Brandon S Allbery KF8NH
openFile "foo" ReadMode >>= \handle -> (hGetContents handle >>= (\s -> hClose handle >> putStr s)) [2]
This is a classic example of the dangers of hGetContents (and, more generally, of unsafeInterleaveIO).
Which makes me wonder why it isn't an error to hClose a semi-closed handle as well? The reason that springs to mind is that most systems (or at least Linux) has for some unfathomable reason an arbitrary, low, and fixed limit on the number of open files, and it is therefore sometimes necessary or desirable to close files before opening new ones.
In general, you should use lazy I/O only for "quick and dirty" stuff and avoid it for serious programming.
I must admit that I use lazy I/O all the time - usually in the form of 'readFile' rather than 'hGetContents'.
You can get many of the benefits of lazy I/O without the nondeterminacy by using iteratee-based I/O (http://hackage.haskell.org/package/iteratee).
I think it is fair to say that iteratees are a bit more involved. -k -- If I haven't seen further, it is by standing in the footprints of giants