hGetContents reads the entire contents of the stream till the end (although lazily). The return value of hGetContents is logically the entire contents of the stream. That it has not read it completely is only a part of its laziness, so the result does not depend upon when the caller stops consuming the result. That is why the handle is semi-closed; logically the handle is already at the end of the stream.

A complete parser to parse the header and the body has to be used on the entire contents, or some function which knows how to find the header end and stop there has to be used.

Regards,
Abhay

On Sat, Jun 14, 2008 at 9:48 PM, Sebastiaan Visser <sfvisser@cs.uu.nl> wrote:
Hi,

I've got a question about lazy IO in Haskell. The most well known
function to do lazy IO is the `hGetContents', which lazily reads all the
contents from a handle and returns this as a regular [Char].

The thing with hGetContents is that is puts the Handle in a semi-closed
state, no one can use the handle anymore. This behaviour is
understandable from the point of safety; it is not yet determined when
the result of hGetContents will actually be computed, using the handle
in the meantime is undesirable.

The point is, I think I really have a situation in which I want to use
the handle again `after' a call to hGetContents. I think I can best
explain this using a code example.

 readHttpMessage :: IO (Headers, Data.ByteString.Lazy.ByteString)
 readHttpMessage = do
   myStream <- <accept http connection from client>
   request <- hGetContents myStream
   header <- parseHttpHeader request
   bs <- Data.ByteString.Lazy.hGetContents myStream
   return (header, body)

The Data.ByteString.Lazy.hGetContents in the example above obviously
fails because the handle is semi-closed.

So, what I am trying to do here is apply a parser (on that consumes
Char's) to the input stream until it has succeeded. After this I want to
collect the remainings of the stream in a lazy ByteString, or maybe even
something else.

I tried to open the handler again using some internal handle hackery,
but this failed (luckily). In the module GHC.IO there is a function
`lazyRead' that more or less seems to do what I want. But I'll guess
there is a good reason for not exporting it.

Does anyone know a pattern in which I can do this easily?

Thanks,

--
Sebastiaan

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