c праздничком

Cedric Beust on Scala, updated

во дворце у снежной королевы

scala > implicit def ski[env,a,b](ef:env=>a=>b) = new { def S(es:env=>a) = (e:env) => ef(e)(es(e))}

scala> S _
res24: ((Nothing) => (Nothing) => Nothing) => ((Nothing) => Nothing) => (Nothing) => Nothing = <function1>

Не выходит каменный цветок.

code snippet

Still struggling implementing McBride/Paterson in Scala.

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))

Kind of funny how one has to bypass the lack of Hindley-Milner... oh, whatever.

Will explain all this later, when I'm done with McBride-Paterson.