CODEWITHSUNDEEP
scalascala-collections
qonf
qonf

Reputation: 1091

How to succinctly map Map to Map

"map map to map", what a mouthful.

Anyway, I can do:

val list = List(1, 2, 3)
val list2 = list map (item => item + 1)
println(list2)
// List(2, 3, 4)

Why does the following not work?:

val ages = Map("alice" -> 21, "bob" -> 22)
val ages2 = ages map ((name, age) => (name, age + 1))

, and how do I go about making it work?

EDIT: I figured out that I had to do:

val ages2 = ages map {case (name, age) => (name, age + 1)}

, but that doesn't answer my first question, which is why my naive first approach does not work.

Upvotes: 2

Views: 498

Answers (1)

Yuval Itzchakov
Yuval Itzchakov

Reputation: 149538

It doesn't work because the Scala compiler doesn't automatically decompose the Tuple2 (which is the type of the key value pairs) in your Map.

This would work:

val ages2 = ages.map(pair => (pair._1, pair._2 + 1))

As it uses the tuple argument as a whole.

As you've noticed, using partial function syntax also works and allows you to automatically decompose the tuple:

val ages2 = ages map { case (name, age) => (name, age + 1) }

This is currently a limitation of the compiler. With dotty (the new new Scala compiler), this will be provided out of the box.

Upvotes: 5

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