On Thu, Oct 18, 2007 at 20:58:45 +1000, Matthew Brecknell wrote: >Magnus Therning: >> hasEmpty s = let >> _first_empty = s !! 0 == '\n' >> _last_empty = (reverse s) !! 1 == '\n' >> in _first_empty || _last_empty >> >> loadAndCheck fp = liftM hasEmpty $ readFile fp >> >> main = getArgs >>= filterM loadAndCheck >>= mapM_ putStrLn >> The one problem I had was that running this on /all/ files in my WP >> resulted in an exception: >> >> *** Exception: ./wp-includes/images/smilies/icon_mrgreen.gif: >> openFile: resource exhausted (Too many open files) >> >> Is there some (easy) way to avoid this while still using readFile? > >The perils of lazy IO: readFile closes its file descriptor only if it >is is forced to read to end of file. Otherwise, you rely on the garbage >collector to run the finaliser on the file handle sometime after it >becomes unreachable, but since file handle exhaustion does not trigger >garbage collection, there are no guarantees. See also the System.IO >documentation. > >For an extremely hackish (and untested) solution to your problem, >replace the expression (_first_empty || _last_empty) with (_last_empty >|| _first_empty). If you can explain why that works, then you advance >to the next level in your training!
I can see how that could have worked. Lazy thinking indeed :-) I
wonder if the fact that it doesn't work should be considered a bug?
Just to make sure I changed (broke) my code so that the very last
character would be read (in case there is an almost crazy level of
optimisation in ghc):
_last_empty = (reverse s) !! 0 == '\n'
Still no cigar :(
>For a less hackish solution, you need to do a bit more work. Again, this
>is untested.
>
>> loadAndCheck fn = bracket (openFile fn ReadMode) hClose checkContents
>> checkContents fh = do { s <- hGetContents fh; return $! hasEmpty s }
>
>Note the explicit close, and "return $!" to ensure that hasEmpty has
>done its work before the file is closed.
Ah, `bracket` is a handy function to know!
And it does work!
/M
--
Magnus Therning (OpenPGP: 0xAB4DFBA4)
magnus＠therning．org Jabber: magnus．therning＠gmail．com
http://therning.org/magnus
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