[Haskell-beginners] Confused about lazy IO

Gesh hseG gesh at gesh.uni.cx
Fri Mar 15 00:16:14 CET 2013

I was bitten by this a couple of weeks ago.
What I was told back then boils down to the following:
- The proper way to reason about lazy IO is that nothing that doesn't need
to be
  read is ever read - in much the same way as the only part of an infinite
list that
  gets evaluated is the prefix that you use in your program.
- When reading and writing to the same file, it is better in general to
make the
  program transactional by writing to a temp file and then renaming it to
the old file.

On Fri, Mar 15, 2013 at 12:52 AM, Jacek Dudek <jzdudek at gmail.com> wrote:

> Q: When trying to compose the following IO actions:
> readFile fileName >>= \ contents ->
> writeFile fileName (f contents)
> I get an exception: openFile: permission denied.
> However the following works:
> readFile fileName >>= \ contents ->
> (mapM_ putStrLn . lines) contents >>
> writeFile fileName (f contents)
> I'm assuming it has something to do with lazy IO, and the second
> action in the second version forces fileName to be read completely and
> to be closed.
> Why do I need to do that? I thought lazy IO was implemented in such a
> way that you were safe to INTERPRET the IO action as having been fully
> performed. (And so I would have been safe to interpret that, in the
> first version, in the first action, the file was released.)
> If that's not the case, then:
> (1) What is the proper way to reason about lazy IO?
> (2) Is there some action less arbitrary that what I cooked up in the
> second version that explicitly instructs to release a file or some
> other resource?
> Regards, Jacek.
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