On Sat, Aug 14, 2010 at 2:38 AM, michael rice
The program below takes a text file and unwraps all lines to 72 columns, but I'm getting an end of file message at the top of my output.
How do I lose the EOF?
Michael
While many other people have shown you why you need not necessarily answer this question, I think it'd be helpful for you to hear the answer anyway. Your message is being produced because you are trying to getLine when there is no input left. This raises an exception, which, because it is not handled by your program, prints a diagnostic message and exits. Strangely, it prints this before the output of your program - this isn't terribly important, but for the sake of completeness, it's because of the different buffering characteristics of stdout and stderr, which confusingly mean that even though your program produces output and then produces an error, the error is printed immediately while the output waits until the program is terminated to be produced. I think. Something like that, anyway. So, how do you avoid the exception? You can use System.IO.isEOF [1] to check if there is input available on the standard input handle: main = do eof <- isEOF when (not eof) realMain -- when from Control.Monad, see also: unless where realMain = do realStuff Or you can let getLine throw the exception, but catch it and deal with it yourself, rather than letting it kill your program. Catching it is a little less simple, but is considerably more flexible and powerful. The exceptions situation in Haskell is somewhat complicated by the fact that the mechanism used by haskell98 has been improved upon, but the new extensible mechanism is incompatible with the old so both have to hang around. Have a look at Control.Exception [2] and System.IO.Error [3]. In either case you have 'catch' as a sort of basic operation, and more convenient things like 'try' which sort of turn an exception into a pure Either result. I'd do something like this: main = do result <- try getLine case result of Left err -> return () -- or do something diagnostic Right "" -> putStrLn "" >> main Right line -> doStuffWith line On a more general note, your main function looks a little suspect because it *always* recurses into itself - there's no termination condition. A question nearly as important as "how does my program know what to do" is "how does it know when to stop" :) [1] http://hackage.haskell.org/packages/archive/base/4.2.0.1/doc/html/System-IO.... [2] http://hackage.haskell.org/packages/archive/base/4.2.0.1/doc/html/Control-Ex... [3] http://hackage.haskell.org/packages/archive/base/4.2.0.1/doc/html/System-IO-...