How can I make a counter without Monad?

[Nicolas Oury]

Thanks for your help.

>> Are there other ways to implement a counter in Haskell?
>
> Using a State monad?
>


If I use your example on :

test   = let Node x l = enumeratedTree ( Node 'a' [undefined, Node 'b' 
[]])
			in tail l

GHCI answers
[Node (*** Exception: Prelude.undefined
A monadic counter imposes an order of evaluation.
In my program, I don't care about the order of the numbers.
I only want them to be all different.
I think a monad is too restrictive for what I need.


>> From some of my code:
>
>     let enumeratedTree =
>             (`evalState` (0::Int)) $ (`mapTreeM` t) $
> 		    \x -> do n <- next
> 			     return (n, x)
> 	next = do a <- get; put $! succ a; return a
>
> where
>
>     mapTreeM :: Monad m => (a -> m b) -> Tree a -> m (Tree b)
>     mapTreeM f (Node a ts) = do
>         b <- f a
>         ts' <- mapM (mapTreeM f) ts
>         return (Node b ts')
>
> (which could also be an instance of a popular non-standard FunctorM
> class)
>
> Best regards
> Tomasz
>