Scala: “any” and “all” functions

68
votes

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.

4
scalafunctional-programming
If you think Haskell is 99% SML, you either haven't gotten to monads yet or rate underlying principles way higher than the way actual code looks and works like (e.g. you'd also consider Java 99% C++). - user395760
how does haskell or SML are related to this question ( perhaps I'm missing the obvious ) - OscarRyz
well, let’s say that 99% of SML is in Haskell ;) - flying sheep
i remember writing similar (=functional) code in SML. That’s why i’m not completely alien to functional programming and not disturbed when having to pass a function as parameter to another) - flying sheep
Haskell and SML are quite different since Haskell is lazy by default. - Kim Stebel

4 Answers

121
votes

  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.

6
votes

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
2
votes
  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.

1
votes

How about exists:

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

It's on Traversable.