Checking for correct invocation of a command line haskell program

Running our example:

   thartman@linodehaskell:~/learning/haskell/UnixTools/arghandling>./args.sh
   Checking for correct invocation of a command line / shell haskell program

     We run a nice example,

     alongside example of "ugly" argument checking that is simpler to write,
     but gives a less user friendly error message when the program is invoked incorrectly

   **********
   nice arg handling, no errors:
   "0 1 2"

   ugly arg handling, no errors:
   "0 1 2"

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

   ugly arg handling, errors:
   *** Exception: user error (Pattern match failure in do expression at arghandling-ugly.hs:3:10-29)

Source code...

args.sh:

   thartman@linodehaskell:~/learning/haskell/UnixTools/arghandling>cat args.sh
   cat <<EOF
   Checking for correct invocation of a command line / shell haskell program

     We run a nice example,

     alongside example of "ugly" argument checking that is simpler to write,
     but gives a less user friendly error message when the program is invoked incorrectly

   **********
   EOF

   echo nice arg handling, no errors:
   runghc arghandling-nice.hs 0 1 2
   echo
   echo

   echo ugly arg handling, no errors:
   runghc arghandling-ugly.hs 0 1 2
   echo
   echo

   echo nice arg handling, errors:
   runghc arghandling-nice.hs 0 1
   echo
   echo

   echo ugly arg handling, errors:
   runghc arghandling-ugly.hs 0 1

   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]