-- Arithmetic Mean of a sequence of numbers

module Mean where

import Data.Monoid

-- the naive solution: with speace leak

mean :: (Fractional a) => [a] -> a
mean xs
  = sum xs / fromIntegral (length xs)

-- --------------------
--
-- the solution with monoids and a single pass over the sequence

mean' :: (Fractional a) => [a] -> a
mean' xs =
  (\ (s, l) -> getSum s / fromIntegral (getSum l))
  . mconcat
  . fmap (\ x -> (Sum x, Sum (1::Int)) )
  $ xs

-- --------------------
--
-- :set +s for statistics
--
-- no meaningful timings within ghci
--
-- () -> Double: no result caching

t1, t1' :: () -> Double

t1  () = mean  [1.0 .. 1000000.0]
t1' () = mean' [1.0 .. 1000000.0]

-- --------------------