[Haskell-cafe] getContents and lazy evaluation

[Donn Cave]

Quoth Julien Oster <haskell at lists.julien-oster.de>:
...
| But what happens when two processes use the same file and one process is
| writing into it using lazy IO which didn't happen yet? The other process
| wouldn't see its changes yet.

That's actually a much more general problem, one that I imagine applies
to hPutStr et al. too.  Application level writes are ordinarily buffered
in process space by the I/O library, so output from an ordinary C program
may not appear on disk (or in kernel space disk I/O buffer) until just
before the program exits.

| As for two processes writing to the same file at the same time, very bad
| things may happen anyway. Sure, lazy IO prevents doing communication
| between running processes using plain files, but why would you do
| something like that?

Quite a few reasons, depending on how you define communication.  You
might even be tempted to use hGetContents in such cases.  For example,
one common way to share a file is to interlock around some resource,
and when you acquire the lock, you read the file (get its contents)
and release the lock.

	Donn Cave, donn at drizzle.com