[Haskell-cafe] Strange behaviour with writeFile

[Donald Bruce Stewart]

cmb21:
> fo/haskell-cafe>,
> 	<mailto:haskell-cafe-request at haskell.org?subject=subscribe>
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> 
> 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!

lazy IO at play. One quick fix would be to use strict IO:

    import qualified Data.ByteString.Char8 as S
    import Data.ByteString                      (ByteString)

    main = do
        ans <- S.readFile "t"           -- strict file IO
        print (S.length ans)            -- current size

        let (x,xs) = S.splitAt 10 ans   -- "parse"
        S.writeFile "t" xs

        ans' <- S.readFile "t"
        print (S.length ans')            -- new size

$ time ./a.out
2487212
2487202



This comes up often enough that I think we should have a strict readFile for Strings

-- Don