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Jonathan Cast  
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 More options Feb 16 2008, 9:48 pm
Newsgroups: fa.haskell
From: Jonathan Cast <jonathancc...@fastmail.fm>
Date: Sun, 17 Feb 2008 02:48:56 UTC
Local: Sat, Feb 16 2008 9:48 pm
Subject: Re: [Haskell-cafe] Doubting Haskell
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On 16 Feb 2008, at 5:04 PM, Donn Cave wrote:

> On Feb 16, 2008, at 3:46 PM, Philippa Cowderoy wrote:

>> On Sat, 16 Feb 2008, Alan Carter wrote:

>>> I'm a Haskell newbie, and this post began as a scream for help.

>> Extremely understandable - to be blunt, I don't really feel that  
>> Haskell
>> is ready as a general-purpose production environment unless users are
>> willing to invest considerably more than usual. Not only is it not as
>> "batteries included" as one might like, sometimes it's necessary  
>> to build
>> your own batteries!

> Ironically, the simple task of reading a file is more work than I  
> expect
> precisely because I don't want to bother to handle exceptions.  I  
> mean,
> in some applications it's perfectly OK to let an exception go to  
> the top.

> But in Haskell, you cannot read a file line by line without writing an
> exception handler, because end of file is an exception!  as if a  
> file does
> not normally have an end where the authors of these library functions
> came from?

I agree 100%; to make life tolerable around Haskell I/O, I usually  
end up binding the moral equivalent of

tryJust (\ exc -> case exc of
                              IOException e | isEOFError e -> return ()
                              _ -> Nothing) $
            getLine

somewhere at top level and then calling that where it's needed.

> For the author of the original post ... can't make out what you  
> actually
> found and tried, so you should know about "catch" in the Prelude, the
> basic exception handler.

Also, you might need to know that bracket nests in various ways:

bracket openFile hClose $ bracket readLine cleanUpLine $ proceed

There's also finally, for when the first argument to bracket is  
ommitted, and (>>) for when the second argument is :)

jcc

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