JavaScript、Python、Ruby、Swift 和 Scala 中的 Option/Maybe、Either 和 Future Monad

 class Monad { // pure :: a -> M a pure = () => { throw "pure method needs to be implemented" } // flatMap :: # M a -> (a -> M b) -> M b flatMap = (x) => { throw "flatMap method needs to be implemented" } // map :: # M a -> (a -> b) -> M b map = f => this.flatMap(x => new this.pure(f(x))) } export class Option extends Monad { // pure :: a -> Option a pure = (value) => { if ((value === null) || (value === undefined)) { return none; } return new Some(value) } // flatMap :: # Option a -> (a -> Option b) -> Option b flatMap = f => this.constructor.name === 'None' ? none : f(this.value) // equals :: # M a -> M a -> boolean equals = (x) => this.toString() === x.toString() } class None extends Option { toString() { return 'None'; } } // Cached None class value export const none = new None() Option.pure = none.pure export class Some extends Option { constructor(value) { super(); this.value = value; } toString() { return `Some(${this.value})` } }

 class Monad: # pure :: a -> M a @staticmethod def pure(x): raise Exception("pure method needs to be implemented") # flat_map :: # M a -> (a -> M b) -> M b def flat_map(self, f): raise Exception("flat_map method needs to be implemented") # map :: # M a -> (a -> b) -> M b def map(self, f): return self.flat_map(lambda x: self.pure(f(x))) class Option(Monad): # pure :: a -> Option a @staticmethod def pure(x): return Some(x) # flat_map :: # Option a -> (a -> Option b) -> Option b def flat_map(self, f): if self.defined: return f(self.value) else: return nil class Some(Option): def __init__(self, value): self.value = value self.defined = True class Nil(Option): def __init__(self): self.value = None self.defined = False nil = Nil()

 class Monad # pure :: a -> M a def self.pure(x) raise StandardError("pure method needs to be implemented") end # pure :: a -> M a def pure(x) self.class.pure(x) end def flat_map(f) raise StandardError("flat_map method needs to be implemented") end # map :: # M a -> (a -> b) -> M b def map(f) flat_map(-> (x) { pure(f.call(x)) }) end end class Option < Monad attr_accessor :defined, :value # pure :: a -> Option a def self.pure(x) Some.new(x) end # pure :: a -> Option a def pure(x) Some.new(x) end # flat_map :: # Option a -> (a -> Option b) -> Option b def flat_map(f) if defined f.call(value) else $none end end end class Some < Option def initialize(value) @value = value @defined = true end end class None < Option def initialize() @defined = false end end $none = None.new()

 import Foundation enum Maybe<A> { case None case Some(A) static func pure<B>(_ value: B) -> Maybe<B> { return .Some(value) } func flatMap<B>(_ f: (A) -> Maybe<B>) -> Maybe<B> { switch self { case .None: return .None case .Some(let value): return f(value) } } func map<B>(f: (A) -> B) -> Maybe<B> { return self.flatMap { type(of: self).pure(f($0)) } } }

 import language.higherKinds trait Monad[M[_]] { def pure[A](a: A): M[A] def flatMap[A, B](ma: M[A])(f: A => M[B]): M[B] def map[A, B](ma: M[A])(f: A => B): M[B] = flatMap(ma)(x => pure(f(x))) } object Monad { def apply[F[_]](implicit M: Monad[F]): Monad[F] = M implicit val myOptionMonad = new Monad[MyOption] { def pure[A](a: A) = MySome(a) def flatMap[A, B](ma: MyOption[A])(f: A => MyOption[B]): MyOption[B] = ma match { case MyNone => MyNone case MySome(a) => f(a) } } } sealed trait MyOption[+A] { def flatMap[B](f: A => MyOption[B]): MyOption[B] = Monad[MyOption].flatMap(this)(f) def map[B](f: A => B): MyOption[B] = Monad[MyOption].map(this)(f) } case object MyNone extends MyOption[Nothing] case class MySome[A](x: A) extends MyOption[A]

