Simple List Comprehensions in Swift

SwiftComprehension

infix operator <- { associativity right precedence 140 }
infix operator |? { associativity right precedence 140 }

func |? <S>(source: [S], predicates: [(s: S) -> Bool]) -> [S] {
    var ps = predicates
    return count(ps) > 0 ? (filter(source, ps.removeLast())) |? ps : source
}

func <-<S, T>(transform: (s: S) -> T, source: [S]) -> [T] {
    return map(source, transform)
}

// In Haskell, this would be [ x * 2 | x <- [1..20], x `mod` 2 == , x `mod` 3 == 0 ]
let output = { $0 * 2 } <- [Int](1...20) |? [{ $0 % 2 == 0 }, { $0 % 3 == 0 }]

println(output) // => [12, 24, 36]

// TODO: In Haskell, you can also draw from multiple lists:
// [ x*y | x <- [1..20], y <- [5..25]]