# [Haskell-cafe] Why is Haskell flagging this?

michael rice nowgate at yahoo.com
Thu Dec 23 04:21:13 CET 2010

```Thanks for the tip, Ozgur. It worked for me. Is this what you had in mind, Ryan?

Michael

==============

import System.Random

type GeneratorState = State StdGen
data Craps a = Roll a | Win a | Lose a deriving (Show)

genRandomR :: Random a => (a,a) -> GeneratorState a
genRandomR = state . randomR

rollDie :: GeneratorState Int
rollDie = genRandomR (1,6)

roll2Dice :: GeneratorState Int
roll2Dice = liftM2 (+) rollDie rollDie

f :: Craps [Int] -> GeneratorState (Craps [Int])
f (Roll []) = do g0 <- get
let (throw1,g1) = runState roll2Dice g0
put g1
case throw1 of
2 -> return (Lose [throw1])
3 -> return (Lose [throw1])
7 -> return (Win [throw1])
11 -> return (Win [throw1])
_ -> do g1 <- get
let (throw2,g2) = runState roll2Dice g1
put g2
if throw2 == throw1
then do return (Win [throw1,throw2])
else
if throw2 == 7
then do return (Lose [throw1,throw2])
else do f (Roll [throw1,throw2])
f (Roll z@(throw1:throws)) = do g0 <- get
let (throw,g1) = runState roll2Dice g0
put g1
if throw == throw1
then do return (Win (z ++ [throw]))
else
if throw == 7
then do return (Lose (z ++ [throw]))
else do f (Roll (z ++ [throw]))

--- On Wed, 12/22/10, Ozgur Akgun <ozgurakgun at gmail.com> wrote:

From: Ozgur Akgun <ozgurakgun at gmail.com>
Subject: Re: [Haskell-cafe] Why is Haskell flagging this?
To: "Ryan Ingram" <ryani.spam at gmail.com>
Date: Wednesday, December 22, 2010, 7:37 PM

On 22 December 2010 20:02, Ryan Ingram <ryani.spam at gmail.com> wrote:

Interesting.  In that case,

state f = StateT \$ \s -> Identity (f s)

allows "state" to replace "State" in that code.
Ozgur

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