CODEWITHSUNDEEP
scala
ChucK
ChucK

Reputation: 2134

Scala: What does mean to pass a Set to the map function of a set

I've just seen I can do:

Set(1, 2, 3).map(Set(1))

which yields the result:

Set(true, false)

But I thought that the map function can only take another function not a new Set. If anything I'd expect this to return a set of sets. What's going on and what does the result mean?

Upvotes: 14

Views: 5936

Answers (3)

missingfaktor
missingfaktor

Reputation: 92026

  • Set[A] extends Function1[A, Boolean].
  • Map[K, V] extends PartialFunction[K, V] which in turn extends Function1[K, V].
  • Seq[A] extends PartialFunction[Int, A] which in turn extends Function1[Int, A].

So in Scala collections library, sets, sequences, and maps are themselves functions.

  • apply method in Set[_] tests set membership.
  • apply method in Map[_, _] returns the value associated with the given key. Throws error if the key is absent.
  • apply method in Seq[_] returns the element at given index. Throws error for an invalid index.

The following two are equivalent:

Set(1, 2, 3).map(Set(1))

Set(1, 2, 3).map(x => Set(1).contains(x))

So are the following two:

Seq(1, 2, 3).collect(someMap)

Seq(1, 2, 3) collect { 
  case x if someMap contains x => someMap(x)
}

And also the following two:

Seq(1, 2, 3).map(someSeq)

Seq(1, 2, 3).map(x => someSeq(x))

Upvotes: 5

Jörg W Mittag
Jörg W Mittag

Reputation: 369458

But I thought that the map function can only take another function not a new Set.

A Set is a function. It is a function from its elements to booleans: when you pass it an element, it tells you whether that element is part of the Set.

Set(1, 2, 3).map(Set(1))

Iterates over the Set(1, 2, 3), passing each each element to the Set(1). I.e. it first asks "is 1 a member of set {1}", which is true, then it asks the same question for 2 and 3, which is false.

So, the result is Set(true, false, false), which of course is just Set(true, false).

Similarly, a sequence is a function from integers to elements, a map is a function from keys to values.

Upvotes: 20

tenshi
tenshi

Reputation: 26566

Set is also function - it extends Function1. See Inherited section in Scaladoc:

http://www.scala-lang.org/api/current/scala/collection/immutable/Set.html

Upvotes: 5

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