[Haskell-cafe] Delaling with State StateT and IO in the same function

[Andrew Wagner]

I don't know if this will help or not, but there's a basic StateT
example on the haskell wiki that you could look at, to see how to deal
with State in general. The code is at
http://www.haskell.org/haskellwiki/Simple_StateT_use and is thanks to
Don Stewart. Maybe I'll just paste the code with a few more comments
(with the warning that I'm a newbie as well):

import Control.Monad.State

main :: IO ()
main = runStateT code [1..] >> return ()
-- Here, the state is just a simple stack of integers. runStateT is
the equivalent
-- in the StateT monad of runState in the State monad

code :: StateT [Integer] IO ()
code = do
    x <- pop         -- pop an item out of the stack
    io $ print x      -- now in the INNER monad, perform an action
    return ()

--
-- pop the next unique off the stack
--
pop :: StateT [Integer] IO Integer
-- This type signature is correct, but it's the reason you have to be
-- careful with StateT. pop really has nothing to do with IO, but it has
-- been 'tainted' by IO because it's being done together with it
pop = do
    (x:xs) <- get     -- get the list that's currently in the stack
    put xs              -- put back all but the first
    return x            -- return the first

io :: IO a -> StateT [Integer] IO a
-- transform an action from being in the inner monad (in this case IO), to
-- being in the outer monad. since IO is so familiar, it's been written already
-- and it's called liftIO
io = liftIO

Gurus, please check my comments to be sure I haven't said something stupid!
Hope this helps.
Andrew

On 2/26/07, Alfonso Acosta <alfonso.acosta at gmail.com> wrote:
> On 2/27/07, Kirsten Chevalier <catamorphism at gmail.com> wrote:
> > So what if you changed your netlist function so that the type
> > sig would be:
> >
> > netlist :: DT.Traversable f =>
> >           (State s (S HDPrimSignal) -> State s  v ) -> -- new
> >           (State s (Type,v) -> S v -> State s ())   -> -- define
> >           State s (f HDPrimSignal) ->                  -- the graph
> >           IO (State s ())
> >
>
> > Or why not:
> >
> > netlist :: DT.Traversable f =>
> >           (State s (S HDPrimSignal) -> State s  v ) -> -- new
> >           (State s (Type,v) -> S v -> State s ())   -> -- define
> >           State s (f HDPrimSignal) ->                  -- the graph
> >           IO s
> >
>
> Uhm, this looks better, I'll try with this one and see what I get, I
> anyway suspect I'll have a hard time because of the nested monads
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