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

michael rice nowgate at yahoo.com
Thu Jan 13 21:17:41 CET 2011

{- 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.


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