F# Monadic tree labeller

open StateMonad

type 'a tree = | Leaf of 'a
               | Node of 'a tree * 'a tree
            
          
// F# Monadic tree labeller 

let rec private mNode x =
    match x with
    | Leaf(c) -> state { let! x = GetState
                         do! SetState (x + 1)
                         return Leaf(c,x) }
                         
    | Node(l,r) -> state { let! l = mNode(l)
                           let! r = mNode(r)
                           return Node(l,r) }
                    
let mLabel x = Execute(mNode(x))


// F# Non-Monadic manual state passing tree labeller

let Label a s = 
     let rec Lab' a s = 
                match a with 
                |Leaf(c) -> (Leaf(c,s), s + 1) 
                
                |Node(l,r) -> let l' = Lab' l s 
                              let r' = Lab' r (snd l')
                              (Node(fst l', fst r'), (snd r')) 
     fst (Lab' a s)