[Haskell] Re: [Haskell-cafe] ANNOUNCE: enumerator, an alternative iteratee package

[Judah Jacobson]

On Sat, Aug 21, 2010 at 10:58 AM, John Millikin <jmillikin at gmail.com> wrote:
> I think the docs are wrong, or perhaps we're misunderstanding them.
> Magnus is correct.
>
> Attached is a test program which listens on two ports, 42000 (blocking
> IO) and 42001 (non-blocking). You can use netcat, telnet, etc, to send
> it data. The behavior is as Magnus describes: bytes from
> hGetNonBlocking are available immediately, while hGet waits for a full
> buffer (or EOF) before returning.
>
> This behavior obviously makes hGet unsuitable for enumHandle; my
> apologies for not understanding the problem sooner.

You should note that in ghc>=6.12, hWaitForInput tries to decode the
next character of input based on to the Handle's encoding.  As a
result, it will block if the next multibyte sequence is incomplete,
and it will throw an error if a multibyte sequence gets split between
two chunks.

I worked around this problem in Haskeline by temporarily setting stdin
to BinaryMode; you may want to do something similar.

Also, this issue caused a bug in bytestring with ghc-6.12:
http://hackage.haskell.org/trac/ghc/ticket/3808
which will be resolved by the new function 'hGetBufSome' (in ghc-6.14)
that blocks only when there's no data to read:
http://hackage.haskell.org/trac/ghc/ticket/4046
That function might be useful for your package, though not portable to
other implementations or older GHC versions.

Best,
-Judah