f# - F# 闭包中的可变变量

Question

我正在尝试移植一些代码形式 java do F#，它会在给定点周围生成一个多维点网格。我想出了这个:

let gridGenerator midpoint stepSize steps = 
    seq {
        let n = Array.length midpoint
        let direction = Array.create n -steps
        let mutable lastIndex = n-1
        while lastIndex>=0 do
            let next = midpoint |> Array.mapi (fun i x -> x+ direction.[i]*stepSize)
            while lastIndex>=0 && direction.[lastIndex]=steps do 
                direction.[lastIndex]<- (-steps)
                lastIndex<-lastIndex-1;        
            if lastIndex>=0 then
                direction.[lastIndex]<-direction.[lastIndex]+1;
                lastIndex <- n-1;
            yield next;
    }

除了此代码非常重要(我将不胜感激提示如何修复它)，我收到一个编译错误:

Program.fs(18,15): error FS0407: The mutable variable 'lastIndex' is used in an invalid way. Mutable variables cannot be captured by closures. Consider eliminating this use of mutation or using a heap-allocated mutable reference cell via 'ref' and '!'.

我该如何解决这个错误？我怎样才能让它更实用？

示例:对于中点 [|0.0, 1.0|] 、步长 0.5 和我期望的步长 1 (实际上以任何顺序)

seq{[|-0.5, 0.5|], [|-0.5, 1.0|], [|-0.5, 1.5|], [|0.0, 0.5|], [|0.0, 1.0|], [|0.0, 1.5|], [|0.5, 0.5|], [|0.5, 1.0|], [|0.5, 1.5|]}

另请注意，这将执行多次，因此性能至关重要。

Answer 1

let gridGenerator midpoint stepSize steps =
    seq {
        let n = Array.length midpoint
        let direction = Array.create n -steps
        let lastIndex = ref (n - 1)
        while !lastIndex >= 0 do
            let next = midpoint |> Array.mapi (fun i x -> x + direction.[i] * stepSize)
            while !lastIndex >= 0 && direction.[!lastIndex] = steps do
                direction.[!lastIndex] <- -steps
                decr lastIndex
            if !lastIndex >= 0 then
                direction.[!lastIndex] <- direction.[!lastIndex] + 1
                lastIndex := n - 1
            yield next
    }

?
ref 非常适合此类用途，并且不被视为可变变量(因为它们不是)。