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]

See also:

• Simple unix tools
• Error reporting strategies
• Command line options in haskell
• string fill-ins (discussion at haskell cafe)