Haskell type class with state

typeclassstate.hs

{-# LANGUAGE ScopedTypeVariables #-}

import Control.Concurrent.MVar

class Foo a where
  mkFoo :: IO a
  readFoo :: a -> IO Int
  operateOnFoo :: a -> IO ()
  
newtype Bar = Bar (MVar Int)
newtype Baz = Baz (MVar Int)
 
instance Foo Bar where
  mkFoo = newMVar 0 >>= \i -> return $ Bar i
  readFoo (Bar mv) = readMVar mv
  operateOnFoo (Bar mv) =
    modifyMVar_ mv $ \i -> return (i+1)

instance Foo Baz where
  mkFoo = newMVar 10 >>= \i -> return $ Baz i
  readFoo (Baz mv) = readMVar mv
  operateOnFoo (Baz mv) =
    modifyMVar_ mv $ \i -> return (i*2)


mkBar :: IO Bar
mkBar = mkFoo

test1 :: IO ()
test1 = do
  bar <- mkBar        -- i=0
  operateOnFoo bar    -- i=1
  operateOnFoo bar    -- i=2
  i <- readFoo bar
  print i             -- prints 2


mkBaz :: IO Baz
mkBaz = mkFoo

test2 :: IO ()
test2 = do
  baz <- mkBaz        -- i=10
  operateOnFoo baz    -- i=20
  operateOnFoo baz    -- i=40
  i <- readFoo baz
  print i             -- prints 40