From: Ramakrishnan Muthukrishnan <ram@rkrishnan.org>
Date: Mon, 29 Dec 2014 06:14:55 +0000 (+0530)
Subject: hw12: wip
X-Git-Url: https://git.rkrishnan.org/components/com_hotproperty/flags//%22news.html/%22?a=commitdiff_plain;h=e30ed55755e2cbe5e22de0a16313d7b97023532c;p=yorgey.git

hw12: wip
---

diff --git a/hw12/12-monads.pdf b/hw12/12-monads.pdf
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diff --git a/hw12/Risk.hs b/hw12/Risk.hs
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+++ b/hw12/Risk.hs
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+{-# LANGUAGE GeneralizedNewtypeDeriving #-}
+
+module Risk where
+
+import Control.Applicative
+import Control.Monad.Random
+import Control.Monad(replicateM, liftM)
+import Data.List(sort)
+
+------------------------------------------------------------
+-- Die values
+
+newtype DieValue = DV { unDV :: Int } 
+  deriving (Eq, Ord, Show, Num)
+
+first :: (a -> b) -> (a, c) -> (b, c)
+first f (a, c) = (f a, c)
+
+instance Random DieValue where
+  random           = first DV . randomR (1,6)
+  randomR (low,hi) = first DV . randomR (max 1 (unDV low), min 6 (unDV hi))
+
+die :: Rand StdGen DieValue
+die = getRandom
+
+------------------------------------------------------------
+-- Risk
+
+type Army = Int
+
+data Battlefield = Battlefield { attackers :: Army, defenders :: Army }
+
+-- battle :: Battlefield -> Rand StdGen Battlefield
+battle (Battlefield {attackers = a, defenders = d}) =
+  let rolla = replicateM (min (a - 1) 3) die
+      rolld = replicateM (min d 2) die
+      zipped = liftA2 zip rolla rolld
+  in
+