Passing Map as (flatMap) mapping function causes error

Question

I have a map:

  val mnem = Map(
    '2' -> "ABC", '3' -> "DEF", '4' -> "GHI", '5' -> "JKL",
    '6' -> "MNO", '7' -> "PQRS", '8' -> "TUV", '9' -> "WXYZ")

I'm curious why I can't pass this map as a mapping function considering that a Map is both a collection and a function such as below

val digits = "2345"                       //> digits  : String = 2345

digits flatMap mnem                       //> ERROR

But I can do this

digits flatMap(mnem(_))                   //> res0: String = ABCDEFGHIJKL

I was trying to do something analogous to this:

digits map mnem                           //> res1: scala.collection.immutable.IndexedSeq[String] = Vector(ABC, DEF, GHI, 
                                              //| JKL)    

Answer 1

Map[Char, String] is a function (Char => String).

trait Map[A, +B] extends ... MapLike[A, B, Map[A, B]] 
trait MapLike[A, +B, ...] extends PartialFunction[A, B] ...
trait PartialFunction[-A, +B] extends (A) ⇒ B

This works fine:

val f: Char => String = mnem

You'll get the same error with digits flatMap f.

flatMap on String takes Char ⇒ GenTraversableOnce[B] as parameter. And String is not a GenTraversableOnce, so (Char => String) (and Map[Char, String]) is not Char ⇒ GenTraversableOnce[B].

Scala rewrites Map.apply and Map(_) as x => Map(x) and then applies an implicit conversion to function result:

x => augmentString(Map(x))

It can convert Map(x) to get function of type Char ⇒ GenTraversableOnce[Char], but it can't convert existing function of type Char => String to Char ⇒ GenTraversableOnce[Char] since there is no such implicit conversion is scope.

Answer 2

To get the equivalent of the line

digits flatMap(mnem(_)) // mnem's apply method is the actual method passed to flatMap

you want

digits flatMap mnem.apply