[Haskell-beginners] State monad to help pass around game settings

[i.caught.air at gmail.com]

I have a metro to catch but small interesting additions worth looking into:

Using `modify` and lenses, you get niceties like this to update the state:

modify $ scolor .~ color

Don’t forget to use ReaderT over StateT if you only need some configuration to be passed implicitly and wont mutate.

Last note, `StateT s m a` relies fairly heavily on understanding the essence of monads. Your goal is to build a computation in which you’re given the ability to keep and mutate a state `s`. At the end of the day, something has to “run” that computation, which will carry out its effects `m`. produce a 
result `a`, and possibly give you the final state `s` if needed as well. (See runStateT vs. execStateT vs. evalStateT).

Cheers,
Alex.

From: David McBride
Sent: April 7, 2017 8:34 AM
To: The Haskell-Beginners Mailing List - Discussion of primarily beginner-level topics related to Haskell
Subject: Re: [Haskell-beginners] State monad to help pass around game settings

The basic outline for using StateT for settings is the following.
Hopefully this will give you an idea of how to get started.

import Control.Monad.State

data Color = White | Red deriving (Enum, Show)
data Shape = Square deriving (Enum, Show)

data Stuff = Stuff deriving Show

data Settings = Settings {
  sColor :: Color,
  sShape :: Shape
} deriving Show

data MyApp = MyApp {
  settings :: Settings,
  otherStuff :: Stuff
} deriving Show

main = do
  (_, settings) <- runStateT  proc (MyApp (Settings White Square) Stuff)
  print settings


-- A reusable prompt function.
prompt :: String -> [a] -> (Char -> a) -> IO a
prompt question opts c2r = do
  putStrLn question
  mapM undefined opts
  c <- getChar
  let r = c2r c -- turn a Char into a Shape or a Color.
  return r

proc :: StateT MyApp IO ()
proc = do
  getColor
  getShape

getColor :: StateT MyApp IO ()
getColor = do
  color <- liftIO $ prompt "What color would you like?" [Red, White] undefined
  MyApp settings otherstuff <- get
  put $ (MyApp (settings { sColor = color })) otherstuff

getShape :: StateT MyApp IO ()
getShape = undefined

On Thu, Apr 6, 2017 at 9:26 PM, Dave Martin <davemartinnyc at aol.com> wrote:
> I'm trying to write a game with a "settings menu" where the user can adjust
> gameplay options. Right now I pass all the settings around as parameters.
> I'm trying to figure out how to use the State monad to simplify this task,
> but I can't figure out how to start. Or maybe my whole design approach is
> wrongheaded, and not in keeping with best practices. Haskell is my first
> language. This is the kind of thing I have now:
>
> mainM color shape =
>   putStrLn "\n\nMain Menu" >>
>   (putStrLn . unlines) [
>     "(1) Set",
>     "(2) Display",
>     "(3) Quit"] >>
>   putStr "? " >>
>   getChar >>= \c ->
>     case c of
>       '1' -> set color shape
>       '2' -> display color shape
>       '3' -> return ()
>       _ -> mainM color shape
>
> set color shape =
>   putStrLn "\n\nSettings" >>
>   (putStrLn . unlines) [
>     "(1) Color",
>     "(2) Shape",
>     "(3) Main Menu"] >>
>   putStr "? " >>
>   getChar >>= \c ->
>     case c of
>       '1' -> setColor color shape
>       '2' -> setShape color shape
>       '3' -> mainM color shape
>       _ -> set color shape
>
> setColor color shape =
>   putStr ("\n\nColor is " ++ color ++ ". New color? ") >>
>   getLine >>= \cs ->
>   set cs shape
>
> setShape color shape =
>   putStr ("\n\nShape is " ++ shape ++ ". New shape? ") >>
>   getLine >>= \cs ->
>   set color cs
>
> display color shape =
>   putStrLn ("\n\nColor is " ++ color ++ ". Shape is " ++ shape ++ ".") >>
>   mainM color shape
>
>
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