Checking for correct invocation of a command line haskell program
Jump to navigation
Jump to search
The printable version is no longer supported and may have rendering errors. Please update your browser bookmarks and please use the default browser print function instead.
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-nice.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:
- Simple unix tools
- Command line options in haskell: http://leiffrenzel.de/papers/commandline-options-in-haskell.html