Question

In F# array2Ds are mutable which doesn’t make for the most functional code. At the moment, I’m hiding this by doing a Array2D.copy of my inputs when I want to make a change and return the array. i.e.

let test (x : int[,]) =
    let y = Array2D.copy x
    y.[0,0] <- 3

Is there a better approach that makes test more functional?

Answer 1

You can always use high-order functions on Array2D module to work with Array2D in a functional style. Particularly, Array2D.mapi operating on array indices is suitable for this task:

let test (xs: int[,]) =
   xs |> Array2D.mapi (fun i j x -> if i=0 && j=0 then 3 else x) 

EDIT:

Generally speaking, your test function is fine since you can limit side effects inside the function only. You need to be more careful in the case of nested arrays; however, writing your own functions with controlled side effects usually gives better performance than using high-order functions for Array2D.