Checking for correct invocation of a command line haskell program
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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: Simple unix tools