Reviewing Haskell by writing Unix commands

date.hs

import System.Time
import System.Locale

main = getClockTime >>=
       toCalendarTime >>=
       putStrLn . formatCalendarTime defaultTimeLocale "%c"

echo.hs

import System.Environment

main = getArgs >>= putStrLn . unwords

false.hs

import System.Exit

main = exitFailure

pwd.hs

import System.Directory

main = getCurrentDirectory >>= putStrLn

sort.hs

import Data.List

main = getContents >>= putStrLn . unlines . sort . lines

tee.hs

import System.Environment

main = getArgs >>= \args -> (
         getContents >>= \s -> (
           tee args s
         )
       )

tee :: [String] -> String -> IO ()
tee [] s = putStr s
tee (filename:rest) s = writeFile filename s >> tee rest s

true.hs

import System.Exit

main = exitWith ExitSuccess

uniq.hs

main = getContents >>= \x -> putStrLn $ unlines $ uniq $ lines x

uniq :: (Eq a) => [a] -> [a]
uniq [] = []
uniq (x:[]) = [x]
uniq (x:xs) = if x == head xs then uniq xs else x:(uniq xs)

yes.hs

import System.Environment

main = getArgs >>= putStrLn . yes

yes :: [String] -> String
yes xs = unlines $ repeat $ unwords $ normalizeArguments xs

normalizeArguments :: [String] -> [String]
normalizeArguments [] = ["y"]
normalizeArguments xs = xs