[Haskell-cafe] StateT and modify
pnsteiner at gmail.com
Wed Nov 8 05:48:24 EST 2006
i have a basic question regarding StateT encapsulating IO and the
my scenario is similar to the following simple code snippet:
> import Control.Monad.State
> type MyState = StateT Int IO
> test = evalStateT foo 0
> foo = do
> modify $ (+) 1
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, like in the following, obviously defunct snippet:
> test = evalStateT bar 0
> bar = do
> modify $ myAdd 1
> myAdd :: Int -> Int -> IO Int
> myAdd x y = do
> putStr "in myAdd\n"
> return $ x + y
this fails because (myAdd :: Int -> Int -> IO Int) does not match the
required modify argument type (Int -> Int -> Int) for MyState.
Couldn't match expected type `Int' against inferred type `IO Int'
In the second argument of `($)', namely `myAdd 1'
In the expression: modify $ (myAdd 1)
In a 'do' expression: modify $ (myAdd 1)
is it possible to 'lift' StateT modify into the inner monad (IO in my case)?
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