Checking for correct invocation of a command line haskell program

From HaskellWiki

   thartman@linodehaskell:~/learning/haskell/UnixTools/arghandling>./args.sh
   This demonstrates checking for correct invocation of a command line / shell haskell program
   It runs a version which gives a "user friendly" error message when invoked incorrectly,
   alongside an example of "ugly" argument checking that is slightly simpler to write,
   but gives a less user friendly error message

   **********
   runghc arghandling-nice.hs 0 1 2
   "0 1 2"

   runghc arghandling-ugly.hs 0 1 2
   "0 1 2"

   runghc arghandling-ugly.hs 0 1
   *** Exception: args length does not equal 3. args: : ["0","1"]
   usage example: $ runghc arghandling-nice.hs firstarg secondarg thirdarg

   runghc arghandling-ugly.hs 0 1
   *** Exception: user error (Pattern match failure in do expression at arghandling-ugly.hs:3:10-29)

   thartman@linodehaskell:~/learning/haskell/UnixTools/arghandling>cat arghandling-nice.hs
   import System

   main = do args <- getArgs
             let usagemsg = "usage example: $ runghc arghandling-nice.hs firstarg secondarg thirdarg"
             case args of
               [first,second,third] -> process first second third
               _                    -> error $ "args length does not equal 3. args: : " ++ ( show args ) ++ "\n" ++ usagemsg

   process a b c = print $ unwords [a,b,c]

   thartman@linodehaskell:~/learning/haskell/UnixTools/arghandling>cat arghandling-ugly.hs
   import System

   main = do [first,second,third] <- getArgs
             process first second third

   process a b c = print $ unwords [a,b,c]

See also: Libraries and tools/Operating system