wai-0.0.0: Web Application Interface.




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, generality and type safety. To address performance, this library is built on Source for the request body and Enumerator for the response bodies. The advantages of this approach over lazy IO have been debated elsewhere.

Nonetheless, many people find these data structures difficult to work with. For that reason, this library includes the Network.Wai.Enumerator module to provide more familiar abstractions, including lazy IO.

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.

Unlike other approaches, this package declares many data types to assist in type safety. This feels more inline with the general Haskell spirit.

A final note: please remember when using this package that, while your application my 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.


Data types

For the data types below, you should only use the Show and Read instances for debugging purposes. Each datatype (excepting UrlScheme) has associated functions for converting to and from strict ByteStrings; these are approrpiate for generating content.

data Method Source

HTTP request method. This data type is extensible via the Method constructor. Request methods are case-sensitive, and comparison is achieved by converting to a ByteString via methodToBS.

URL scheme (http versus https)

data UrlScheme Source



HTTP protocol versions

data HttpVersion Source

Version of HTTP protocol used in current request. This data type is extensible via the HttpVersion constructor. Comparison is achieved by converting to a ByteString via httpVersionToBS.

httpVersionFromBS :: ByteString -> HttpVersionSource

This function takes the information after "HTTP/". For example:

 httpVersionFromBS (pack "1.0") == Http10

httpVersionToBS :: HttpVersion -> ByteStringSource

Returns the version number, for example:

 unpack (httpVersionToBS Http10) == "1.0"

Request header names

data RequestHeader Source

Headers sent from the client to the server. Clearly, this is not a complete list of all possible headers, but rather a selection of common ones. If other headers are required, they can be created with the RequestHeader constructor.

The naming rules are simple: removing any hyphens from the actual name, and if there is a naming conflict with a ResponseHeader, prefix with Req.

Equality determined by conversion via requestHeaderToBS. Remember, headers are case sensitive.

Response header names

data ResponseHeader Source

Headers sent from the server to the client. Clearly, this is not a complete list of all possible headers, but rather a selection of common ones. If other headers are required, they can be created with the ResponseHeader constructor.

if there is a naming conflict with a ResponseHeader, prefix with Req.

Equality determined by conversion via responseHeaderToBS. Remember, headers are case sensitive.

Response status code

data Status Source

This attempts to provide the most common HTTP status codes, not all of them. Use the Status constructor when you want to create a status code not provided.

The Eq instance tests equality based only on the numeric status code value. See statusCode.



newtype Source Source

This is a source for ByteStrings. It is a function (wrapped in a newtype) that will return Nothing if the data has been completely consumed, or return the next ByteString from the source along with a new Source to continue reading from.

Be certain not to reuse a Source! It might work fine with some implementations of Source, while causing bugs with others.




newtype Enumerator Source

An enumerator is a data producer. It takes two arguments: a function to enumerate over (the iteratee) and an accumulating parameter. As the enumerator produces output, it calls the iteratee, thereby avoiding the need to allocate large amounts of memory for storing the entire piece of data.

Normally in Haskell, we can achieve the same results with laziness. For example, an inifinite list does not require inifinite memory storage; we simply get away with thunks. However, when operating in the IO monad, we do not have this luxury. There are other approaches, such as lazy I/O. If you would like to program in this manner, please see Network.Wai.Enumerator, in particular toLBS.

That said, let's address the details of this particular enumerator implementation. You'll notice that the iteratee is a function that takes two arguments and returns an Either value. The second argument is simply the piece of data generated by the enumerator. The Either value at the end is a means to alert the enumerator whether to continue or not. If it returns Left, then the enumeration should cease. If it returns Right, it should continue.

The accumulating parameter (a) has meaning only to the iteratee; the enumerator simply passes it around. The enumerator itself also returns an Either value; a Right means the enumerator ran to completion, while a Left indicates early termination was requested by the iteratee.

Enumerators are not required to be resumable. That is to say, the Enumerator may only be called once. While this requirement puts a bit of a strain on the caller in some situations, it saves a large amount of complication- and thus performance- on the producer.




runEnumerator :: forall a. (a -> ByteString -> IO (Either a a)) -> a -> IO (Either a a)

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
pathInfo :: 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.

queryString :: ByteString

If no query string was specified, this should be empty.

serverName :: ByteString
serverPort :: Int
requestHeaders :: [(RequestHeader, ByteString)]
urlScheme :: UrlScheme
requestBody :: Source
errorHandler :: String -> IO ()
remoteHost :: ByteString

The client's host information.

data Response Source




status :: Status
responseHeaders :: [(ResponseHeader, ByteString)]
responseBody :: Either FilePath Enumerator

A common optimization is to use the sendfile system call when sending files from the disk. This datatype facilitates this optimization; if Left is returned, the server will send the file from the disk by whatever means it wishes. If Right, it will call the Enumerator.

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.