import Monad from './monad'; export class Either extends Monad { // pure :: a -> Either a pure = (value) => { return new Right(value) } // flatMap :: # Either a -> (a -> Either b) -> Either b flatMap = f => this.isLeft() ? this : f(this.value) isLeft = () => this.constructor.name === 'Left' } export class Left extends Either { constructor(value) { super(); this.value = value; } toString() { return `Left(${this.value})` } } export class Right extends Either { constructor(value) { super(); this.value = value; } toString() { return `Right(${this.value})` } } // attempt :: (() -> a) -> M a Either.attempt = f => { try { return new Right(f()) } catch(e) { return new Left(e) } } Either.pure = (new Left(null)).pure

from monad import Monad class Either(Monad): # pure :: a -> Either a @staticmethod def pure(value): return Right(value) # flat_map :: # Either a -> (a -> Either b) -> Either b def flat_map(self, f): if self.is_left: return self else: return f(self.value) class Left(Either): def __init__(self, value): self.value = value self.is_left = True class Right(Either): def __init__(self, value): self.value = value self.is_left = False

require_relative './monad' class Either < Monad attr_accessor :is_left, :value # pure :: a -> Either a def self.pure(value) Right.new(value) end # pure :: a -> Either a def pure(value) self.class.pure(value) end # flat_map :: # Either a -> (a -> Either b) -> Either b def flat_map(f) if is_left self else f.call(value) end end end class Left < Either def initialize(value) @value = value @is_left = true end end class Right < Either def initialize(value) @value = value @is_left = false end end

import Foundation enum Either<A, B> { case Left(A) case Right(B) static func pure<C>(_ value: C) -> Either<A, C> { return Either<A, C>.Right(value) } func flatMap<D>(_ f: (B) -> Either<A, D>) -> Either<A, D> { switch self { case .Left(let x): return Either<A, D>.Left(x) case .Right(let x): return f(x) } } func map<C>(f: (B) -> C) -> Either<A, C> { return self.flatMap { Either<A, C>.pure(f($0)) } } }

package monad sealed trait MyEither[+E, +A] { def flatMap[EE >: E, B](f: A => MyEither[EE, B]): MyEither[EE, B] = Monad[MyEither[EE, ?]].flatMap(this)(f) def map[EE >: E, B](f: A => B): MyEither[EE, B] = Monad[MyEither[EE, ?]].map(this)(f) } case class MyLeft[E](e: E) extends MyEither[E, Nothing] case class MyRight[A](a: A) extends MyEither[Nothing, A] // ... implicit def myEitherMonad[E] = new Monad[MyEither[E, ?]] { def pure[A](a: A) = MyRight(a) def flatMap[A, B](ma: MyEither[E, A])(f: A => MyEither[E, B]): MyEither[E, B] = ma match { case MyLeft(a) => MyLeft(a) case MyRight(b) => f(b) } }

 import Monad from './monad'; import { Either, Left, Right } from './either'; import { none, Some } from './option'; export class Future extends Monad { // constructor :: ((Either err a -> void) -> void) -> Future (Either err a) constructor(f) { super(); this.subscribers = []; this.cache = none; f(this.callback) } // callback :: Either err a -> void callback = (value) => { this.cache = new Some(value) while (this.subscribers.length) { const subscriber = this.subscribers.shift(); subscriber(value) } } // subscribe :: (Either err a -> void) -> void subscribe = (subscriber) => (this.cache === none ? this.subscribers.push(subscriber) : subscriber(this.cache.value)) toPromise = () => new Promise( (resolve, reject) => this.subscribe(val => val.isLeft() ? reject(val.value) : resolve(val.value)) ) // pure :: a -> Future a pure = Future.pure // flatMap :: (a -> Future b) -> Future b flatMap = f => new Future( cb => this.subscribe(value => value.isLeft() ? cb(value) : f(value.value).subscribe(cb)) ) } Future.async = (nodeFunction, ...args) => { return new Future(cb => nodeFunction(...args, (err, data) => err ? cb(new Left(err)) : cb(new Right(data))) ); } Future.pure = value => new Future(cb => cb(Either.pure(value))) // traverse :: [a] -> (a -> Future b) -> Future [b] Future.traverse = list => f => list.reduce( (acc, elem) => acc.flatMap(values => f(elem).map(value => [...values, value])), Future.pure([]) )

