[web-devel] Re: [Haskell-cafe] HSGI: Haskell Server Gateway Interface

[Felix Martini]

[Original discussion in haskell-cafe:
http://www.haskell.org/pipermail/haskell-cafe/2009-February/055270.html]

Manlio Perillo:
> Here are the problems:
>
> 1) supported HTTP methods are fixed, since you use a custom data type

(Btw. It is not my proposal, Johan Tibell has written it. I'll mail
him to notify him about this discussion.)

I agree with that. In my own framework prototype i have

data HttpMethod = OPTIONS
                          | GET
                          | HEAD
                          | POST
                          | PUT
                          | DELETE
                          | TRACE
                          | CONNECT
                          | Extension String
    deriving (Eq, Show)

> 2) no support for other possible CGI variables
>   (or simply custom variables: I use the environ dictionary to store
>   application configuration, and I think it is a great thing).

A user dictionary / association list could be useful. A Trie is
probably faster than a Map so i am not sure if the Map type should be
used. Could also be implemented by the frameworks themselves.

> 3) No support for optimized file serving, like with WSGI file_wrapper

Would be nice to have provided it is cross-platform.

> 4) No support for keeping "complex" state (like database connection) in
>   the environ.
>
>   I have read some examples with CGI package, where a custom monad
>   transformer is used.
>   I'm not sure if this solution is flexible and "scalable".

This should be a task for the framework itself. Using a ReaderT or
StateT transformer is very common in Haskell.

> 5) I can't find documentation about the type of the response.
>   It is rather clear that it is
>      (ByteString, [(ByteString, ByteString)], Enumerator)
>
>   but this should be documented somewhere.

It's a wiki so feel free to change it or add more documentation.


About the enumerator, using it is indeed debatable because it is not
immediately obvious how it works, but i haven't seen anything for
Haskell that works better. Like other haskellers have said lazy
bytestrings with IO is asking for trouble. I have been bitten by it
myself and it is hard to debug. Imho lazy bytestrings are only useful
when the data generator is a pure function.

Regards,
Felix