PartialFunction.swift

struct PartialFunction<A, B> {
    let f: A throws -> B
    init(_ f: A throws -> B) {
        self.f = f
    }
}

extension Optional {
    func collect<B>(f: PartialFunction<Wrapped, B>) -> Optional<B> {
        if let res = try? self.map(f.f) {
            return res
        } else {
            return .None
        }
    }
}

enum PartialFunctionError: ErrorType {
    case MatchErorr
}

let f = PartialFunction<Int, String> { a in
    switch a {
    case 1:
        return "hello!!"
    default:
        throw PartialFunctionError.MatchErorr
    }
}

Optional<Int>(1).collect(f) // hello!
Optional<Int>(2).collect(f) // nil