How do I use State
to mimic the behaviour of List.zipWithIndex
? What I have come up with so far (which doesn't work) is:
def numberSA[A](list : List[A]) : State[Int, List[(A, Int)]] = list match {
case x :: xs => (init[Int] <* modify((_:Int) + 1)) map { s : Int => (x -> s) :: (numberSA(xs) ! s) }
case Nil => state( (i : Int) => i -> nil[(A, Int)] )
}
This is based very loosely on the state example. As I said, it does not work:
scala> res4
res5: List[java.lang.String] = List(one, two, three)
scala> numberSA(res4) ! 1
res6: List[(String, Int)] = List((one,1), (two,1), (three,1))
I can get it to work by changing a line of the case statement:
case x :: xs => (init[Int]) map { s : Int => (x -> s) :: (numberSA(xs) ! (s + 1)) }
But this just feels wrong. Can anyone help?
EDIT - more playing around has got me to this
def numberSA[A](list : List[A]) : State[Int, List[(A, Int)]] = {
def single(a : A) : State[Int, List[(A, Int)]] = (init[Int] <* modify((_ : Int) 开发者_如何学JAVA+ 1)) map { s : Int => List(a -> s) }
list match {
case Nil => state( (_ : Int) -> nil[(A, Int)] )
case x :: xs => (single(x) <**> numberSA(xs)) { _ ::: _ }
}
}
Can it be improved? Can it be generalized to containers other than List
(and, if so, what typeclasses are needed?)
EDIT 2 - I have now generalized it, albeit a bit clunkily
def index[M[_], A](ma : M[A])
(implicit pure : Pure[M], empty : Empty[M], semigroup : Semigroup[M[(A, Int)]], foldable : Foldable[M])
: State[Int, M[(A, Int)]] = {
def single(a : A) : State[Int, M[(A, Int)]] = (init[Int] <* modify((_ : Int) + 1)) map { s : Int => pure.pure(a -> s) }
foldable.foldLeft(ma, state( (_ : Int) -> empty.empty[(A, Int)] ), { (s : State[Int, M[(A, Int)]],a : A) => (s <**> single(a)) { (x,y) => semigroup.append(x,y)} } )
}
Or the very similar:
def index[M[_] : Pure : Empty : Plus : Foldable, A](ma : M[A])
: State[Int, M[(A, Int)]] = {
import Predef.{implicitly => ??}
def single(a : A) : State[Int, M[(A, Int)]] = (init[Int] <* modify((_ : Int) + 1)) map { s : Int => ??[Pure[M]].pure(a -> s) }
??[Foldable[M]].foldLeft(ma, state( (_ : Int) -> ??[Empty[M]].empty[(A, Int)] ), { (s : State[Int, M[(A, Int)]],a : A) => (s <**> single(a)) { (x,y) => ??[Plus[M]].plus(x,y)} } )
}
def index[M[_]:Traverse, A](m: M[A]) =
m.traverse[({type λ[x] = State[Int,x]})#λ, (A, Int)](a =>
state(i => (i + 1, (a, i)))) ! 0
Or even...
def index[M[_]:Traverse, A](m: M[A]) =
m.traverse[({type λ[x] = State[Int,x]})#λ, (A, Int)](a =>
(Lens.self[Int] += 1) map ((a, _)) ! -1
See The Essence of the Iterator Pattern for more on traversing with State.
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