average_monoid.hs

module Main where

import Data.Monoid

data Average a = Average { sum :: a, count :: a } deriving (Show)

instance (Fractional a) => Monoid (Average a) where
  mempty = Average (fromIntegral 0) (fromIntegral 0)
  mappend (Average s1 c1) (Average s2 c2) = Average (s1 + s2) (c1 + c2)

average :: (Eq a, Fractional a) => Average a -> a
average (Average sum 0) = error "Divide by zero, bad!"
average (Average sum count) = sum / count

main :: IO ()
main = do
  let avg = mempty :: Average Float
  let avg2 = avg <> (Average 1 1)
  let avg3 = avg2 <> (Average 3 1)
  print avg3
  print $ average avg3