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Setup.lhs hack_input and body use bytestring; custom removed add remote host
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Hack: a Haskell Webserver Interface

Hack is a port of Ruby's Rack webserver interface.


It's now recommended to use Hack2 for new projects.





Separation of concerns:

  • hack: interface spec
  • hack-middleware: building blocks
  • hack-handler: server backends


type Application = Env -> IO Response


import Hack
import Hack.Handler.Happstack
import Data.ByteString.Lazy.Char8 (pack)

app :: Application
app = \env -> return $
  Response 200 [ ("Content-Type", "text/plain") ] (pack "Hello World")

main = run app


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.
  • remoteHost: The remote host of this request
  • http: 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 http headers, intended to be used by handlers and middleware.
  • hackCache: High performance cache, could be used by handlers and middleware.

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.


  • 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 hack

pick a backend

cabal install hack-handler-happstack

Create a Hack app

put the following code in Main.hs

import Hack
import Hack.Handler.Happstack
import Data.ByteString.Lazy.Char8 (pack)

app :: Application
app = \env -> return $ Response 
    { status  = 200
    , headers = [ ("Content-Type", "text/plain") ]
    , body    = pack "Hello World"

main = run app


runghc Main.hs

It should be running on now.


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.Happstack
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 (<<<)


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 ()


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


Discuss:Web programming with Haskell

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