How does Scala's groupBy identity work?

Question

I was browsing around and found a question about grouping a String by it\'s characters, such as this:

The input:

\"aaabbbccccdd\"
         


        

Answer 1

To understand this just call scala repl with -Xprint:typer option:

val res2: immutable.Map[Char,String] = augmentString(str).groupBy[Char]({
   ((x: Char) => identity[Char](x))
});

Scalac converts a simple String into StringOps with is a subclass of TraversableLike which has a groupBy method:

def groupBy[K](f: A => K): immutable.Map[K, Repr] = {
    val m = mutable.Map.empty[K, Builder[A, Repr]]
    for (elem <- this) {
      val key = f(elem)
      val bldr = m.getOrElseUpdate(key, newBuilder)
      bldr += elem
    }
    val b = immutable.Map.newBuilder[K, Repr]
    for ((k, v) <- m)
      b += ((k, v.result))

    b.result
  }

So groupBy contains a map into which inserts chars return by identity function.