Ch 15.15 (Monoid Exercises)

08-Mem.hs

module Mem where

import Data.Monoid hiding ((<>))
import Data.Semigroup

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

instance Monoid a => Monoid (Mem s a) where
  mempty = Mem $ \x -> (mempty, x)
  mappend (Mem f) (Mem f') =
    Mem $ \x -> 
--    Taking the first values of the (a,s) tuples
--    returned from applying each function, I should
--    get two 'a's that I can mappend together?
      ( (fst $ f x) `mappend` (fst $ f' x)
--    Then I am taking the s (second of the (a,s) tuple)
--    returned from applying f', and applying that to f
      , (snd $ f $ snd $ f' x)
      )

f' = Mem $ \s -> ("hi", s + 1)

main :: IO ()
main = do
  let rmzero = runMem mempty 0
      rmleft = runMem (f' <> mempty) 0
      rmright = runMem (mempty <> f') 0
  print $ rmleft
  print $ rmright
  print $ (rmzero :: (String, Int))
  print $ rmleft == runMem f' 0
  print $ rmright == runMem f' 0