CODEWITHSUNDEEP
scalaclosures
armundle
armundle

Reputation: 1179

Understanding closure in scala?

I am not even sure if this is about closures, but I cannot reason about e in the following code.

type Set = Int => Boolean

/* characteristic function of Set */
def contains(s: Set, elem: Int): Boolean = s(elem)

/* definition of a singleton set, returning a set of only one given element  */
def singletonSet(elem: Int): Set = e => e == elem

In the above code, where does the e come from? How can I reason about that?

Upvotes: 1

Views: 70

Answers (1)

Yuval Itzchakov
Yuval Itzchakov

Reputation: 149538

This isn't about closure. e simply represents an element of type Int as dictated by the declaration of the Set type, which is an alias for a Function1[Int, Bool]. Think of e as a placeholder for future Int values which you'll pass. This is simply anonymous function syntax.

When you call singletonSet, you'll have to pass the Int parameter which e will reflect:

println(contains(singletonSet(1), 1)) // yields true
println(contains(singletonSet(1), 2)) // yields false

The same would work if you passed an anonymous function directly:

println(contains(e => e == 1, 1))

Upvotes: 2

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