Question

I am trying to port some code form java do F# that generates a grid of multidimensional points around a given point. I came up with this:

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;
    }

Apart from this code being horribly imperative (I would be grateful for hints how to fix it), I am getting a compilation error:

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 ‘!’.

How can I fix this error? How can I make it more functional?

EXAMPLE: For midpoint [|0.0, 1.0|], step size 0.5 and steps 1 I expect (in any order really)

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|]}

Please also note that this will be executed many times, so performance is critical.

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‘s are very good for such uses, and are not considered mutable variables (because they are not).