[Haskell-cafe] Why is Haskell flagging this?

[Ryan Ingram]

Haha, not exactly.

You can replace

sj <- get
let (a, sk) = runState something sj
put sk

with

a <- something

Also, you don't need do notation for single statements; "do return x" is
just "return x"


On Wed, Dec 22, 2010 at 7:21 PM, michael rice <nowgate at yahoo.com> wrote:

> Thanks for the tip, Ozgur. It worked for me. Is this what you had in mind,
> Ryan?
>
> Michael
>
> ==============
>
> import Control.Monad.State.Lazy
>
> import Control.Monad
> 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>
> Cc: haskell-cafe at haskell.org, "Daniel Fischer" <
> daniel.is.fischer at googlemail.com>
> Date: Wednesday, December 22, 2010, 7:37 PM
>
>
> see also:
> http://hackage.haskell.org/packages/archive/mtl/latest/doc/html/Control-Monad-State-Lazy.html#v:state
>
> On 22 December 2010 20:02, Ryan Ingram <ryani.spam at gmail.com<http://mc/compose?to=ryani.spam@gmail.com>
> > wrote:
>
> Interesting.  In that case,
>
> state f = StateT $ \s -> Identity (f s)
>
> allows "state" to replace "State" in that code.
>
>
> Ozgur
>
> -----Inline Attachment Follows-----
>
>
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