Averages/Simple moving average: Difference between revisions

Stateful function using the state monad to keep track of state

{{works with|GHC|7.8.3}}

<lang Haskell>

import Control.Monad

import Control.Monad.State

period :: Int

period = 3

type SMAState = [Float]

computeSMA :: Float -> State SMAState Float

computeSMA x = do

previousValues <- get

let values = previousValues ++ [x]

let newAverage = if length values <= period then (sum values) / (fromIntegral $ length remainingValues :: Float)

else (sum remainingValues) / (fromIntegral $ length remainingValues :: Float)

where remainingValues = dropIf period values

put $ dropIf period values

return newAverage

dropIf :: Int -> [a] -> [a]

dropIf x xs = drop ((length xs) - x) xs

demostrateSMA :: State SMAState [Float]

demostrateSMA = mapM computeSMA [1, 2, 3, 4, 5, 5, 4, 3, 2, 1]

main :: IO ()

main = putStrLn $ (show result)

where

(result, _) = runState demostrateSMA []

</lang>

outputs:

<pre>

[1.0,1.5,2.0,3.0,4.0,4.6666665,4.6666665,4.0,3.0,2.0]

</pre>