 from monad import Monad from option import nil, Some from either import Either, Left, Right from functools import reduce import threading class Future(Monad): # __init__ :: ((Either err a -> void) -> void) -> Future (Either err a) def __init__(self, f): self.subscribers = [] self.cache = nil self.semaphore = threading.BoundedSemaphore(1) f(self.callback) # pure :: a -> Future a @staticmethod def pure(value): return Future(lambda cb: cb(Either.pure(value))) def exec(f, cb): try: data = f() cb(Right(data)) except Exception as err: cb(Left(err)) def exec_on_thread(f, cb): t = threading.Thread(target=Future.exec, args=[f, cb]) t.start() def async(f): return Future(lambda cb: Future.exec_on_thread(f, cb)) # flat_map :: (a -> Future b) -> Future b def flat_map(self, f): return Future( lambda cb: self.subscribe( lambda value: cb(value) if (value.is_left) else f(value.value).subscribe(cb) ) ) # traverse :: [a] -> (a -> Future b) -> Future [b] def traverse(arr): return lambda f: reduce( lambda acc, elem: acc.flat_map( lambda values: f(elem).map( lambda value: values + [value] ) ), arr, Future.pure([])) # callback :: Either err a -> void def callback(self, value): self.semaphore.acquire() self.cache = Some(value) while (len(self.subscribers) > 0): sub = self.subscribers.pop(0) t = threading.Thread(target=sub, args=[value]) t.start() self.semaphore.release() # subscribe :: (Either err a -> void) -> void def subscribe(self, subscriber): self.semaphore.acquire() if (self.cache.defined): self.semaphore.release() subscriber(self.cache.value) else: self.subscribers.append(subscriber) self.semaphore.release()

 require_relative './monad' require_relative './either' require_relative './option' class Future < Monad attr_accessor :subscribers, :cache, :semaphore # initialize :: ((Either err a -> void) -> void) -> Future (Either err a) def initialize(f) @subscribers = [] @cache = $none @semaphore = Queue.new @semaphore.push(nil) f.call(method(:callback)) end # pure :: a -> Future a def self.pure(value) Future.new(-> (cb) { cb.call(Either.pure(value)) }) end def self.async(f, *args) Future.new(-> (cb) { Thread.new { begin cb.call(Right.new(f.call(*args))) rescue => e cb.call(Left.new(e)) end } }) end # pure :: a -> Future a def pure(value) self.class.pure(value) end # flat_map :: (a -> Future b) -> Future b def flat_map(f) Future.new(-> (cb) { subscribe(-> (value) { if (value.is_left) cb.call(value) else f.call(value.value).subscribe(cb) end }) }) end # traverse :: [a] -> (a -> Future b) -> Future [b] def self.traverse(arr, f) arr.reduce(Future.pure([])) do |acc, elem| acc.flat_map(-> (values) { f.call(elem).map(-> (value) { values + [value] }) }) end end # callback :: Either err a -> void def callback(value) semaphore.pop self.cache = Some.new(value) while (subscribers.count > 0) sub = self.subscribers.shift Thread.new { sub.call(value) } end semaphore.push(nil) end # subscribe :: (Either err a -> void) -> void def subscribe(subscriber) semaphore.pop if (self.cache.defined) semaphore.push(nil) subscriber.call(cache.value) else self.subscribers.push(subscriber) semaphore.push(nil) end end end

