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Checking for correct invocation of a command line haskell program

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* [[Simple unix tools]]
 
* [[Simple unix tools]]
 
* [http://leiffrenzel.de/papers/commandline-options-in-haskell.html Command line options in haskell]
 
* [http://leiffrenzel.de/papers/commandline-options-in-haskell.html Command line options in haskell]
  +
* [http://groups.google.de/group/fa.haskell/browse_thread/thread/6359767a8e64bc5d/e5286a2783c22bbe?lnk=st&q=haskell+string+fill+ins&rnum=1&hl=en#e5286a2783c22bbe string fill-ins (discussion at haskell cafe)]

Revision as of 13:30, 16 April 2007

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

1 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)

2 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]

3 See also: