stupid me, that works and is more flexible than cale's solution. thanks!
On 11/8/06, Nicolas Frisby <[EMAIL PROTECTED]> wrote:
Applying lift outside of modifyM is not a problem. It can seem a bit tricky with the function types around. Try > modifyM $ lift . myAdd 1 instead of > modifyM $ myAdd 1 Cale's should certainly work fine and lead to more concise code for what you're after. Just thought I'd mention this in case your needs change. Good luck, Nick On 11/8/06, Peter Steiner <[EMAIL PROTECTED]> wrote: > 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 <[EMAIL PROTECTED]> 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 <[EMAIL PROTECTED]> wrote: > > > >>>>> "Peter" == Peter Steiner <[EMAIL PROTECTED]> writes: > > > > > > Peter> On 11/8/06, Bulat Ziganshin <[EMAIL PROTECTED]> 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. > > > > > > _______________________________________________ > > > Haskell-Cafe mailing list > > > Haskell-Cafe@haskell.org > > > http://www.haskell.org/mailman/listinfo/haskell-cafe > > > > > _______________________________________________ > > Haskell-Cafe mailing list > > Haskell-Cafe@haskell.org > > http://www.haskell.org/mailman/listinfo/haskell-cafe > > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe@haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe >
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