BSD-3-Clause licensed by Pavan Rikhi
Maintained by [email protected]
This version can be pinned in stack with:wai-middleware-clacks-,3995

Module documentation for


wai-middleware-clacks Build Status

“You know they’ll never really die while the Trunk is alive.”

wai-middleware-clacks is a middleware that inserts an arbitrary X-Clacks-Overhead header into every Wai response. From GNU Terry Pratchett:

In Terry Pratchett’s Discworld series, the clacks are a series of semaphore towers loosely based on the concept of the telegraph. Invented by an artificer named Robert Dearheart, the towers could send messages “at the speed of light” using standardized codes. Three of these codes are of particular import:

  • G: send the message on
  • N: do not log the message
  • U: turn the message around at the end of the line and send it back again

When Dearheart’s son John died due to an accident while working on a clacks tower, Dearheart inserted John’s name into the overhead of the clacks with a “GNU” in front of it as a way to memorialize his son forever (or for at least as long as the clacks are standing.)

Keeping the legacy of Sir Terry Pratchett alive forever. For as long as his name is still passed along the Clacks, Death can’t have him.


To use this package to keep the legacy of Terry Pratchett alive, simply pass your wai application to clacks gnuTerryPratchett before passing it to the run function:

import Network.Wai.Handler.Warp (run)
import Network.Wai.Middleware.Clacks (clacks, gnuTerryPratchett)

import MyLib.App (myApp)

main :: IO ()
main = run 8080 $ clacks gnuTerryPratchett myApp

You can use the Clacks type to build a custom configuration for the clacks function, allowing you to pass anything into the header:

import Data.List.NonEmpty (NonEmpty(..))
import Network.Wai (Middleware)
import Network.Wai.Middleware.Clacks (Clacks(..), clacks)

myClacks :: Middleware
myClacks = clacks $ Clacks $ "GNU Ada Lovelace" :| ["GNU Hoban Washburne", "GNU Shephard Book"]


You can build the project with stack:

stack build

For development, you can enable fast builds with file-watching, documentation-building, & test-running:

stack test --haddock --fast --file-watch --pedantic

To build & open the documentation, run

stack haddock --open wai-middleware-clacks






  • Add upper bounds to library dependencies


  • Initial release