On Tue, 2009-10-27 at 17:00 +0100, Daniel Fischer wrote:
> Look at the sources:
>
> hGetLine :: Handle -> IO ByteString
> hGetLine h = wantReadableHandle "Data.ByteString.hGetLine" h $ \ handle_ -> do
> case haBufferMode handle_ of
> NoBuffering -> error "no buffering"
> _other -> hGetLineBuffered handle_
>
> where
> ...
>
> So, in ghci, stdin isn't buffered and ByteString can't cope with that.
> Why exactly, I don't know.
By comparison if we look at System.IO.hGetLine we see:
hGetLine :: Handle -> IO String
hGetLine h = do
m <- wantReadableHandle "hGetLine" h $ \ handle_ -> do
case haBufferMode handle_ of
NoBuffering -> return Nothing
LineBuffering -> do
l <- hGetLineBuffered handle_
return (Just l)
BlockBuffering _ -> do
l <- hGetLineBuffered handle_
return (Just l)
case m of
Nothing -> hGetLineUnBuffered h
Just l -> return l
So there is something special about an unbuffered hGetLine. I can't
remember exactly but when we implemented hGetLine for ByteString I guess
we didn't understand / couldn't be bothered with the weird unbuffered
case.
The System.IO code in ghc-6.10 also notes:
-- ToDo: the unbuffered case is wrong: it doesn't lock the handle for
-- the duration.
In ghc-6.12 I think the unbuffered mode for input was simply abolished
so there is no longer a hGetLineUnBuffered case.
Duncan
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