Hi Ross, Thanks for going the extra mile. A lot of what you did I haven't seen before, so it's going to take me some time to go through it. But I'll be back. Michael --- On Thu, 4/23/09, Ross Mellgren <rmm-haskell@z.odi.ac> wrote: From: Ross Mellgren <rmm-haskell@z.odi.ac> Subject: Re: [Haskell-cafe] Random number example To: "michael rice" <nowgate@yahoo.com> Cc: haskell-cafe@haskell.org Date: Thursday, April 23, 2009, 11:51 AM So there are a couple problems. First is you are trying to rebind prelude functions, when instead you should be creating an instance of Monad. This requires a bit of shuffling because without language extensions you can't instance Monad Random for your type of Random, as it is a type synonym. So, changing the type synonym to a newtype and instancing monad, you get: module Rand9b where import Control.Applicative (Applicative(..), (<$>), (<*>))import Control.Monad (ap, liftM) type Seed = Intnewtype Random a = Rand { unRand :: (Seed -> (a, Seed)) } randomNext :: Seed -> SeedrandomNext rand = if newRand > 0 then newRand else newRand + 2147483647    where newRand = 16807 * lo - 2836 * hi          (hi,lo) = rand `divMod` 127773 rollDie :: Random IntrollDie = Rand $ \ seed -> ((seed `mod` 6) + 1, randomNext seed) instance Monad Random where    (>>=) = randomBind    return = randomReturn instance Functor Random where    fmap = liftM instance Applicative Random where    pure = return    (<*>) = ap randomBind :: Random a -> (a -> Random b) -> Random bm `randomBind` g = Rand $ \seed0 ->   let (result1, seed1) = unRand m $ seed0      (result2, seed2) = unRand (g result1) $ seed1  in (result2, seed2) randomReturn :: a -> Random arandomReturn x = Rand $ \ seed0 -> (x, seed0) sumTwoDice :: Random IntsumTwoDice = (+) <$> rollDie <*> rollDie I also threw in instances of Functor and Applicative, so that I could simplify sumTwoDice using applicative form (much nicer, no? Applicative is totally rockin') Now you need one more thing, a way to convert a series of Random actions into a pure function: runRandom :: Seed -> Random a -> arunRandom s f = fst . unRand f $ s  which now makes what you want to do in GHCi easy and well wrapped: Prelude> :reload[1 of 1] Compiling Rand9b           ( rand9b.hs, interpreted )Ok, modules loaded: Rand9b.*Rand9b> runRandom 0 sumTwoDice3 Hope this helps,-Ross On Apr 23, 2009, at 11:28 AM, michael rice wrote: I pretty much followed the sequence of steps that led to this final code (see below), but will be looking it over for a while to make sure it sinks in. In the meantime, I get this when I try to use it (sumTwoDice) at the command line: [michael@localhost ~]$ ghci rand9 GHCi, version 6.10.1: http://www.haskell.org/ghc/  :? for help Loading package ghc-prim ... linking ... done. Loading package integer ... linking ... done. Loading package base ... linking ... done. [1 of 1] Compiling Main             ( rand9.hs, interpreted ) Ok, modules loaded: Main. *Main> sumTwoDice <interactive>:1:0:     No instance for (Show (Seed -> (Int, Seed)))       arising from a use of `print' at <interactive>:1:0-9     Possible fix:       add an instance declaration for (Show (Seed -> (Int, Seed)))     In a stmt of a 'do' expression: print it *Main> Can I employ a 'do' expression from the command line? Also, can I now use functions (>>) (>>=) and 'return' defined in the Prelude and still have this code work? Michael ================== {-# LANGUAGE NoImplicitPrelude #-} import Prelude hiding ((>>), (>>=), return) type Seed = Int type Random a = Seed -> (a, Seed) randomNext :: Seed -> Seed randomNext rand = if newRand > 0 then newRand else newRand + 2147483647     where newRand = 16807 * lo - 2836 * hi           (hi,lo) = rand `divMod` 127773 rollDie :: Random Int rollDie seed = ((seed `mod` 6) + 1, randomNext seed) (>>) :: Random a -> Random b -> Random b (>>) m n = \seed0 ->   let (result1, seed1) = m seed0       (result2, seed2) = n seed1   in (result2, seed2) (>>=) :: Random a -> (a -> Random b) -> Random b (>>=) m g = \seed0 ->   let (result1, seed1) = m seed0       (result2, seed2) = (g result1) seed1   in (result2, seed2) return :: a -> Random a return x = \seed0 -> (x, seed0) sumTwoDice :: Random Int sumTwoDice = rollDie >>= (\die1 -> rollDie >>= (\die2 -> return (die1 + die2))) _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe