Cristiano Paris wrote:
Daniel Fischer wrote:
I would separate the reading of headers and bodies, reopening the files whose body is needed, for some (maybe compelling) reason he wants to do it differently.
Yes, that's the way Haskell forces you to do that as it's the only way for you to go safe.
I don't think it has anything to do with Haskell. How would you do this in C? You'd pass a flag indicating whether to read the whole file or just the header. You can do the same in Haskell, of course, no lazy IO needed. The body remains undefined if the flag indicates header only. Even better wrap the body in a Maybe.
But, if you know more about your code, you can use unsafe(Perform|Interleave)IO to assure the compiler that everything's right.
I have a hard time believing this is possible, if you demand that the files should not stay opened indefinitely. How is the runtime supposed to know whether to close the file or not? What you /can/ do is use unsafePerformIO to lazily re-open, read the body, and close the file, as soon as the body gets demanded. However, this is ugly and not advised. Cheers Ben