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

[David McBride]

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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