[Haskell-beginners] Using StateT with List monad

[Brent Yorgey]

On Sat, Nov 24, 2012 at 02:52:42PM +0100, Nathan Hüsken wrote:
> Hey,
> 
> I have an example function in the list monad.
> 
> twoSucc :: Int -> [Int]
> twoSucc i = [i+1, i+2]
> 
> Now I want write something similar with the StateT monad transformer.
> 
> twoSucc :: StateT Int [] ()
> twoSucc = do
>   i <- get
>   put (i+1) -- how do I put [i+1,i+2] here?
> 
> As mentioned in the comment, when doing
> 
> runStateT twoSucc n
> 
> it outputs [((),n+1)] (which makes sense). How do I write it that it
> behaves similar to the original twoSucc and outputs [((),n+1),((),n+2)]?

StateT Int [] is an instance of MonadPlus which gives you access to
the nondeterminism of the list monad.  So you can do

  twoSucc :: StateT Int [] ()
  twoSucc = do
    i <- get
    put (i+1) `mplus` put (i+2)

Intuitively, mplus splits the computation into two independent
parallel branches.

-Brent