How do I import Control.Monad.State? I see this note in http://en.wikibooks.org/wiki/Haskell/Understanding_monads/State   Note: in some package systems used for GHC, the Control.Monad.State module is in a separate package, usually indicated by MTL (Monad Transformer Library). Michael ===============rand.hs ============== import Control.Monad.State type GeneratorState = State StdGen rollDie :: GeneratorState Int rollDie = do generator <- get              let( value, newGenerator ) = randomR (1,6) generator              put newGenerator              return value ======================= [michael@localhost ~]$ ghci rand GHCi, version 6.12.1: http://www.haskell.org/ghc/  :? for help Loading package ghc-prim ... linking ... done. Loading package integer-gmp ... linking ... done. Loading package base ... linking ... done. Loading package ffi-1.0 ... linking ... done. rand.hs:1:7:     Could not find module `Control.Monad.State':       Use -v to see a list of the files searched for. Failed, modules loaded: none. Prelude>