[Haskell-cafe] Doubting Haskell

Jonathan Cast jonathanccast at fastmail.fm
Sat Feb 16 21:48:24 EST 2008

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) $

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 :)


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