Observations

implicit def ski[env,a,b](ef:env=>a=>b) = new { val S = (ea:env=>a) => (e:env) => ef(e)(ea(e))}
def K[env,a](x:a) = (gamma:env) => x
type E = Any => Int
def KEnv[X](x: X) = K[E, X](x)

val add = (i: Int) => (j: Int) => i+j

def fetch(x: String) = (env: E) => env(x)
trait Expr
case class Var(x: String)        extends Expr
case class Val(i: Int)           extends Expr
case class Add(p: Expr, q: Expr) extends Expr

def eval(exp: Expr): (E => Int) = exp match {
  case Var(x) => fetch(x)
  case Val(i) => KEnv(i)
  case Add(p,q) => KEnv(add) S eval(p) S eval(q)
}

eval(Add(Var("one"),Add(Var("three"), Val(11))))(Map("one" -> 1, "two" -> 2, "three" -> 3))

def K[env,a](x:a) = (gamma:env) => x
implicit def ski[env,a,b](ef:env=>a=>b) = new { 
  val S = (ea:env=>a) => (gamma:env) => ef(gamma)(ea(gamma))
}

val add = (i: Int) => (j: Int) => i+j
  type Env = String => Int
  def fetch(x: String) = (env: Env) => env(x)

  trait Expr
  case class Var(x: String)        extends Expr
  case class Val(i: Int)           extends Expr
  case class Add(p: Expr, q: Expr) extends Expr
  def Ke[T](x:T) = K[Env, T](x)
  
  def eval(exp: Expr): (Env => Int) = exp match {
    case Var(x) => fetch(x)
    case Val(i) => Ke(i)
    case Add(p,q) => (Ke(add) S (eval(p))) S (eval(q))
  }

eval(Add(Var("one"),Add(Var("three"), Val(11))))(Map("one" -> 1, "two" -> 2, "three" -> 3))

  def fileContent(key: String): Either[Any, String] = 
    BootConfig.settings.get(key).toRight.
    right.flatMap(probablyFile).
    right.flatMap(_.probablyText)

sassa_nf

  import scala.collection.immutable._

  trait Functor[T[_]]{
    def map[A,B](f:A=>B)(ta:T[A]):T[B]
  }

  trait Applicative[T[_]] extends Functor[T]{
    def pure[A](a:A):T[A]
    def ap[A,B](tf:T[A=>B])(ta:T[A]):T[B]
  }

  implicit def applicablize[A, B](fs: Set[A=>B]) = new {
    def <*>(as: Set[A]) = (for (f <- fs; a <- as) yield f(a)).toSet
  }

  implicit object AppSet extends Applicative[Set] {
    def pure[A](a: A) = Set(a)
    def ap[A,B](fs:Set[A=>B])(as:Set[A]):Set[B] = (for (f <- fs; a <- as) yield f(a)).toSet
    def map[A,B](f:A=>B)(as:Set[A]) = ap(pure(f))(as)
  }

  val p = AppSet.pure((s:String) => "'" + s + "'")
  val p2 = AppSet.pure((first: String) => (second:String) => first + " " + second)
  val names = Set("Jedi", "Michael Valentine")
  p <*> names
  val lasts = Set("Smith", "Frazer")
  p2 <*> names <*> lasts

def process(user: User, phone: Phone) = { val s = user.name + ": " + phone.number; println(s); s }

for (user  ← db.getUser(userId); // обычно это максимум один юзер
     phone ← db.getPhones(userId) // телефонов может быть и несколько (драма, если написано на джаве)
    ) process(user, phone)

select user.name, phone.number from user, phone where user.user.id = phone.userId

def process (user: User) (phone:Phone) = { val s = user.name + ": " + phone.number; println(s); s }

for (user  ← db.getUser(userId);
     f     = process(user);
     phone ← db.getPhones(userId);
    ) f(phone)

val fs     = db.getUser(userId).map(process)
val phones = db.getPhones(userId)
for (f ← fs; phone ← phones) f(phone)

val fs:     Set[Phone => String] = db.getUser(userId).map(process)
val phones: Set[Phone]           = db.getPhones(userId)

  def ap[A, B](f: F[A => B], a: F[A]): F[B]

val fs     = db.getUser(userId).map(process)
val phones = db.getPhones(userId)
val result = fs ap phones

val processes = Set(process)
ap(processes, db.getUser(userId))

  class Set[A]} {
  ...
  def ap[B](f: F[A => B], a: F[A]): F[B]

  trait Functor[T[_]]{
    def map[A,B](f:A=>B)(ta:T[A]):T[B]
  }

  trait Applicative[T[_]] extends Functor[T]{
    def pure[A](a:A):T[A]
    def ap[A,B](tf:T[A=>B])(ta:T[A]):T[B]
  }

