How do I define type Random [Int] for rollNDice in Exercise 1, given
the code below?
Michael
============
Exercises
1. Implement rollNDice :: Int -> Random [Int] from the previous
subsection with >>= and return.
NOTE: Since >>= and return are already present in the Prelude, you
may want to use import Prelude hiding ((>>=),return) to avoid
compilation errors.
=================
{-# 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)))
rollNDice :: Int -> Random [Int]
--- On Thu, 4/23/09, michael rice <nowg...@yahoo.com> wrote:
From: michael rice <nowg...@yahoo.com>
Subject: Re: [Haskell-cafe] Random number example
To: "Ross Mellgren" <rmm-hask...@z.odi.ac>
Cc: haskell-cafe@haskell.org
Date: Thursday, April 23, 2009, 5:49 PM
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-hask...@z.odi.ac> wrote:
From: Ross Mellgren <rmm-hask...@z.odi.ac>
Subject: Re: [Haskell-cafe] Random number example
To: "michael rice" <nowg...@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 = Int
newtype Random a = Rand { unRand :: (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 = 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 b
m `randomBind` g = Rand $ \seed0 ->
let (result1, seed1) = unRand m $ seed0
(result2, seed2) = unRand (g result1) $ seed1
in (result2, seed2)
randomReturn :: a -> Random a
randomReturn x = Rand $ \ seed0 -> (x, seed0)
sumTwoDice :: Random Int
sumTwoDice = (+) <$> 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 -> a
runRandom 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 sumTwoDice
3
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:
[mich...@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)))
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