Checking for correct invocation of a command line haskell program
This page shows checking for correct invocation of a command line / shell haskell program. The first example is a version which gives a "user friendly" error message when invoked incorrectly, the second is an example of "ugly" argument checking that is slightly simpler to write, but gives a less user friendly error message
Usage examples
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)
The code
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]
arghandling-ugly.hs
import System
main = do [first,second,third] <- getArgs
process first second third
process a b c = print $ unwords [a,b,c]