Scala：“任何”和“所有”功能

Question

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.

Answer 1

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.

Answer 2

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

Answer 3

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.

Answer 4

How about exists :

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

It's on Traversable .