Question

Could someone propose better and/or more elegant implementation of this:

let each xs = 
    let rec each' acc left right = 
        match right with
        | [] -> acc
        | right ->  let new_left  = left @ [List.hd right]
                    let next   = List.tl right
                    let result = (List.hd right), left @ next
                    each' (result::acc) new_left next
    each' [] [] xs

It do that:

> each [1..3];;
val it : (int * int list) list = [(3, [1; 2]); (2, [1; 3]); (1, [2; 3])]

This function could return the result in reverse direction, too. The idea is to get all elements as tuples with an element and list of rest elements.

Answer 1

The semantic is slightly different here, but from the example you give Set might be a good fit:

let each xs =
    let X = set xs                           
    [ for x in xs -> (x, X - set [x]) ]


> fsi.AddPrinter( fun (x:Set<int>) -> sprintf "%A" (Set.to_list x))
> each [1..3];;
> val it : (int * Set<int>) list = [(1, [2; 3]); (2, [1; 3]); (3, [1; 2])]

// Edited as per comments.