gistfile1.swift

func catOptionals<T,S where S:SequenceType, S.Generator.Element == Optional<T>>(optionals: S) -> [T] {
    return reduce(optionals, []) { (acc, value: T?) -> [T] in
        switch value {
        case let .Some(v): return acc + [v]
        default: return acc
        }
    }
}
 

let a: [Int?] = [4,7,nil,8]
catOptionals(a)
// returns [4,7,8]

Note that array+array in Swift has lousy performance compared to Haskell's List prepend. It forces a lot of real copying (not just copy-on-write). This function has O(n^2) performance if I remember correctly.

I guess I'll stick with a O(2n) pass with a filter then map will be better than a O(n^2) for now.