Map and filter implemented with a version of reduce taking list as its reduce argument.

map_filter_reduce.swift

func reduce<T,R>(list: [T], block: (([R],T) -> R?)) -> [R]? {
    var acc = [R]()
    for x in list {
        if let val = block(acc, x) {
           acc += [val]
        }
    }
    
    return acc
}

func filter<T>(list: [T], filterPass: (T -> Bool)) -> [T]?{
    return reduce(list) {
        return filterPass($1) ? $1 : nil
    }
}

func map<T,R>(list:[T], mapClosure:(T -> R)) -> [R]? {
    return reduce(list) {
        return mapClosure($1)
    }
}