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 --- On Thu, 1/13/11, Daniel Fischer wrote: From: Daniel Fischer Subject: Re: [Haskell-cafe] Monad transformer: apply StateT to List monad To: haskell-cafe@haskell.org Cc: "michael rice" Date: Thursday, January 13, 2011, 3:44 PM On Thursday 13 January 2011 21:17:41, michael rice wrote:

{- From: http://en.wikibooks.org/wiki/Haskell/Monad_transformers if for instance we apply StateT to the List monad, a function that returns a list (i.e., a computation in the List monad) can be lifted into StateT s [], where it becomes a function that returns a StateT (s -> [(a,s)]). That is, the lifted computation produces multiple (value,state) pairs from its input state. -}

import Control.Monad.Trans.State.Lazy

type GeneratorState = StateT Int

-- a function in the list monad f :: Int -> [Int] f n = [0..n]

Will someone please demonstrate the above comment from the wiki page.

lift (f n) = StateT (\s -> [(k,s) | k <- [0 .. n]]) Generally, lift list = StateT (\s -> zip list (repeat s))

Michael

Thanks, John. Too late to do much now, but it looks like what I was needing, and more.  May have more questions after I examine it more closely tomorrow. Michael --- On Fri, 1/14/11, John Millikin wrote: From: John Millikin Subject: Re: [Haskell-cafe] Monad transformer: apply StateT to List monad To: "michael rice" Cc: haskell-cafe@haskell.org Date: Friday, January 14, 2011, 1:00 AM 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 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