On Wed, Feb 25, 2009 at 12:15 PM, Neil Mitchell wrote:

Hi,

I want to read a file using Data.Binary, and I want to read the file strictly - i.e. when I leave the read file I want to guarantee the handle is closed. The reason is that (possibly immediately after) I need to write to the file. The following is the magic I need to use - is it all necessary, is it guaranteed correct, should I use something else?

       src <- decodeFile "_make/_make"        Map.size mp `seq` performGC

Thanks for all the helpful replies to the last post, I think I've nearly come up with a minimal test case I can give you, or at least characterise where the issue might lie. But I'll post to that thread when I've got the details.

Thanks

Neil

Funnily enough, I was just grappling with this issue in Yi. My solution was to make the read strict, so my function looked like this: readDB ∷ YiM ArticleDB readDB = io $ (dbLocation >>= r) `catch` (λ_ → return empty) where r x = fmap (decode · BL.fromChunks · return) $ B.readFile x -- gwern

Neil Mitchell wrote:

Hi,

I want to read a file using Data.Binary, and I want to read the file strictly - i.e. when I leave the read file I want to guarantee the handle is closed. The reason is that (possibly immediately after) I need to write to the file. The following is the magic I need to use - is it all necessary, is it guaranteed correct, should I use something else?

src <- decodeFile "_make/_make" Map.size mp `seq` performGC

With binary 0.5, src <- decodeFile "_make/_make" return $! src should close the file, assuming that all the data is read from the file, thanks to this patch: Mon Aug 25 23:01:09 CEST 2008 Don Stewart * WHNF the tail of a bytestring on decodeFile, will close the resource For older versions, import qualified Data.Binary.Get as Get data EOF = EOF instance Binary EOF where get = do eof <- Get.isEmpty return (if eof then EOF else error "EOF expected") put EOF = return () ... (src, EOF) <- decodeFile "_make/_make" accomplishes the same effect. Btw, contrary to what Duncan said, Get is a lazy monad (lazy in its actions, that is): instance Binary EOF where get = do eof <- Get.isEmpty when (not eof) error "EOF expected" return EOF put EOF = return () does not help, because the result (EOF) does not depend on the value returned by isEmpty. The idea of using isEmpty for closing the file is not perfect though; due to the lazy nature of Get, there's a stack overflow lurking below: main = do encodeFile "w.bin" [0..1000000 :: Int] m <- decodeFile "w.bin" print $ foldl' (+) 0 (m :: [Int]) One idea to fix this is to force the read data before checking for EOF, as follows: data BinaryRNF a = BinaryRNF a instance (NFData a, Binary a) => Binary (BinaryRNF a) where get = (\a -> rnf a `seq` BinaryRNF a) `fmap` get put (BinaryRNF a) = put a main = do encodeFile "w.bin" [0..1000000 :: Int] (BinaryRNF m, EOF) <- decode `fmap` L.readFile "w.bin" print $ foldl' (+) 0 (m :: [Int]) HTH, Bertram