[Haskell-cafe] StateT and modify

[Peter Steiner]

hi haskellers,

i have a basic question regarding StateT encapsulating IO and the
modify function.

my scenario is similar to the following simple code snippet:

> import Control.Monad.State
>
> type MyState = StateT Int IO
>
> test = evalStateT foo 0
>
> foo = do
>    modify $ (+) 1
>    get

i would like to be able to debug what's happening inside the modifier
function. that's why i want to be able to use a modifier that's in the
IO monad, like in the following, obviously defunct snippet:

> test = evalStateT bar 0
>
> bar = do
>    modify $ myAdd 1
>    get
>
> myAdd :: Int -> Int -> IO Int
> myAdd x y = do
>    putStr "in myAdd\n"
>    return $ x + y

this fails because (myAdd :: Int -> Int -> IO Int) does not match the
required modify argument type (Int -> Int -> Int) for MyState.

    Couldn't match expected type `Int' against inferred type `IO Int'
    In the second argument of `($)', namely `myAdd 1'
    In the expression: modify $ (myAdd 1)
    In a 'do' expression: modify $ (myAdd 1)

is it possible to 'lift' StateT modify into the inner monad (IO in my case)?

regards,
peter.