-- -*- haskell -*-

-- smallest
-- the smallest running program doing nothing
main = putStr ""

-- hello world
-- print a simple string on stdout
main = putStrLn "Hello World"

-- argv
-- access command line parameters (no segmentation fault accepted, nor silent exception, so some languages must explicitly check the presence of the argument)
import System
main = do l <- getArgs
          putStrLn (head l)


-- env
-- access environment variable
import System
main = getEnv "HOME" >>= putStrLn


-- test file exists
-- return exit code error (non zero) if a file does not exist
import Directory
import System
main = catch (getPermissions "/etc/mtab") (\_ -> exitFailure)


-- test file readable
-- return exit code error (non zero) if a file is not readable
import System
main = catch (readFile "/etc/mtab") (\_ -> exitFailure)


-- formatting
-- print integers in a simple formatted string
main = putStrLn $ show a ++ " + " ++ show b ++ " = " ++ show(a + b) 
    where a=1
          b=2


-- system
-- call an external program and check the return value
import System
import IO
import Monad
main = 
    do ret <- system "false"
       when (ret /= ExitSuccess) (hPutStrLn stderr "false failed")
       system "echo done"


-- sed in place
-- remove #-comments from a file (modifying the file, i.e. in place)
import System
import System.IO
import Monad
import Control.Exception

c = unlines . map (takeWhile (/= '#')) . lines
fop f n = do l <- fmap f (readFile n)
             evaluate (length l)
             writeFile n l

main = getArgs >>= (fop c) . head


-- compile what must be
-- find and compile .c files into .o when the .o is old or absent
import System
import System.IO
import System.Directory
import Data.List
import Control.Monad

gmt = getModificationTime
op f = do catch (do ct <- gmt f; ot <- gmt o;
                    when (ct > ot) comp)
                (\_ -> comp)
 where o = take (length f - 2) f ++ ".o"
       comp = do putStrLn ("Compiling "++f++" to "++o)
                 system ("gcc -c -o "++o++" "++f)

main = do files <- getCurrentDirectory >>= getDirectoryContents
          mapM_ op $ filter (".c"`isSuffixOf`) files