Hack2: a Haskell Webserver Interface (V2)
Hack2 is a port of Ruby's Rack webserver interface.
Version
2011.6.10
Introduction
Idea
Separation of concerns:
- hack2: interface spec
- hack2-middleware: building blocks
- hack2-handler: server backends
Design
type Application = Env -> IO Response
Demo
{-# LANGUAGE OverloadedStrings #-}
import Hack2
app :: Application
app = \env -> return $
Response 200 [ ("Content-Type", "text/plain") ] "Hello World"
Spec
The Environment
- requestMethod: The HTTP request method, e.g.
GET
, POST
.
- scriptName: The initial portion of the request URL‘s “path” that corresponds to the application object, so that the application knows its virtual “location”. This may be an empty string, if the application corresponds to the “root” of the server.
- pathInfo: The remainder of the request URL‘s “path”, designating the virtual “location” of the request‘s target within the application. This may be an empty string, if the request URL targets the application root and does not have a trailing slash. This value may be percent-encoded when I originating from a URL.
- queryString: The portion of the request URL that follows the ?, if any. May be empty.
- serverName, serverPort: When combined with scriptName and pathInfo, these variables can be used to complete the URL. Note, however, that
Host
in http field, if present, should be used in preference to serverName for reconstructing the request URL. serverName and serverPort can never be empty, and so are always required.
- httpHeaders: Variables corresponding to the client-supplied HTTP request headers (e.g. "Accept"). The presence or absence of these variables should correspond with the presence or absence of the appropriate HTTP header in the request.
- hackVersion: The list of
Int
, representing this version of Hack
- hackUrlScheme:
HTTP
or HTTPS
, depending on the request URL.
- hackInput: The body of the request.
- hackErrors: The error stream.
- hackHeaders: None standard http headers.
The Response
- status: This is an HTTP status. It must be greater than or equal to 100.
- headers: The header must not contain a Status key, contain keys with : or newlines in their name, contain keys names that end in - or _, but only contain keys that consist of letters, digits, _ or - and start with a letter. The values of the header must be Strings, consisting of lines (for multiple header values) separated by “\n”. The lines must not contain characters below 037.
- body: The body of the response.
Properties
- The scriptName, if non-empty, must start with /
- The pathInfo, if non-empty, must start with /
- One of scriptName or pathInfo must be set. pathInfo should be / if scriptName is empty. scriptName never should be /, but instead be empty.
1 minute tutorial
update cabal
cabal update
install hack
cabal install hack2
pick a backend
cabal install hack2-handler-happstack
Create a Hack app
put the following code in Main.hs
{-# LANGUAGE OverloadedStrings #-}
import Hack2
import Hack2.Handler.HappstackServer
app :: Application
app = \env -> return $ Response
{ status = 200
, headers = [ ("Content-Type", "text/plain") ]
, body = "Hello World"
}
main = run app
run
runghc Main.hs
It should be running on http://127.0.0.1:3000 now.
Middleware
(below is waiting to be finished)
demo usage of middleware
install hack-contrib:
cabal install hack-contrib
put the following in Main.hs
. This code uses the URLMap
middleware to route both /hello
and /there
to the say
application.
import Hack
import Hack.Handler.HappstackServer
import Hack.Contrib.Utils
import Hack.Contrib.Middleware.URLMap
import Data.ByteString.Lazy.Char8 (pack)
import Data.Default
say :: Application
say = \env -> return $ def {body = pack $ show env, status = 200}
app :: Application
app = url_map [("/hello", say), ("/there", say)] empty_app
main = run app
create a middleware
inside Hack.hs:
type Middleware = Application -> Application
since Haskell has curry, middleware api can be of type
Anything -> Application -> Application
just pass an applied middleware into a chain.
finally the source code of Config.hs
:
module Hack.Contrib.Middleware.Config (config) where
import Hack
config :: (Env -> Env) -> Middleware
config alter app = \env -> app (alter env)
Use the middleware stack
From Hack.Contrib.Utils
:
-- usage: use [content_type, cache] app
use :: [Middleware] -> Middleware
use = reduce (<<<)
Handlers
Once an application is written using Hack, it should work on any web server that provides a Hack handler.
A handler should expose at least one function of type:
run :: Application -> IO ()
Upgrade
With every new release, any library links to hack should be recompiled against the new version, usually as simple as:
cabal install linked_lib --reinstall
Links