Thank you all for your answers. I'll try to be more careful with lazy
evaluations while doing IO operations from now on.
Best regards
André
On Fri, Aug 12, 2005 at 02:54:03PM +0200, Lemmih wrote:
On 8/12/05, David Roundy <[EMAIL PROTECTED]> wrote:
On Fri, Aug 12, 2005 at 09:17:32AM -0300, Andr Vargas Abs da Cruz wrote:
readDataFromFile filename = do
bracket (openFile filename ReadMode) hClose
(\h -> do contents <- hGetContents h
return (lines contents))
The question is: if i try to run this, it returns me nothing (just
an empty list). Why does this happen ? When i remove the "return" and
put a "print" instead, it prints everything as i expected.
I think that this may be because hGetContents puts the file in a
semi-closed state (and reads the contents lazily). Try removing the
hClose (replacing it with (return ()). Lazy IO is nice, but gets
tricky at times.
Or better yet, use 'readFile'.
Indeed, that would be simpler, but it's still lazy IO, and if you're
unaware of that you'll still get confused when you try to delete the file
after reading the lines you wanted out of it, and it fails on windows, but
not on POSIX systems... unless you read the entire file, or computed its
length.
Glynn Clements wrote:
hGetContents reads the file lazily; it won't actually read anything
until you try to "consume" the result. However, by that point, you
will have called hClose.
In general, you shouldn't use hClose in conjunction with lazy I/O
(hGetContents etc) unless you are certain that the data will have been
read.
When you put the "print" in place of the return, you force the data to
be consumed immediately, so the issue doesn't arise.
Arthur Baars wrote: Hi André,
The problem is that hGetContents does lazy reading of the handle.
I you do:
readDataFromFile "test.txt" >>= print
the handle is closed by hClose (in readDataFromFile) before "print"
demands the contents from the handle.
Just don't close the Handle explicitly.
This code works:
readDataFromFile filename =
do h <-openFile filename ReadMode
contents <- hGetContents h
return (lines contents)
You do not really need to close it, because hGetContents "semi-closes"
the handle. Read more:
http://www.haskell.org/onlinereport/io.html (Section 21.2.2
Semi-Closed Handles)
http://users.aber.ac.uk/afc/stricthaskell.html#semiclosed
Cheers,
Arthur
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