wai-3.0.1: 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:




type Application = Request -> (Response -> IO ResponseReceived) -> IO ResponseReceivedSource

The WAI application.

Note that, since WAI 3.0, this type is structured in continuation passing style to allow for proper safe resource handling. This was handled in the past via other means (e.g., ResourceT). As a demonstration:

 app :: Application
 app req respond = bracket_
     (putStrLn "Allocating scarce resource")
     (putStrLn "Cleaning up")
     (respond $ responseLBS status200 [] "Hello World")

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 signature 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 ResponseReceived Source

A special datatype to indicate that the WAI handler has received the response. This is to avoid the need for Rank2Types in the definition of Application.

It is highly advised that only WAI handlers import and use the data constructor for this data type.

Since 3.0.0


data Request Source

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


defaultRequest :: RequestSource

A default, blank request.

Since 2.0.0

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

Request accessors

requestMethod :: Request -> MethodSource

Request method such as GET.

httpVersion :: Request -> HttpVersionSource

HTTP version such as 1.1.

rawPathInfo :: Request -> ByteStringSource

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 :: Request -> ByteStringSource

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.

requestHeaders :: Request -> RequestHeadersSource

A list of header (a pair of key and value) in an HTTP request.

isSecure :: Request -> BoolSource

Was this request made over an SSL connection?

Note that this value will not tell you if the client originally made this request over SSL, but rather whether the current connection is SSL. The distinction lies with reverse proxies. In many cases, the client will connect to a load balancer over SSL, but connect to the WAI handler without SSL. In such a case, isSecure will be False, but from a user perspective, there is a secure connection.

remoteHost :: Request -> SockAddrSource

The client's host information.

pathInfo :: Request -> [Text]Source

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

queryString :: Request -> QuerySource

Parsed query string information

requestBody :: Request -> IO ByteStringSource

Get the next chunk of the body. Returns an empty bytestring when the body is fully consumed.

vault :: Request -> VaultSource

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

requestBodyLength :: Request -> RequestBodyLengthSource

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

Since 1.4.0

requestHeaderHost :: Request -> Maybe ByteStringSource

The value of the Host header in a HTTP request.

Since 2.0.0

requestHeaderRange :: Request -> Maybe ByteStringSource

The value of the Range header in a HTTP request.

Since 2.0.0

strictRequestBody :: Request -> IO ByteStringSource

Get the request body as a lazy ByteString. However, do not use any lazy I/O, instead reading the entire body into memory strictly.

Since 3.0.1

lazyRequestBody :: Request -> 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


type StreamingBody = (Builder -> IO ()) -> IO () -> IO ()Source

Represents a streaming HTTP response body. It's a function of two parameters; the first parameter provides a means of sending another chunk of data, and the second parameter provides a means of flushing the data to the client.

Since 3.0.0

data FilePart Source

Information on which part to be sent. Sophisticated application handles Range (and If-Range) then create FilePart.


Response composers

responseBuilder :: Status -> ResponseHeaders -> Builder -> ResponseSource

Creating Response from Builder.

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.

responseStream :: Status -> ResponseHeaders -> StreamingBody -> ResponseSource

Creating Response from a stream of values.

In order to allocate resources in an exception-safe manner, you can use the bracket pattern outside of the call to responseStream. As a trivial example:

 app :: Application
 app req respond = bracket_
     (putStrLn "Allocating scarce resource")
     (putStrLn "Cleaning up")
     $ respond $ responseStream status200 [] $ \write flush -> do
         write $ fromByteString "Hello\n"
         write $ fromByteString "World\n"

Note that in some cases you can use bracket from inside responseStream as well. However, placing the call on the outside allows your status value and response headers to depend on the scarce resource.

Since 3.0.0

responseRaw :: (IO ByteString -> (ByteString -> IO ()) -> IO ()) -> Response -> ResponseSource

Create a response for a raw application. This is useful for "upgrade" situations such as WebSockets, where an application requests for the server to grant it raw network access.

This function requires a backup response to be provided, for the case where the handler in question does not support such upgrading (e.g., CGI apps).

In the event that you read from the request body before returning a responseRaw, behavior is undefined.

Since 2.1.0

Response accessors

responseToStream :: Response -> (Status, ResponseHeaders, (StreamingBody -> IO a) -> IO a)Source

Converting the body information in Response to a StreamingBody.