Custom 404 with servant

(last edited 2020-11-29)

I'm rebuilding my website + blog using servant. It's mostly for educational purposes since there exist loads of great solutions for my needs.

Servant is meant to create API from datatypes. It leverage http content negotation mechanism to render a haskell type to the correct representation.

The servant doc is pretty good, but one thing I couldn't find is how to do a custom 404 page. The idea is to use Raw to plug a custom WAI application and put it as the last handler of the api. The tricky part is then to construct the response using only warp's primitives.

import Network.HTTP.Types (status404)
import Network.Wai (Application, responseLBS)
import Text.Blaze.Renderer.Utf8 (renderMarkup)

type WebsiteAPI =
  -- some servant endpoints
  :<|> Servant.Raw


websiteImplementation =
  -- handler implementation
  :<|> pure custom404


-- A Raw server as simply a Wai Application, which is a synonym for
--  Network.Wai.Request ->
--  (Network.Wai.Response -> IO ResponseReceived) ->
--  ResponseReceived
custom404 :: Application
custom404 _request sendResponse =
  sendResponse $
    responseLBS
      status404
      [("Content-Type", "text/html; charset=UTF-8")]
      (renderMarkup Errors.notFound)

Where Errors.notFound is a simple view of type Text.Blaze.Html5.Markup. For example:

notFound = docTypeHtml $ body $ h1 "not found"

This doesn't do correct content negotiation though. In my case it's not an issue. Otherwise a regular servant handler with the possible content type would be better, although I don't know how to do a default content-type.

You can find the actual source code behind this site (as of 2020-11-29) for the API type, the server implementation and the custom 404 handler.

Special thanks to Jonathan Moregård for pointing out some errors in a previous version.