基本斯卡拉州议题 [英] Basic Scalaz State question
问题描述
如何使用 State
模仿 List.zipWithIndex
的行为?我到目前为止(不起作用)的是:
def numberSA [A](list:List [A]):State [Int,List [(A,Int)]] =列表匹配{
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)])
}
这非常基于状态示例。正如我所说,它不起作用:
scala> res4
res5:List [java.lang.String] =列表(一,二,三)
scala> numberSA(res4)! 1
res6:List [(String,Int)] = List((one,1),(two,1),(three,1))
我可以通过更改case语句的一行来实现它:
case x :: xs => (init [Int])map {s:Int => (x - > s)::(numberSA(xs)!(s + 1))}
但这只是感觉不对。任何人都可以帮忙吗?
编辑 - 让更多人玩这个游戏
def numberSA [A](list:List [A]):State [Int,List [(A,Int)]] = {
def single(a:A):State [Int,List [(A,Int)]] =(init [Int] <* modify((_:Int)+ 1))map {s:Int =>列表(a - > s)}
列表匹配{
case Nil => state((_:Int) - > nil [(A,Int)])
case x :: xs => (single(x)** numberSA(xs)){_ ::: _}
}
}
是否可以改进?是否可以推广到 List
之外的容器(如果需要,需要什么类型的类?)
编辑2 - 现在我已经概括了它,尽管有点笨拙
<$ c [c],def index [M [_],A](ma:M [A])
(隐式纯:Pure [M],empty:空[M],半群:Semigroup [M [ ,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 => (p,int) - > empty.mpty [(A,Int)]),{(s:State [Int ,M((A,I))],a:A)=>(s **单(a)){(x,y)=> semigroup.append(x,y)}} )
}
或者非常相似:
def index [M [_]:Pure:Empty:Plus:Foldable,A](ma:M [A])
:State [Int,M [(A,Int)]] = {
import Predef。{implicitly => (a:A):State [Int,M [(A,Int)]] =(init [Int]< * modify((_:Int)+ 1))map { s:Int =>折叠[M]]。foldLeft(ma,state((_:Int) - > [空[[]]纯净的(a-> s)}
[ (A,Int)]],空([A,Int)]),{(s:State [Int,M [(A,Int)]],a:A)=>(s< )){(x,y)=> ?? [Plus [M]]。plus(x,y)}})
}
def index [M [_]:遍历,A](m:M [A ] =
m.traverse [({typeλ[x] = State [Int,x]})#λ,(A,Int)](a =>
state(i => ;(i + 1,(a,i))))! 0
甚至...
def index [M [_]:遍历,A](m:M [A])=
m.traverse [({typeλ[x] = State [Int, (a,)))(a =>
(Lens.self [Int] + = 1)map((a,_))!-1
code>
请参阅迭代器模式的本质,了解更多关于状态遍历的信息。
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) + 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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