scala - Map versus FlatMap on String

Question

Listening to the Collections lecture from Functional Programming Principles in Scala, I saw this example:

scala> val s = "Hello World"

scala> s.flatMap(c => ("." + c)) // prepend each element with a period
res5: String = .H.e.l.l.o. .W.o.r.l.d

Then, I was curious why Mr. Odersky didn't use a map here. But, when I tried map, I got a different result than I expected.

scala> s.map(c => ("." + c))
res8: scala.collection.immutable.IndexedSeq[String] = Vector(.H, .e, .l, .l, .o, 
                                                          ". ", .W, .o, .r, .l, 

I expected that above call to return a String, since I'm map-ing, i.e. applying a function to each item in the "sequence," and then returning a new "sequence."

However, I could perform a map rather than flatmap for a List[String]:

scala> val sList = s.toList
sList: List[Char] = List(H, e, l, l, o,  , W, o, r, l, d)

scala> sList.map(c => "." + c)
res9: List[String] = List(.H, .e, .l, .l, .o, ". ", .W, .o, .r, .l, .d)

Why was a IndexedSeq[String] the return type of calling map on the String?

Answer 1

The reason for this behavior is that, in order to apply "map" to a String, Scala treats the string as a sequence of chars (IndexedSeq[String]). This is what you get as a result of the map invocation, where for each element of said sequence, the operation is applied. Since Scala treated the string as a sequence to apply map, that is what mapreturns.

flatMap then simply invokes flatten on that sequence afterwards, which then "converts" it back to a String