How do I sort a collection of Lists in lexicographic order in Scala?

Question

If A has the Ordered[A] trait, I\'d like to be able to have code that works like this

val collection: List[List[A]] = ... // constr         


        

Answer 1

In 2.8, you should be able to just do collection.sorted. sorted takes an implicit Ordering parameter. Any type that implements Ordered has a corresponding Ordering (thanks to the implicit conversion Ordering.ordered). There is also the implicit Ordering.Iterable that makes an Iterable[T] have an Ordering if T has an Ordering.

However, if you try this it doesn't work:

scala> def sort[A <: Ordered[A]](coll: List[List[A]]) = coll.sorted

:5: error: could not find implicit value for parameter ord: Ordering[List[A]]
       def sort[A <: Ordered[A]](coll: List[List[A]]) = coll.sorted
                                                             ^

You need to explicitly specify that you want the Ordering[Iterable[A]]:

def sort[A <: Ordered[A]](coll: List[List[A]]) = coll.sorted[Iterable[A]]

I'm not sure why the compiler can't find the Ordering[Iterable[A]] if the element type of the collection is List[A].