Code Examples for

Programming in Scala, Fourth Edition

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26 Extractors

Sample run of chapter's interpreter examples

26.1 An example: extracting email addresses


def isEMail(s: String): Boolean def domain(s: String): String def user(s: String): String
if (isEMail(s)) println(user(s) + " AT " + domain(s)) else println("not an email address")
EMail(user, domain)
// In file extractors/IsEmail.scala s match { case EMail(user, domain) => println(user + " AT " + domain) case _ => println("not an email address") }
ss match { case EMail(u1, d1) :: EMail(u2, d2) :: _ if (u1 == u2) => ... ... }

26.2 Extractors


// In file extractors/EMail.scala object EMail { // The injection method (optional) def apply(user: String, domain: String) = user + "@" + domain // The extraction method (mandatory) def unapply(str: String): Option[(String, String)] = { val parts = str split "@" if (parts.length == 2) Some(parts(0), parts(1)) else None } }
object EMail extends ((String, String) => String) { ... }
unapply("John@epfl.ch") equals Some("John", "epfl.ch") unapply("John Doe") equals None
selectorString match { case EMail(user, domain) => ... }
EMail.unapply(selectorString)
val x: Any = ... x match { case EMail(user, domain) => ... }
// In file extractors/Misc.scala EMail.unapply(EMail.apply(user, domain))
Some(user, domain)
EMail.unapply(obj) match { case Some(u, d) => EMail.apply(u, d) }

26.3 Patterns with zero or one variables


// In file extractors/Twice.scala object Twice { def apply(s: String): String = s + s def unapply(s: String): Option[String] = { val length = s.length / 2 val half = s.substring(0, length) if (half == s.substring(length)) Some(half) else None } }
// In file extractors/UpperCase.scala object UpperCase { def unapply(s: String): Boolean = s.toUpperCase == s }
// In file extractors/Test2.scala def userTwiceUpper(s: String) = s match { case EMail(Twice(x @ UpperCase()), domain) => "match: " + x + " in domain " + domain case _ => "no match" }
scala> userTwiceUpper("DIDI@hotmail.com") res0: String = match: DI in domain hotmail.com scala> userTwiceUpper("DIDO@hotmail.com") res1: String = no match scala> userTwiceUpper("didi@hotmail.com") res2: String = no match

26.4 Variable argument extractors


// In file extractors/DomainTest.scala dom match { case Domain("org", "acm") => println("acm.org") case Domain("com", "sun", "java") => println("java.sun.com") case Domain("net", _*) => println("a .net domain") }
// In file extractors/Domain.scala object Domain { // The injection method (optional) def apply(parts: String*): String = parts.reverse.mkString(".") // The extraction method (mandatory) def unapplySeq(whole: String): Option[Seq[String]] = Some(whole.split("\\.").reverse.toSeq) }
// In file extractors/DomainTest.scala def isTomInDotCom(s: String): Boolean = s match { case EMail("tom", Domain("com", _*)) => true case _ => false }
scala> isTomInDotCom("tom@sun.com") res3: Boolean = true scala> isTomInDotCom("peter@sun.com") res4: Boolean = false scala> isTomInDotCom("tom@acm.org") res5: Boolean = false
// In file extractors/ExpandedEMail.scala object ExpandedEMail { def unapplySeq(email: String) : Option[(String, Seq[String])] = { val parts = email split "@" if (parts.length == 2) Some(parts(0), parts(1).split("\\.").reverse.toSeq) else None } }
scala> val s = "tom@support.epfl.ch" s: String = tom@support.epfl.ch scala> val ExpandedEMail(name, topdom, subdoms @ _*) = s name: String = tom topdom: String = ch subdoms: Seq[String] = WrappedArray(epfl, support)

26.5 Extractors and sequence patterns


List() List(x, y, _*) Array(x, 0, 0, _)
package scala object List { def apply[T](elems: T*) = elems.toList def unapplySeq[T](x: List[T]): Option[Seq[T]] = Some(x) ... }
List() List(1, 2, 3)

26.6 Extractors versus case classes


case C(...)

26.7 Regular expressions


scala> import scala.util.matching.Regex
scala> val Decimal = new Regex("(-)?(\\d+)(\\.\\d*)?") Decimal: scala.util.matching.Regex = (-)?(\d+)(\.\d*)?
scala> val Decimal = new Regex("""(-)?(\d+)(\.\d*)?""") Decimal: scala.util.matching.Regex = (-)?(\d+)(\.\d*)?
scala> val Decimal = """(-)?(\d+)(\.\d*)?""".r Decimal: scala.util.matching.Regex = (-)?(\d+)(\.\d*)?
package scala.runtime import scala.util.matching.Regex class StringOps(self: String) ... { ... def r = new Regex(self) }
scala> val Decimal = """(-)?(\d+)(\.\d*)?""".r Decimal: scala.util.matching.Regex = (-)?(\d+)(\.\d*)? scala> val input = "for -1.0 to 99 by 3" input: String = for -1.0 to 99 by 3 scala> for (s <- Decimal findAllIn input) | println(s) -1.0 99 3 scala> Decimal findFirstIn input res7: Option[String] = Some(-1.0) scala> Decimal findPrefixOf input res8: Option[String] = None
scala> val Decimal(sign, integerpart, decimalpart) = "-1.23" sign: String = - integerpart: String = 1 decimalpart: String = .23
scala> val Decimal(sign, integerpart, decimalpart) = "1.0" sign: String = null integerpart: String = 1 decimalpart: String = .0
scala> for (Decimal(s, i, d) <- Decimal findAllIn input) | println("sign: " + s + ", integer: " + | i + ", decimal: " + d) sign: -, integer: 1, decimal: .0 sign: null, integer: 99, decimal: null sign: null, integer: 3, decimal: null

26.8 Conclusion

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