks if the received object is a java.lang.Integer in most cases).

2. Literal patterns, as in case 0 or case "" . Again, nothing special about strings, this works for all literals.

Answer 2

java.lang.String is enriched with scala.collection.immutable.StringOps ( http://www.scala-lang.org/api/2.11.8/#scala.collection.immutable.StringOps ) which mix scala.collection.immutable.StringLike ( http://www.scala-lang.org/api/2.11.8/#scala.collection.immutable.StringLike ) in. There you can find complementary methods, like apply.

String is a bit special as well, you can convert it to list of Char s, and use List extractors then like case List(a,b) or case x:xs on a String , bearing in mind that a and b will be Char s; x: Char and xs: List[Char]

All primitive types have Rich* classes (eg scala.runtime.RichBoolean , scala.runtime.RichByte ).

Value classes mechanics is used to enrich all of the above mentioned types ( http://docs.scala-lang.org/overviews/core/value-classes.html ). In compile time they have a wrapper type, like RichBoolean or RichInt but in runtime they are pure Boolean or Int types. In such way avoiding overhead of creating runtime objects.

Answer 3

val x: Any = 5
def f[T](v: T) = v match {
  case _: Int    => "Int"
  case _: String => "String"
  case _         => "Unknown"
}

Answer 4

You don't have to define unapply in class to able to use that class in switch/case -style pattern matching. unapply is used to deconstruct object, so, if you want to match switch in List -style ( case x:xs ), you should use unapply/unapplySeq . Good example here are regexps, are they are constructed from strings -- "something".r (note .r in the end).