Re: [Haskell-cafe] Re: StateT and modify

8 Nov 2006

      cale's solution worked fine for me (i forgot to cc this list in my response).

i have troubles getting your modifyM to compile, and i do not really
understand how it might without somehow lifting the function into the
inner monad.
...
import Control.Monad.State
type MyState = StateT Int IO
test = evalStateT bar 0
modifyM :: (MonadState s m) => (s -> m s) -> m ()
modifyM f = do
   s <- get
   s' <- f s
   put s'
bar :: MyState Int
bar = do
   modifyM $ myAdd 1
   get
myAdd :: Int -> Int -> IO Int
myAdd x y = do
   putStr "in myAdd\n"
   return $ x + y
fails with:

    Couldn't match `StateT Int IO' against `IO'
      Expected type: StateT Int IO
      Inferred type: IO
    In a 'do' expression: modifyM $ (myAdd 1)
    In the definition of `bar':
        bar = do
                modifyM $ (myAdd 1)
                get

and applying lift is not possible outside of modifyM.
what am i doing wrong?

regards,
peter.

On 11/8/06, Nicolas Frisby  wrote:
...
Regardless of what monad is transformed by StateT, I think the OP's
issue remains.
modify below is straight from Gill's source at
http://darcs.haskell.org/packages/
modify :: (MonadState s m) => (s -> s) -> m ()
modify f = do
        s <- get
        put (f s)
we could add
modifyM :: (MonadState s m) => (s -> m s) -> m ()
modifyM f = do
        s <- get
        s' <- f s
       put s'
which I think you could use...
modifyM is just a bit more flexible than Cale's liftModify, I think.
On 11/8/06, Max Vasin  wrote:
...
...
...
...
...
> "Peter" == Peter Steiner  writes:
Peter> On 11/8/06, Bulat Ziganshin  wrote:
...
...
Hello Peter,
Wednesday, November 8, 2006, 1:48:24 PM, you wrote:
...
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
for debugging there is 'trace' function which don't needs IO
monad
Peter> thanks. i am aware of trace, but the potentially messed up
Peter> execution order makes it very hard for me to get useful
Peter> information out of the resulting trace. besides, IO will
Peter> scale to more elaborate logging mechanisms later on...
If all you want from IO is logging why not just use MonadWriter?
--
WBR,
Max Vasin.
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