[Book] [Ch15] Exercise 8

module Reader where

import Data.Monoid

newtype Mem s a =
  Mem {
    runMem :: s -> (a,s)
  }

instance Monoid a => Monoid (Mem s a) where
  mempty = Mem $ \s -> (mempty, s)
  mappend m1 m2 =
    Mem $ \s -> let
      (a1, s1) = runMem m1 s
      (a2, s2) = runMem m2 s1
      in (a1 <> a2, s2)

fc :: Mem Integer String
fc = Mem $ \s -> ("hi", s + 1)

main :: IO ()
main = do
  print $ runMem (fc <> mempty) 0                -- ("hi",1)
  print $ runMem (mempty <> fc) 0                -- ("hi",1)
  print (runMem mempty 0 :: (String, Int))       -- ("",0)
  print $ runMem (fc <> mempty) 0 == runMem fc 0 -- True
  print $ runMem (mempty <> fc) 0 == runMem fc 0 -- True