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

On Sat, Nov 24, 2012 at 5:52 AM, Nathan Hüsken 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