[Haskell-beginners] Confused about lazy IO

[Kim-Ee Yeoh]

On Fri, Mar 15, 2013 at 5:52 AM, Jacek Dudek <jzdudek at gmail.com> wrote:
> readFile fileName >>= \ contents ->
> writeFile fileName (f contents)

You're reading and writing to the /same/ file back to back. With Lazy
I/O. Those two just don't mix.

The "permission denied" probably stems from the attempted write
clashing with the previous exclusive read handle.

> I thought lazy IO was implemented in such a way that you were safe to
> INTERPRET the IO action as having been fully performed.

Nope.

The usual gotcha lying for the unwary is

do
    h <- openFile fname1 ReadMode
    s <- hGetContents h
    hClose h
    writeFile fname2 s

In the case fname1==fname2, a different surprise lies in store as
you've just witnessed.

Importing System.IO.Strict from the strict package should fix all of the above.

-- Kim-Ee