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module Utility.Observed where
import qualified Data.ByteString.Lazy as L
import qualified Data.ByteString as S
import System.IO
import System.IO.Unsafe
import Foreign.Storable (Storable(sizeOf))
{- This is like L.hGetContents, but after each chunk is read, an action
- is run to observe the size of the chunk.
-
- Note that the observer is run in unsafeInterleaveIO, which means that
- it can be run at any time. It's even possible for observers to run out
- of order, as different parts of the ByteString are consumed.
-
- All the usual caveats about using unsafeInterleaveIO apply to the observers,
- so use caution.
-}
hGetContentsObserved :: Handle -> (Int -> IO ()) -> IO L.ByteString
hGetContentsObserved h observe = lazyRead
where
lazyRead = unsafeInterleaveIO loop
loop = do
c <- S.hGetSome h defaultChunkSize
if S.null c
then do
hClose h
return $ L.empty
else do
observe $ S.length c
{- unsafeInterleaveIO causes this to be
- deferred until the data is read from the
- ByteString. -}
cs <- lazyRead
return $ L.append (L.fromChunks [c]) cs
{- Same default chunk size Lazy ByteStrings use. -}
defaultChunkSize :: Int
defaultChunkSize = 32 * k - chunkOverhead
where
k = 1024
chunkOverhead = 2 * sizeOf (undefined :: Int) -- GHC specific
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