Checking for correct invocation of a command line haskell program
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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]