递归计算表达式

previous question 我被告知如何重写我的计算表达式,以便它使用尾部递归。我重写了我的代码,但仍然得到了StackOverflowException。为了定位问题,我使用状态单子(取自 this blog entry

type State<'a, 's> = State of ('s -> 'a * 's)

let runState (State s) initialState = s initialState

let getState = State (fun s -> (s,s))
let putState s = State (fun _ -> ((),s))

type StateBuilder() =
  member this.Return a = State (fun s -> (a, s))
  member this.Bind(m, k) = 
    State (fun s -> let (a,s') = runState m s in runState (k a) s')
  member this.ReturnFrom a = a
let state = new StateBuilder()

let s max = 
    let rec Loop acc = state {
        let! n = getState
        do! putState (n + 1)
        if acc < max then
            return! Loop (acc + 1)
        else return acc
        }
    Loop 0

runState (s 100000) 0

这将再次抛出StackOverflowException,尽管循环函数可以使用尾部递归(?)。我想国家建设者班出了点问题。我试着用延迟法做些什么。把所有东西都包在一个额外的lambda里,没有成功。

type State<'a, 's> = State of ('s -> 'a * 's)

let runState (State s) initialState = s initialState

let getState = fun () -> State (fun s -> (s,s))
let putState s = fun () -> State (fun _ -> ((),s))

type StateBuilder() =
  member this.Delay(f) = fun () -> f()
  member this.Return a = State (fun s -> (a, s))
  member this.Bind(m, k) = 
    fun () -> State (fun s -> let (a,s') = runState (m ()) s in runState ((k a) ()) s')
  member this.ReturnFrom a = a
let state = new StateBuilder()

let s max = 
    let rec Loop acc = state {
        let! n = getState
        do! putState (n + 1 - acc)
        if acc < max then
            return! Loop (acc + 2)
        else return acc
        }
    Loop 0

runState (s 100000 ()) 0