CODEWITHSUNDEEP
scalatraitsmixins
Nicolas Cailloux
Nicolas Cailloux

Reputation: 448

Scala anonymous mixin?

when debugging I had to inspect the scala.collection.GenSetLike file. I encountered this little bit of code in the interface: 

trait GenSetLike[A, +Repr] 
extends GenIterableLike[A, Repr]
  with (A => Boolean)
  with Equals
  with Parallelizable[A, parallel.ParSet[A]] {
  ...
}

What does this (A => Boolean) mix means? I tried searching with various keywords but could not find something satisfactory.
This syntax indicated a function that takes an A and returns a Boolean, but I can't see the meaning in the trait mixin context...

My first guess was that is related to stuff like constructor or apply, but this is just an intuition.

Please teach me!

Upvotes: 2

Views: 130

Answers (2)

Eugene Platonov
Eugene Platonov

Reputation: 3285

Set in Scala implements Function1[A, Boolean] (A => Boolean) interface overriding apply to be same as contains, so sets in Scala can be used as functions. 

> val s = Set(1,2,3) 
s: Set[Int] = Set(1, 2, 3)

> s(2) 
res1: Boolean = true

> s.apply(3) 
res2: Boolean = true

> s(23) 
res3: Boolean = false

This is, btw, the reason why Set is invariant.

Upvotes: 1

Nagarjuna Pamu
Nagarjuna Pamu

Reputation: 14825

A => Boolean is the syntactic sugar for Function1 trait

Scala REPL

scala> def f[A] = (a: A) => true
f: [A]=> A => Boolean

The above lambda can also be written as 

scala> def f[A]: Function1[A, Boolean] = (a: A) => true
f: [A]=> A => Boolean

Upvotes: 1

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