如何建模递归函数类型? [英] How to model recursive function types?
问题描述
我很好奇这段代码将如何在 Scala 中建模.
I'm curious how this code would be modelled in Scala.
这是在golang中,并且是递归函数类型:
This is in golang, and it is a recursive function type:
type walkFn func(*int) walkFn
所以上面只是对类型的定义,walk函数是一个函数,它接受一个指向整数的指针,并返回一个walk函数.
So the above is just a defintion of the type, a walk function is a function that takes a pointer to an integer, and returns a walk function.
一个示例实现是:
func walkForward(i *int) walkFn {
*i += rand.Intn(6)
return pickRandom(walkEqual, walkBackward)
}
func walkBackward(i *int) walkFn {
*i += -rand.Intn(6)
return pickRandom(walkEqual, walkForward)
}
你可以在这里运行这样的代码:http://play.golang.org/p/621lCnySmy
You can run code like this here: http://play.golang.org/p/621lCnySmy
是否可以在 Scala 中编写类似这种模式的内容?
Is it possible to write something like this pattern in Scala?
推荐答案
这是可能的.您可以使用存在类型来欺骗"scala 的循环引用限制:
It's possible. You can use existential types to "cheat" scala's cyclic reference restriction:
type A[T <: A[_]] = Int => (Int, T)
lazy val walkEqual: A[A[_]] = (i: Int) =>
(i + Random.nextInt(7) - 3, if (Random.nextBoolean) walkForward else walkBackward)
lazy val walkForward: A[A[_]] = (i: Int) =>
(i + Random.nextInt(6), if (Random.nextBoolean) walkEqual else walkBackward)
lazy val walkBackward: A[A[_]] = (i: Int) =>
(i - Random.nextInt(6), if (Random.nextBoolean) walkEqual else walkForward)
def doWalk(count: Int, walkFn: A[_] = walkEqual, progress: Int = 0): Unit =
if (count > 0) {
val (nextProgress, nextStep: A[_] @unchecked) = walkFn(progress)
println(nextProgress)
doWalk(count - 1, nextStep, nextProgress)
}
结果:
scala> doWalk(10)
2
5
2
0
-3
-5
-4
-8
-8
-11
或者像@Travis Brown 一样:
Or like in @Travis Brown addition:
val locations = Stream.iterate[(Int,A[_] @unchecked)](walkEqual(0)) {
case (x: Int, f: A[_]) => f(x)
}.map(_._1)
scala> locations.take(20).toList
res151: List[Int] = List(-1, 1, 1, 4, 1, -2, 0, 1, 0, 1, 4, -1, -2, -4, -2, -1, 2, 1, -1, -2)
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