Tower of Hanoi in F#

hanoi.fs

type Peg =  
    | A
    | B
    | C
 
type Move = 
    | Move of int * Peg * Peg
    static member toString m = 
        match m with
        | Move(n, p1, p2) when n <> 1 -> sprintf "move %i %A %A" n p1 p2
        | Move(_, p1, p2) -> sprintf "%A->%A" p1 p2
 
let move n p1 p2 = Move(n, A, B)
 
// solve with: move n A B |> hanoi
let hanoi x =
    let pegs = Set.ofList [A; B; C]
    
    let free (p1 : Peg) (p2: Peg) = 
        pegs |> Set.remove p1 |> Set.remove p2 |> Set.toList |> List.head
       
    let rec expand move = 
        match move with
        | Move (n, a, b) when n = 1 -> [Move (n, a, b)]
        | Move (n, a, b) -> 
            let f = free a b
            expand (Move(n-1, a, f)) @ [Move(1, a, b)] @ expand (Move(n-1, f, b))
 
    let moves = expand x
 
    List.map Move.toString moves