Lifting 'f' into StateT -- you get a list of (result, state) pairs. Since the state is never modified, the second half of each pair is identical:
-------------------------------------------------------------------------- import Control.Monad.State f :: Int -> [Int] f n = [0..n] -- lifting 'f' into State, I use 'Char' for the state so you -- can see which param it is liftedF :: Int -> StateT Char [] Int liftedF n = lift (f n) -- prints [(0,'a'),(1,'a'),(2,'a'),(3,'a'),(4,'a')] -- -- 4 is n , 'a' is the state main = print (runStateT (liftedF 4) 'a') -------------------------------------------------------------------------- Lifting 'tick' into ListT -- you get a single pair, the first half is a list with one value, which is whatever 'tick' returned: -------------------------------------------------------------------------- import Control.Monad.List type GeneratorState = State Int tick :: GeneratorState Int tick = do n <- get put (n + 1) return n liftedTick :: ListT GeneratorState Int liftedTick = lift tick -- prints ([4],5) -- -- 4 is the initial state, 5 is the final state main = print (runState (runListT liftedTick) 4) -------------------------------------------------------------------------- Generally, monad transformers aren't used to add new functionality to existing monadic computations. Instead, they're used with a generic "Monad m =>" (or similar) constraint, and modify how that generic result is returned. For example, a modified version of 'tick' can have any monad (including lists) applied to it: -------------------------------------------------------------------------- tick :: Monad m => StateT Int m Int tick = do n <- get put (n + 1) return n -- prints [(0,1),(1,2),(2,3)] main = print ([0,1,2] >>= runStateT tickTo) -------------------------------------------------------------------------- On Thu, Jan 13, 2011 at 16:38, michael rice <nowg...@yahoo.com> wrote: > Hi Daniel, > > What I need to see is a function, say g, that lifts the function f (in the > List monad) into the StateT monad, applies it to the monad's value, say 1, > and returns a result [0,1]. > > Or, alternatively, code that lifts a function in the State monad, say tick > > import Control.Monad.State > > type GeneratorState = State Int > > tick :: GeneratorState Int > tick = do n <- get > put (n+1) > return n > > into the ListT monad and applies it to a list, say > > lst = [0,1,2] > > producing [(0,1),(1,2),(2,3)]. > > Both would be very helpful. Or maybe I'm missing the concept of monad > transformers altogether and putting them together improperly, like trying to > use a spreadsheet to write a letter? > > Michael >
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