On Jun 14, 2008, at 6:49 PM, Isaac Dupree wrote:
Sebastiaan Visser 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)
that's impure because parseHttpHeader doesn't return anything
telling you how much of the stream it's looked at. Maybe it looked
ahead more than it needed to, thus deleting part of the body. I
was going to suggest, if you can't change parseHttpHeader to use
ByteStrings,
> bs <- Data.ByteString.Lazy.hGetContents myStream
> header <- parseHttpHeader (Data.ByteString.Lazy.unpack bs)
but you still have to get parseHttpHeader (or perhaps if it has
similar friends) to tell you how much of the string it consumed! I
don't know what parsing functions you have available to work with,
so I can't tell you whether it's possible.
It is a regular Parsec parser and I am pretty sure it does not
consume anything other than the header itself. Maybe I could rewrite
my parser to work on Word8's instead of Char's, I don't think HTTP
even allows Unicode characters within HTTP headers.
Thanks. I think I'll try this.
But I'm still curious about how to lazily parse messages with
arbitrary size Unicode headers and plain (possibly) binary bodies.
-Isaac
--
Sebastiaan.
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