[Haskell-cafe] Lazy IO.

Isaac Dupree isaacdupree at charter.net
Sat Jun 14 12:49:38 EDT 2008

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.


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