[Haskell-cafe] Strange behaviour with writeFile

[C.M.Brown]

Hi Neil,

> When you add that extra line the act of writing out the remainer
> causes the rest of the input to be fully evaluated and hence the
> filehandle is closed.

Ah, yes of course :)

I've found that:

let (answer2, remainder) = parseAnswer (force answer)

where

force  :: Eq a => a -> a
force x = if x==x then x else x

Seems to do the trick.

Thanks!
Chris.
>
> On 04/02/07, C.M.Brown <cmb21 at kent.ac.uk> wrote:
> > Hi,
> >
> > I am observing some rather strange behaviour with writeFile.
> >
> > Say I have the following code:
> >
> > answer <- AbstractIO.readFile "filename"
> > let (answer2, remainder) = parseAnswer answer
> > if remainder == "" && answer2 == ""
> >   then do
> >     AbstractIO.putStrLn $ "completed"
> >   else do
> >     AbstractIO.putStrLn answer2
> >     AbstractIO.writeFile "filename" remainder
> >
> > With the above I get an error saying the resources to "filename" are
> > locked. If I add the line "AbstractIO.putStrLn $ show (answer2, remainder)
> > before I call writeFile it suddenly magically works!
> >
> > Has anyone seen strange behaviour like this before?
> >
> > Regards,
> > Chris.
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>