How to use scala strings in list-like pattern matching - string

Question

So I was reading up about how scala lets you treat string as a sequence of chars through its implicit mechanism. I created a generic Trie class for a general element type and wanted to use it's Char based implementation with string like syntax.
import collection.mutable
import scala.annotation.tailrec
case class Trie[Elem, Meta](children: mutable.Map[Elem, Trie[Elem, Meta]], var metadata: Option[Meta] = None) {
def this() = this(mutable.Map.empty)
#tailrec
final def insert(item: Seq[Elem], metadata: Meta): Unit = {
item match {
case Nil =>
this.metadata = Some(metadata)
case x :: xs =>
children.getOrElseUpdate(x, new Trie()).insert(xs, metadata)
}
}
def insert(items: (Seq[Elem], Meta)*): Unit = items.foreach { case (item, meta) => insert(item, meta) }
def find(item: Seq[Elem]): Option[Meta] = {
item match {
case Nil => metadata
case x :: xs => children.get(x).flatMap(_.metadata)
}
}
}
object Trie extends App {
type Dictionary = Trie[Char, String]
val dict = new Dictionary()
dict.insert( "hello", "meaning of hello")
dict.insert("hi", "another word for hello")
dict.insert("bye", "opposite of hello")
println(dict)
}
Weird thing is, it compiles fine but gives error on running:
Exception in thread "main" scala.MatchError: hello (of class scala.collection.immutable.WrappedString)
at Trie.insert(Trie.scala:11)
at Trie$.delayedEndpoint$com$inmobi$data$mleap$Trie$1(Trie.scala:34)
at Trie$delayedInit$body.apply(Trie.scala:30)
at scala.Function0$class.apply$mcV$sp(Function0.scala:34)
at scala.runtime.AbstractFunction0.apply$mcV$sp(AbstractFunction0.scala:12)
at scala.App$$anonfun$main$1.apply(App.scala:76)
at scala.App$$anonfun$main$1.apply(App.scala:76)
at scala.collection.immutable.List.foreach(List.scala:381)
at scala.collection.generic.TraversableForwarder$class.foreach(TraversableForwarder.scala:35)
at scala.App$class.main(App.scala:76)
at Trie$.main(Trie.scala:30)
at Trie.main(Trie.scala)
It's able to implicitly convert String to WrappedString, but that doesn't match the ::. Any workarounds for this?

Answer 1

You can use startsWith as follows:
val s = "ThisIsAString"
s match {
case x if x.startsWith("T") => 1
case _ => 0
}
Or convert your String to List of chars with toList
scala> val s = "ThisIsAString"
s: String = ThisIsAString
scala> s.toList
res10: List[Char] = List(T, h, i, s, I, s, A, S, t, r, i, n, g)
An then use it as any other List
s.toList match {
case h::t => whatever
case _ => anotherThing
}

Answer 2

Your insert method declares item to be a Seq, but your pattern match only matches on List. A string can be implicitly converted to a Seq[Char], but it isn't a List. Use a pattern match on Seq instead of List using +:.
#tailrec
final def insert(item: Seq[Elem], metadata: Meta): Unit = {
item match {
case Seq() =>
this.metadata = Some(metadata)
case x +: xs =>
children.getOrElseUpdate(x, new Trie()).insert(xs, metadata)
}
}
The same applies to your find method.