CODEWITHSUNDEEP
scalafunctional-programming
flying sheep
flying sheep

Reputation: 8942

Scala: “any” and “all” functions

my Haskell* is a bit rusty, so i can imagine that I’m missing the obvious:

def any[A](s: Traversable[A], f: A => Boolean): Boolean = {
    s.foldLeft(false)((bool, elem) => bool || f(elem))
}

Does one of these properties apply to the it?

  1. predefined somewhere in the Scala libs
  2. circumstantial, and faster written as some one-liner
  3. wrong (I didn’t test it, sorry ;))

*actually SML, but that’s 99% the same, but known by nobody under the sun.

Upvotes: 77

Views: 48687

Answers (4)

missingfaktor
missingfaktor

Reputation: 92036

  1. It's predefined and is called exists. And forall would be the "all" function you are looking for.

    scala> Vector(3, 4, 5).exists(_ % 2 == 0)
res1: Boolean = true

scala> Vector(3, 4, 5).forall(_ % 2 == 0)
res2: Boolean = false

  2. You can make it more performant using a for loop with a break (from scala.util.control.Breaks). (See the standard library implementation of exists and forall.)

  3. It's correct.

Upvotes: 141

rafalotufo
rafalotufo

Reputation: 3932

  1. No it isn't predifined with those names. You can use exists from Traversable package.

  2. The biggest disadvantage of your implementation is that will necessary consume all of your traversible, when, for any, if any is true, if could already give you your answer. The same goes for all. But one could easily implement this so that it doesn't evaluate the whole sequence. Another solution would be to implement a monad for this type of operation. Then you would call:

    a and b and c which is equivalent to a.and(b).and(c)

  3. It is correct.

BTW, another function that I find missing is a sum function.

Upvotes: 2

David Winslow
David Winslow

Reputation: 8590

Methods exist on the Traversable trait which are equivalent to any and all:

def all[A](xs: Traversable[A], p: A => Boolean): Boolean = xs forall p

def any[A](xs: Traversable[A], p: A => Boolean): Boolean = xs exists p

Upvotes: 6

Adam Rabung
Adam Rabung

Reputation: 5224

How about exists:

scala> List(1,2,3).exists(_ > 2)
res12: Boolean = true

It's on Traversable.

Upvotes: 1

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