Function which generically takes a type and returns the same type

Question

I am having a tough time understanding why the Scala compiler is unhappy about this function definition:

def trimNonWordCharacters[T <: Iterable[String]](items: T): T =
     items map { _.replaceAll("\\W", "") }

Here is the REPL output:

scala> def trimNonWordCharacters[T <: Iterable[String]](items: T): T =
     items map { _.replaceAll("\\W", "") }
<console>:5: error: type mismatch;
 found   : Iterable[java.lang.String]
 required: T
       def trimNonWordCharacters[T <: Iterable[String]](items: T): T = items map { _.replaceAll("\\W", "") }

The goal is to pass in any implementation of an Iterable and get the same type of back out. Is this possible?

Answer 1

The map method on Iterable returns an Iterable, so even if T is a subclass of Iterable, it's map method will return Iterable.

To get better typing, you'd have to write it like this:

import scala.collection.IterableLike
def trimNonWordCharacters[T <: Iterable[String]](items: T with IterableLike[String, T]): T =
     items map { _.replaceAll("\\W", "") }

However, that won't work either, because there's no information that let a map on T to generate another T. For example, mapping a BitSet into a String cannot result in a BitSet. So we need something else: something that teaches how to build a T from a T, where the mapped elements are of type String. Like this:

import scala.collection.IterableLike
import scala.collection.generic.CanBuildFrom
def trimNonWordCharacters[T <: Iterable[String]]
                         (items: T with IterableLike[String, T])
                         (implicit cbf: CanBuildFrom[T, String, T]): T =
     items map { _.replaceAll("\\W", "") }