...

...

...

...

"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.

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.

_______________________________________________ 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

stupid me, that works and is more flexible than cale's solution. thanks! On 11/8/06, Nicolas Frisby 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 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 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.

_______________________________________________ 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