[Haskell-cafe] Monad transformer: apply StateT to List monad

michael rice nowgate at yahoo.com
Fri Jan 14 01:38:31 CET 2011

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?


--- On Thu, 1/13/11, Daniel Fischer <daniel.is.fischer at googlemail.com> wrote:

From: Daniel Fischer <daniel.is.fischer at googlemail.com>
Subject: Re: [Haskell-cafe] Monad transformer: apply StateT to List monad
To: haskell-cafe at haskell.org
Cc: "michael rice" <nowgate at yahoo.com>
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]])


lift list = StateT (\s -> zip list (repeat s))

> Michael

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