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

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:

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"Peter" == Peter Steiner writes:

Peter> On 11/8/06, Bulat Ziganshin wrote:

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Hello Peter,

Wednesday, November 8, 2006, 1:48:24 PM, you wrote:

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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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