[Haskell-cafe] Re: RFC: A standardized interface between web servers and applications or frameworks (ala WSGI)

Daniel Yokomizo daniel.yokomizo at gmail.com
Mon Apr 14 07:54:07 EDT 2008

On Mon, Apr 14, 2008 at 3:27 AM, Adam Langley <agl at imperialviolet.org> wrote:
> On Sun, Apr 13, 2008 at 6:32 PM, Chris Smith <cdsmith at twu.net> wrote:
>  >  Does old code that handled these headers stop working, just because it
>  >  was looking in the "other" section, but now needs to check a field
>  >  dedicated to that header?
>  Yes, but it would be very sad if we couldn't do common header parsing
>  because of this.
>  I'd suggest that all the headers given in RFC 2616 be parsed and
>  nothing else.

Both request and response accept any entity headers and 7.1 (of RFC
2616) says that a valid entity header is an extension header, which
can be any kind of header.

> That leaves the question of how we would handle the
>  addition of any extra ones in the future. Firstly, packages could
>  depend on a given version of this interface and we declare that the
>  set of handled headers doesn't change within a major version.
>  Better would be some static assertion that the interface doesn't
>  handle some set of headers. Maybe there's a type trick to do this, but
>  I can't think of one, so we might have to settle for a non static:
>  checkUnparsedHeaders :: [String] -> IO ()
>  Which can be put in 'main' (or equivalent) and can call error if
>  there's a mismatch.

Most of the times a Header makes sense in some scenarios and doesn't
in others, so a package level checking is too coarse grained.

IMHO it would be better to create a two layered approach. The bottom
layer handles the request as a bunch of strings, just checks for
structural correctness (i.e. break the headers by line and such)
without checking if the headers are correct. The top layer provides a
bunch of parser combinators to validate, parse and sanitize the
request so a library can create its own contract:

newtype Contract e a = Contract (HttpRequest -> e a)

contract :: Contract Maybe MyRequest
contract = do pragma <- parseHeader "Pragma" (\header -> ...)
                      return $ MyRequest pragma ...

main =
   do request <- readHttpRequest
        sanitized <- enforce contract request

Such approach would be more flexible and extensible. Later other
packages could provide specialized combinators for other RFCs. HTTP is
regularly extended, in RFCs and by private parties experimenting
before writing an RFC, it would be bad if the primary Haskell library
for HTTP didn't support this behavior. Also it's important to notice
that the HTTP spec defines things to be mostly orthogonal, so most of
the headers stand on their own and can be used in combination with
many methods and other headers, every once in a while someone finds a
combination that makes sense and wasn't thought of before.

>  AGL
>  --
>  Adam Langley agl at imperialviolet.org http://www.imperialviolet.org

Best regards,
Daniel Yokomizo.

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