On Tue, Sep 15, 2009 at 9:16 PM, Svein Ove Aas
I have a number of suggestions, some of which conflict with each other, so I'll just throw them out here. Let's see..
:)
First off, the IO monad does indeed enforce sequencing; that's its primary purpose.
So, unsafePerformIO can be thought as an escape to this strict rule.
However, you can ask for it to run I/O out of order, specifically when the value the out-of-order action returns is actually forced (used); that's lazy I/O, and is implemented using unsafeInterleaveIO.
I imagined that. But I hate to have to use unsafePerformIO when hGetContets is already using it.
You would not usually use unsafeInterleaveIO directly, though.
That's the point.
Instead, you'd use an existing wrapper, such as hGetContents. (for Strings, or lazy bytestrings; the strict bytestring variant reasonably has a strict semantics)
One thing to keep in mind about lazy I/O is that the I/O in question can run at any arbitrary time, or not at all; not more than once, though. You must make sure this is safe. For file input, that basically means the file should not change during the program's lifetime.
Ok.
hGetLine is not lazy in this way, but the hGetContents you use is. I'm not sure whether this means your program should work as-is, and I'm not going to examine it closely enough to tell - as you mentioned it's a mockup anyway. Besides..
Strings are also *slow*. What you want for I/O is, when reasonably possible, bytestrings. You'd then use parsec-bytestring, or if possible Data.Binary, to parse said bytestring; the latter is faster (..probably), if more limited in function.
Yes, that was only a first attempt, kind of prototype...
You could use the lazy bytestring hGetContents for this. However...
There is also a bytestring-mmap package on hackage, which outsources the decision of what blocks to load into memory to the OS, and has the best performance overall. Use this.
Oh. And unsafePerformIO is a trap that will kill you. See http://www.girlgeniusonline.com/comic.php?date=20070725 for details.
Thank you for the link. Cristiano