Example filter implementation in Swift.

myFilter

import Foundation

struct FilteringGenerator<T where T:Generator> : Generator {
    let test:(T.Element) -> Bool
    var generator:T
    
    init(_ generator:T, _ test: (T.Element) -> Bool) {
        self.test = test
        self.generator = generator
    }
    
    mutating func next() -> T.Element? {
        var item = generator.next()
        while item && !test(item!) {
            item = generator.next()
        }
        return item
    }
}

struct FilteringSequence<T where T:Sequence> : Sequence {
    
    let sequence:T
    let test:T.GeneratorType.Element -> Bool
    
    init(_ sequence:T, _ test:T.GeneratorType.Element -> Bool) {
        self.sequence = sequence
        self.test = test
    }
    
    func generate() -> FilteringGenerator<T.GeneratorType> {
        return FilteringGenerator(sequence.generate(), test)
    }
}

func myFilter<S : Sequence>(source: S, includeElement: (S.GeneratorType.Element) -> Bool) -> FilteringSequence<S> {
    return FilteringSequence(source, includeElement)
}

myFilterTests

class MyFilterTests: XCTestCase {
    
    func testBuiltIn() {
        self.measureBlock() {
            let x = filter(1..100000) { $0 % 5 == 0 }
            println(reduce(x, 0) { $1 - $0 })
        }
    }
    
    func testCustom() {
        self.measureBlock() {
            let x = myFilter(1..100000) { $0 % 5 == 0 }
            println(reduce(x, 0) { $1 - $0 })
        }
    }
    
    func testRaw() {
        self.measureBlock() {
            var acc = 0
            for i in 1..100000 {
                if i % 5 == 0 {
                    acc = i - acc
                }
            }
            println(acc)
        }
    }
}