wai-1.4.1: Web Application Interface.

Safe HaskellNone




This module defines a generic web application interface. It is a common protocol between web servers and web applications.

The overriding design principles here are performance and generality . To address performance, this library is built on top of the conduit and blaze-builder packages. The advantages of conduits over lazy IO have been debated elsewhere and so will not be addressed here. However, helper functions like responseLBS allow you to continue using lazy IO if you so desire.

Generality is achieved by removing many variables commonly found in similar projects that are not universal to all servers. The goal is that the Request object contains only data which is meaningful in all circumstances.

Please remember when using this package that, while your application may compile without a hitch against many different servers, there are other considerations to be taken when moving to a new backend. For example, if you transfer from a CGI application to a FastCGI one, you might suddenly find you have a memory leak. Conversely, a FastCGI application would be well served to preload all templates from disk when first starting; this would kill the performance of a CGI application.

This package purposely provides very little functionality. You can find various middlewares, backends and utilities on Hackage. Some of the most commonly used include:



WAI interface

data Request Source

Information on the request sent by the client. This abstracts away the details of the underlying implementation.




requestMethod :: Method
httpVersion :: HttpVersion
rawPathInfo :: ByteString

Extra path information sent by the client. The meaning varies slightly depending on backend; in a standalone server setting, this is most likely all information after the domain name. In a CGI application, this would be the information following the path to the CGI executable itself. Do not modify this raw value- modify pathInfo instead.

rawQueryString :: ByteString

If no query string was specified, this should be empty. This value will include the leading question mark. Do not modify this raw value- modify queryString instead.

serverName :: ByteString

Generally the host requested by the user via the Host request header. Backends are free to provide alternative values as necessary. This value should not be used to construct URLs.

serverPort :: Int

The listening port that the server received this request on. It is possible for a server to listen on a non-numeric port (i.e., Unix named socket), in which case this value will be arbitrary. Like serverName, this value should not be used in URL construction.

requestHeaders :: RequestHeaders
isSecure :: Bool

Was this request made over an SSL connection?

This value should not be used, and will be removed in future revisions of WAI. There is no meaningful way that a backend can indicate whether the request is actually over a secure channel, due to issues of reverse proxying.

remoteHost :: SockAddr

The client's host information.

pathInfo :: [Text]

Path info in individual pieces- the url without a hostname/port and without a query string, split on forward slashes,

queryString :: Query

Parsed query string information

requestBody :: Source (ResourceT IO) ByteString
vault :: Vault

A location for arbitrary data to be shared by applications and middleware.

requestBodyLength :: RequestBodyLength

The size of the request body. In the case of a chunked request body, this may be unknown.

Since 1.4.0


data Response Source

Some questions and answers about the usage of Builder here:

Q1. Shouldn't it be at the user's discretion to use Builders internally and then create a stream of ByteStrings?

A1. That would be less efficient, as we wouldn't get cheap concatenation with the response headers.

Q2. Isn't it really inefficient to convert from ByteString to Builder, and then right back to ByteString?

A2. No. If the ByteStrings are small, then they will be copied into a larger buffer, which should be a performance gain overall (less system calls). If they are already large, then blaze-builder uses an InsertByteString instruction to avoid copying.

Q3. Doesn't this prevent us from creating comet-style servers, since data will be cached?

A3. You can force blaze-builder to output a ByteString before it is an optimal size by sending a flush command.


type Middleware = Application -> ApplicationSource

Middleware is a component that sits between the server and application. It can do such tasks as GZIP encoding or response caching. What follows is the general definition of middleware, though a middleware author should feel free to modify this.

As an example of an alternate type for middleware, suppose you write a function to load up session information. The session information is simply a string map [(String, String)]. A logical type signatures for this middleware might be:

 loadSession :: ([(String, String)] -> Application) -> Application

Here, instead of taking a standard Application as its first argument, the middleware takes a function which consumes the session information as well.

data RequestBodyLength Source

The size of the request body. In the case of chunked bodies, the size will not be known.

Since 1.4.0

lazyRequestBody :: Request -> ResourceT IO ByteStringSource

Get the request body as a lazy ByteString. This uses lazy I/O under the surface, and therefore all typical warnings regarding lazy I/O apply.

Since 1.4.1

Response body smart constructors