Wed, 11 Jun 2014

Moving from Wai 2.X to 3.0.

Michael Snoyman has just released version 3.0 of Wai, the Haskell Web Application Interface library which is used with the Yesod Web Framework and anything that uses the Warp web server. The important changes for Wai are listed this blog post. The tl;dr is that removing the Conduit library dependency makes the Wai interface more easily usable with one of the alternative Haskell streaming libraries, like Pipes, Stream-IO, Iterator etc.

As a result of the above changes, the type of a web application changes as follows:

  -- Wai > 2.0 && Wai < 3.0
  type Application = Request -> IO Response

  -- Wai == 3.0
  type Application = Request -> (Response -> IO ResponseReceived) -> IO ResponseReceived

Typically a function of type Application will be run by the Warp web server using one of or associated functions which have type signatures of:

  run :: Port -> Application -> IO ()

  runSettings :: Settings -> Application -> IO ()

  runSettingsSocket :: Settings -> Socket -> Application -> IO ()Source

  runSettingsConnection :: Settings -> IO (Connection, SockAddr) -> Application -> IO ()

Its important to note that the only thing that has changed about these Warp functions is the Application type. That means that if we have a function oldWaiApplication that we want to interface to the new version of Wai, we can just wrap it with the following function:

  newWaiApplication :: Manager -> Request -> (Response -> IO ResponseReceived) -> IO ResponseReceived
  newWaiApplication mgr wreq receiver = oldWaiApplication mgr wreq >>= receiver

and use newWaiApplication in place of oldWaiApplication in the call to whichever of the Warp run functions you are using.

Posted at: 20:16 | Category: CodeHacking/Haskell | Permalink