how to do lazy IO like getContents?
bescoto at stanford.edu
Sat Oct 18 22:35:40 EDT 2003
On Sat, 18 Oct 2003 19:17:06 -0700 (PDT) Hal Daume III <hdaume at ISI.EDU> wrote:
> It is unsafe because, in general, lazy IO is a bad idea. In particular:
> foo f x = do
> h <- openFile x ReadMode
> t <- hGetContents h
> v <- f t
> hClose h
> return t
> will do substantially different things depending on the strictness of
> 'f'. For instance, if 'f' is 'return . head', you might get a
> 'head:: empty list' error, while if 'f' is 'evaluate . head', you won't.
I don't understand the details of your example (for instance, what
does "evaluate" do? I found a reference to it in the GHC manual under
Debug.QuickCheck, but couldn't figure out what it.), but get the
In my case, I wanted to represent a directory tree that was too big to
fit in memory. I was planning on declaring it as
data DirTree = DirTree File [DirTree]
but then I apparently need lazy IO. If lazy IO is bad, is
data DirTree = DirTree File [IO DirTree]
a better way of doing it?
P.S. Thanks for your haskell book. I read it and learned a lot.
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