Command line tool to find the average (arithmetic mean) of the input

average.hs

#! /usr/bin/env stack
-- stack --resolver lts-9.14 --install-ghc script --package base

import Data.Maybe (mapMaybe)
import System.Environment (getArgs)
import Text.Read (readMaybe)

{- USAGE

Pipe input:
$ echo -e "1.3\n2.7    0.8 yeah" | average
1.5999999999999999 # damn floating point

Pass input via args:
$ average 93 99 96 94 95 gunk 95 81 99 79 96 86 90 gloop 95 90 98
92.4

Beware:
$ average 7 NaN 8
NaN

-}

main = do
    args <- getArgs
    input <- if null args then words <$> getContents else pure args
    print $ mean $ mapMaybe readMaybe input
  where
    mean xs = sum xs / fromIntegral (length xs)