gistfile1.txt

#!/usr/bin/env xcrun swift 

// Garth Snyder - August 4, 2014

struct TakeWhile<T: SequenceType> : SequenceType
{
    typealias Element = T.Generator.Element

    let underlyingSequence: T
    let keepGoing: Element -> Bool

    init(_ underlyingSequence: T, _ keepGoing: Element -> Bool) {
        self.underlyingSequence = underlyingSequence
        self.keepGoing = keepGoing
    }

    func generate() -> GeneratorOf<Element> {
        var underlyingGenerator = underlyingSequence.generate()
        return GeneratorOf<Element> {
            if let candidate = underlyingGenerator.next() {
                if self.keepGoing(candidate) {
                    return candidate
                }
            }
            return nil
        }
    }
}

var array = Array(TakeWhile([1, 2, 3, 4]) {$0 < 3})
println(array)

// Also allow takeWhile as a method on Array

extension Array {
    func takeWhile(keepGoing: T -> Bool) -> TakeWhile<Array> {
        return TakeWhile(self, keepGoing)
    }
}

var anotherArray = Array([1, 2, 3, 4].takeWhile {$0 < 3})
println(anotherArray)

// More tests from practicalswift

assert(equal([1, 2, 3, 4, 3, 2, 1].takeWhile { $0 < 3 }, [1, 2]))
assert(equal(["a", "b", "c", "d", "c", "b", "a"].takeWhile { $0 < "d" }, ["a", "b", "c"]))
assert(equal([1.1, 2.2, 3.3, 4.4, 3.3, 2.2, 1.1].takeWhile { $0 < 3 }, [1.1, 2.2]))