 import Foundation let background = DispatchQueue(label: "background", attributes: .concurrent) class Future<Err, A> { typealias Callback = (Either<Err, A>) -> Void var subscribers: Array<Callback> = Array<Callback>() var cache: Maybe<Either<Err, A>> = .None var semaphore = DispatchSemaphore(value: 1) lazy var callback: Callback = { value in self.semaphore.wait() self.cache = .Some(value) while (self.subscribers.count > 0) { let subscriber = self.subscribers.popLast() background.async { subscriber?(value) } } self.semaphore.signal() } init(_ f: @escaping (@escaping Callback) -> Void) { f(self.callback) } func subscribe(_ cb: @escaping Callback) { self.semaphore.wait() switch cache { case .None: subscribers.append(cb) self.semaphore.signal() case .Some(let value): self.semaphore.signal() cb(value) } } static func pure<B>(_ value: B) -> Future<Err, B> { return Future<Err, B> { $0(Either<Err, B>.pure(value)) } } func flatMap<B>(_ f: @escaping (A) -> Future<Err, B>) -> Future<Err, B> { return Future<Err, B> { [weak self] cb in guard let this = self else { return } this.subscribe { value in switch value { case .Left(let err): cb(Either<Err, B>.Left(err)) case .Right(let x): f(x).subscribe(cb) } } } } func map<B>(_ f: @escaping (A) -> B) -> Future<Err, B> { return self.flatMap { Future<Err, B>.pure(f($0)) } } static func traverse<B>(_ list: Array<A>, _ f: @escaping (A) -> Future<Err, B>) -> Future<Err, Array<B>> { return list.reduce(Future<Err, Array<B>>.pure(Array<B>())) { (acc: Future<Err, Array<B>>, elem: A) in return acc.flatMap { elems in return f(elem).map { val in return elems + [val] } } } } }

 package monad import java.util.concurrent.Semaphore class MyFuture[A] { private var subscribers: List[MyEither[Exception, A] => Unit] = List() private var cache: MyOption[MyEither[Exception, A]] = MyNone private val semaphore = new Semaphore(1) def this(f: (MyEither[Exception, A] => Unit) => Unit) { this() f(this.callback _) } def flatMap[B](f: A => MyFuture[B]): MyFuture[B] = Monad[MyFuture].flatMap(this)(f) def map[B](f: A => B): MyFuture[B] = Monad[MyFuture].map(this)(f) def callback(value: MyEither[Exception, A]): Unit = { semaphore.acquire cache = MySome(value) subscribers.foreach { sub => val t = new Thread( new Runnable { def run: Unit = { sub(value) } } ) t.start } subscribers = List() semaphore.release } def subscribe(sub: MyEither[Exception, A] => Unit): Unit = { semaphore.acquire cache match { case MyNone => subscribers = sub :: subscribers semaphore.release case MySome(value) => semaphore.release sub(value) } } } object MyFuture { def async[B, C](f: B => C, arg: B): MyFuture[C] = new MyFuture[C]({ cb => val t = new Thread( new Runnable { def run: Unit = { try { cb(MyRight(f(arg))) } catch { case e: Exception => cb(MyLeft(e)) } } } ) t.start }) def traverse[A, B](list: List[A])(f: A => MyFuture[B]): MyFuture[List[B]] = { list.foldRight(Monad[MyFuture].pure(List[B]())) { (elem, acc) => Monad[MyFuture].flatMap(acc) ({ values => Monad[MyFuture].map(f(elem)) { value => value :: values } }) } } } // ... implicit val myFutureMonad = new Monad[MyFuture] { def pure[A](a: A): MyFuture[A] = new MyFuture[A]({ cb => cb(myEitherMonad[Exception].pure(a)) }) def flatMap[A, B](ma: MyFuture[A])(f: A => MyFuture[B]): MyFuture[B] = new MyFuture[B]({ cb => ma.subscribe(_ match { case MyLeft(e) => cb(MyLeft(e)) case MyRight(a) => f(a).subscribe(cb) }) }) }

