Simon Marlow wrote:
>
> > Arguably, readFile is too strict: why is the file opened, if there
> > aren't any characters needed. What is the motivation for opening the
> > file eagerly?
>
> Once we've started reading lazilly, it's hard to generate errors. The file
> is opened eagerly so that any errors generated (such as if the file doesn't
> exist) can be propogated immediately to the caller.
>
> I checked the Haskell Report, which doesn't seem to specify the correct
> behaviour - I guess this should be clarified. The report also contains the
> sentence "The exceptions raised by the I/O functions in the Prelude are
> defined in the Library Report", but I can't find any reference to
> readFile/writeFile in the library report.
>
> Cheers,
> Simon
I can't seem to work around this behaviour, is it possible to
force readFile to just open a file, read it entirely and close it
before doing the next open ? I tried a few things with seq,
but this doesn't work:
xs <- mapM (\x -> let y = readFile x
in seq y y) (take 1000 (repeat "tmp"));
and neither does this:
let
f = mapM (\x -> let y = readFile x
in seq y y) (take 1000 (repeat "tmp"));
in do {
xs <- seq f f;
return ();
}
Jan