  implicit def applicablize[A, B](fs: Set[A=>B]) = new {
    def <*>(as: Set[A]) = (for (f <- fs; a <- as) yield f(a)).toSet
  }

  implicit object AppSet extends Applicative[Set] {
    def pure[A](a: A) = Set(a)
    def ap[A,B](fs:Set[A=>B])(as:Set[A]):Set[B] = (for (f <- fs; a <- as) yield f(a)).toSet
    def map[A,B](f:A=>B)(as:Set[A]) = ap(pure(f))(as)
  }

  val p = AppSet.pure((s:String) => "'" + s + "'")
  val p2 = AppSet.pure((first: String) => (second:String) => first + " " + second)
  val names = Set("Jedi", "Michael Valentine")
  p <*> names
  val lasts = Set("Smith", "Frazer")
  p2 <*> names <*> lasts

  processes = AppSet.pure(process)
  processes <*> db.getUser(userId)

val result: Set[String] = AppSet.pure(process) <*> db.getUser(userId) <*> db.getPhones(userId)

String myDrive = new File(".").getAbsolutePath().split(":")[0]

User user = DaoFactoryManager.getUserDaoFactory().getUserDao().getUserByUserId(userId); if (user != null) ...

  livejournal.getFriends("ivan_gandhi") match {
    case None          => err.println("ну вот, опять Носик сломал код")
    case Some(friends) => { println("Это всё мои друзья:"); for (friend <- friends) println(" - " + friend) }
  }

for (f   <- livejournal.getFriends("ivan_gandhi");
     fof <- livejournal.getFriends(f)
    ) println(" - " + fof)

livejournal.getFriends("ivan_gandhi").flatMap(f => livejournal.getFriends(f)).foreach(x => println(" - " + x))

for (fOpt   <- livejournal.getFriends("ivan_gandhi");
     f      <- fOpt;
     fofOpt <- livejournal.getFriends(f);
     fof    <- fofOpt
    ) println(" - " + fof)

for (stockOpt   <- etrade.getPortfolio("vpatryshev", "password1");
     stock      <- stockOpt;
     batchOpt   <- etrade.getTrades("vpatryshev", "password1", stock);
     batch      <- batch
    ) println(stock + ": " + batch.size + "@" + batch.purchasePrice)

def optionT[M[_], T](optionalmt: Option[M[T]]) = optionalmt match {
  case None => M.unit(None)                // засунули None внутрь M, получив M[Option[T]]
  case Some(mt) => mt.map(t => Some(t))    // засунули Some() внутрь M, получив M[Option[T]] 
}

akuklev

функтор

монада

единица

умножение

законы монады

примеры

List

Option

Set

исключение

trait Exceptional[T] { def apply(); }
case class Ok[T](value: T)       extends Exceptional[T] { def apply = value }
case class Oops[T](x: Exception) extends Exceptional[T] { def apply = throw x }

def m[T](eet: Exceptional[Exceptional[T]]): Exceptional[T] = eet match {
  case Ok(et) => et // just extract the content which is an Exceptional anyway 
  case ex     => ex // it's the same exception

 
 val serverSocket = try {new ServerSocket(port)} catch {
    case e: java.net.BindException => {
      println("wtf, several copies? what is it? ")
      throw new IllegalStateException("this port, " + port + ", is already taken!")
    }
  }

int fact(int n) {
  return n < 2 ? 1 : n * fact(n-1);
}

int fact(int n) {
  if (n < 0 || n > 12) throw new IllegalArgumentException("won't calculate factorial for " + n + ", should be between 0 and 12");
  return n < 2 ? 1 : n * fact(n-1);
}

for (int f: factorial(n)) {
  System.out.println("factorial(" + n + ") = " + f);
}

List<int> factorial(final int n) {
  if (n < 0 || n > 12) return Collections.emptyList<Integer>();
  return n < 2 ? Arrays.asList(1) :
      factorial(n-1).map(new Function <Integer, Integer>() {
        public Integer apply(Integer k) {
          return n * k;
        }
      });
}

for (f <- factorial(n)) {
  println("factorial(" + n + ") = " + f);
}

def factorial(n: Int): List[Int] = {
  if      (n < 0 || n > 12) List()
  else if (n < 2)           List(1) 
  else                      factorial(n-1) map (x => n * x)
}

def factorial(n: Int): Option[Int] = {
  if      (n < 0 || n > 12) None
  else if (n < 2)           Some(1) 
  else                      factorial(n-1) map (x => n * x)
}