import { Future } from './future'; import { Either, Left, Right } from './either'; import { readFile } from 'fs'; import https from 'https'; const getResponse = url => new Future(cb => https.get(url, res => { var body = ''; res.on('data', data => body += data); res.on('end', data => cb(new Right(body))); res.on('error', err => cb(new Left(err))) })) const getShortResponse = url => getResponse(url).map(resp => resp.substring(0, 200)) Future .async(readFile, 'resources/urls.txt') .map(data => data.toString().split("\n")) .flatMap(urls => Future.traverse(urls)(getShortResponse)) .map(console.log)

 import http.client import threading import time import os from future import Future from either import Either, Left, Right conn = http.client.HTTPSConnection("en.wikipedia.org") def read_file_sync(uri): base_dir = os.path.dirname(__file__) #<-- absolute dir the script is in path = os.path.join(base_dir, uri) with open(path) as f: return f.read() def fetch_sync(uri): conn.request("GET", uri) r = conn.getresponse() return r.read().decode("utf-8")[:200] def read_file(uri): return Future.async(lambda: read_file_sync(uri)) def fetch(uri): return Future.async(lambda: fetch_sync(uri)) def main(args=None): lines = read_file("../resources/urls.txt").map(lambda res: res.splitlines()) content = lines.flat_map(lambda urls: Future.traverse(urls)(fetch)) output = content.map(lambda res: print("\n".join(res))) if __name__ == "__main__": main()

 require './lib/future' require 'net/http' require 'uri' semaphore = Queue.new def read(uri) Future.async(-> () { File.read(uri) }) end def fetch(url) Future.async(-> () { uri = URI(url) Net::HTTP.get_response(uri).body[0..200] }) end read("resources/urls.txt") .map(-> (x) { x.split("\n") }) .flat_map(-> (urls) { Future.traverse(urls, -> (url) { fetch(url) }) }) .map(-> (res) { puts res; semaphore.push(true) }) semaphore.pop

 import Foundation enum Err: Error { case Some(String) } func readFile(_ path: String) -> Future<Error, String> { return Future<Error, String> { callback in background.async { let url = URL(fileURLWithPath: path) let text = try? String(contentsOf: url) if let res = text { callback(Either<Error, String>.pure(res)) } else { callback(Either<Error, String>.Left(Err.Some("Error reading urls.txt"))) } } } } func fetchUrl(_ url: String) -> Future<Error, String> { return Future<Error, String> { callback in background.async { let url = URL(string: url) let task = URLSession.shared.dataTask(with: url!) {(data, response, error) in if let err = error { callback(Either<Error, String>.Left(err)) return } guard let nonEmptyData = data else { callback(Either<Error, String>.Left(Err.Some("Empty response"))) return } guard let result = String(data: nonEmptyData, encoding: String.Encoding.utf8) else { callback(Either<Error, String>.Left(Err.Some("Cannot decode response"))) return } let index = result.index(result.startIndex, offsetBy: 200) callback(Either<Error, String>.pure(String(result[..<index]))) } task.resume() } } } var result: Any = "" let _ = readFile("\(projectDir)/Resources/urls.txt") .map { data -> [String] in data.components(separatedBy: "\n").filter { (line: String) in !line.isEmpty } }.flatMap { urls in return Future<Error, String>.traverse(urls) { url in return fetchUrl(url) } }.map { responses in print(responses) } RunLoop.main.run()

 import scala.io.Source import java.util.concurrent.Semaphore import monad._ object Main extends App { val semaphore = new Semaphore(0) def readFile(name: String): MyFuture[List[String]] = MyFuture.async[String, List[String]](filename => Source.fromResource(filename).getLines.toList, name) def fetch(url: String): MyFuture[String] = MyFuture.async[String, String]( uri => Source.fromURL(uri).mkString.substring(0, 200), url ) val future = for { urls <- readFile("urls.txt") entries <- MyFuture.traverse(urls)(fetch _) } yield { println(entries) semaphore.release } semaphore.acquire }