[Haskell-beginners] Using StateT with List monad

[Karl Voelker]

On Sat, Nov 24, 2012 at 5:52 AM, Nathan Hüsken <nathan.huesken at posteo.de>wrote:

> 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.
>

The simplest addition of StateT would look like this:

twoSucc :: StateT () [] Int
twoSucc i = lift [i+1, i+2]


> twoSucc :: StateT Int [] ()
> twoSucc = do
>   i <- get
>   put (i+1) -- how do I put [i+1,i+2] here?
>

In this transformation, you've moved your Int result from being the monadic
result to being the stored state. I don't know which transformation you
really want.

-Karl
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