Scala provides a rich Collections API. Let's look at the useful groupBy() function. What does groupBy() do? It takes a collection, assesses each item in that collection against a discriminator function, and returns a Map data structure. Each key in the returned map is a distinct result of the discriminator function, and the key's corresponding value is another collection which contains all elements of the original one that evaluate the same way against the discriminator function. So, for example, here is a collection of Strings: val sports = Seq ("baseball", "ice hockey", "football", "basketball", "110m hurdles", "field hockey") Running it through the Scala interpreter produces this output showing our value's definition: sports: Seq[String] = List(baseball, ice hockey, football, basketball, 110m hurdles, field hockey) We can group those sports names by, say, their first letter. To do so, we need a disc