[Haskell-cafe] Re: StateT and modify

[Nicolas Frisby]

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 <pnsteiner at gmail.com> 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 <nicolas.frisby at gmail.com> 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 <max.vasin at gmail.com> wrote:
> > > >>>>> "Peter" == Peter Steiner <pnsteiner at gmail.com> writes:
> > >
> > > Peter> On 11/8/06, Bulat Ziganshin <bulat.ziganshin at gmail.com> 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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