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.
>
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