Re: [Haskell-beginners] Confused about lazy IO

That would have been useful to know three weeks ago. Better late than never I guess. Thanks. On Fri, Mar 15, 2013 at 1:24 AM, Kim-Ee Yeoh wrote:

On Fri, Mar 15, 2013 at 5:52 AM, Jacek Dudek 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

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