Haskell from First Principles - Chapter 15 - 'Mem' Exercise

exercise.md

This gist and its comments are a way of working through an exercise problem found in Chapter 15 of the Haskell Book.

Here's the code from the exercise. Our job is to implement the mempty and mappend functions, which are presented here as undefined:

module Chap15Ex where
import Data.Monoid

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

instance Monoid a => Monoid (Mem s a) where
  mempty = undefined
  mappend = undefined


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

main = do
  let rmzero = runMem mempty 0
      rmleft = runMem (f' <> mempty) 0
      rmright = runMem (mempty <> f') 0
  print $ rmleft -- ("hi, 1)
  print $ rmright -- ("hi", 1)
  print $ (rmzero :: (String, Int)) -- ("", 0)
  print $ rmleft == runMem f' 0 -- True
  print $ rmright == runMem f' 0 -- True

Nice, those look great 👍 I see how you're threading the state through from both functions before returning the final result, that makes a lot of sense!