If I have something like this main = do args <- getArgs let file = args !! 0 -- etc... And I run it without any arguments, I get an error message like this: "Prelude.(!!): index too large". What's the best way to handle an error like this to give a better message back to the user? Peter
On Thursday 29 December 2011, 21:25:31, Peter Hall wrote:
If I have something like this
main = do args <- getArgs let file = args !! 0 -- etc...
And I run it without any arguments, I get an error message like this: "Prelude.(!!): index too large".
What's the best way to handle an error like this to give a better message back to the user?
main = do args <- getArgs case args of [] -> complain _ -> happiness or args <- getArgs if null args then complain else happiness
On 12/29/11 15:42, Daniel Fischer wrote:
On Thursday 29 December 2011, 21:25:31, Peter Hall wrote:
If I have something like this
main = do args <- getArgs let file = args !! 0 -- etc...
And I run it without any arguments, I get an error message like this: "Prelude.(!!): index too large".
What's the best way to handle an error like this to give a better message back to the user?
main = do args <- getArgs case args of [] -> complain _ -> happiness
or
args <- getArgs if null args then complain else happiness
I've always used Control.Monad.when here, to avoid unnecessary nesting (or the alternative "do nothing"). E.g, main = do args <- getArgs when (null args) $ do putStrLn help_text exitWith $ ExitFailure exit_not_enough_args
Thanks,
I was trying this, but it turns out my indentation was wrong; the
'then' and 'else' need to be indented under the 'if', or else it gets
confused about the do block.
Also thanks for the other solutions. I'm looking up cmdargs now.
Peter
On Thu, Dec 29, 2011 at 8:42 PM, Daniel Fischer
On Thursday 29 December 2011, 21:25:31, Peter Hall wrote:
If I have something like this
main = do args <- getArgs let file = args !! 0 -- etc...
And I run it without any arguments, I get an error message like this: "Prelude.(!!): index too large".
What's the best way to handle an error like this to give a better message back to the user?
main = do args <- getArgs case args of [] -> complain _ -> happiness
or
args <- getArgs if null args then complain else happiness
On Thu, Dec 29, 2011 at 9:25 PM, Peter Hall
If I have something like this
main = do args <- getArgs let file = args !! 0 -- etc...
And I run it without any arguments, I get an error message like this: "Prelude.(!!): index too large".
What's the best way to handle an error like this to give a better message back to the user?
Don't use partial functions (functions that can fail on some input) except if you're sure they won't fail (and even then you may be better off not using them...), here you can use pattern matching :
main = do args <- getArgs case args of [file] -> -- etc _ -> error "You should call this program with one file argument !!"
In this particular case (arguments parsing), if you ever find yourself with more complicated argument handling to do, consider using one of the good packages that make this easy and safe to do like cmdargs. -- Jedaï
participants (4)
-
Chaddaï Fouché -
Daniel Fischer -
Michael Orlitzky -
Peter Hall