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Checking for correct invocation of a command line haskell program

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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]