Servant: a callback that runs on any request

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I use Servant. I need to set some callback function that will be executed on every request and it will forward the request processing/handling further (to other handler, as if this callback did not exist at all). I need it to change some IORef MyGlobalState in this callback. How to do it? Some example? Sure, I can call it explicitly in all handlers' bodies but maybe there is some right/canonical way to do it...

servant haskell
1 Answers

Consider this example server:

{-# LANGUAGE DataKinds #-}
{-# LANGUAGE TypeOperators #-}

import Servant
import Servant.API
import Network.Wai
import Network.Wai.Handler.Warp
import Data.IORef
import qualified Data.ByteString.Char8 as C

type API = "one" :> Get '[PlainText] String :<|> "two" :> Get '[PlainText] String

api :: Proxy API
api = Proxy

server :: Server API
server = return "1\n" :<|> return "2\n"

app :: Application
app = serve api server

main = run 3000 app

The app value is a WAI Application, defined by:

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

Conceptually, an Application accepts a Request, and invokes a callback on its Response. WAI supports the concept of Middleware which can wrap an application, allowing preprocessing of every request and post-processing of every response in the IO monad:

type Middleware = Application -> Application

So, you can write a piece of middleware to update an IORef on every received request like so:

counter :: IORef Int -> Middleware
counter cref = convert

  where
    convert :: Application -> Application  -- AKA Middleware
    convert oldapp = newapp

      where
        newapp :: Request -> (Response -> IO ResponseReceived) -> IO ResponseReceived  -- AKA Application
        newapp req respond = do
          n <- atomicModifyIORef cref (\n' -> (n'+1,n'+1))
          putStrLn $ "Request #" ++ show n ++ ": " ++ showRequest req
          oldapp req respond

        showRequest req = C.unpack (requestMethod req) ++ " " ++ C.unpack (rawPathInfo req)

I've broken this up into separate functions to make it clear how the middleware is constructed piece by piece, but the definition of counter can be simplified to the equivalent:

counter :: IORef Int -> Middleware
counter cref oldapp req respond
  = do n <- atomicModifyIORef cref (\n' -> (n'+1,n'+1))
       putStrLn $ "Request #" ++ show n ++ ": " ++ showRequest req
       oldapp req respond

  where
    showRequest req = C.unpack (requestMethod req) ++ " " ++ C.unpack (rawPathInfo req)

Now, all you need to do is wrap your app with the middleware in main:

main = do
  cref <- newIORef (0 :: Int)
  run 3000 $ counter cref app

Full code:

{-# LANGUAGE DataKinds #-}
{-# LANGUAGE TypeOperators #-}

import Servant
import Servant.API
import Network.Wai
import Network.Wai.Handler.Warp
import Data.IORef
import qualified Data.ByteString.Char8 as C

type API = "one" :> Get '[PlainText] String :<|> "two" :> Get '[PlainText] String

api :: Proxy API
api = Proxy

server :: Server API
server = return "1\n" :<|> return "2\n"

app :: Application
app = serve api server

counter :: IORef Int -> Middleware
counter cref oldapp req respond
  = do n <- atomicModifyIORef cref (\n' -> (n'+1,n'+1))
       putStrLn $ "Request #" ++ show n ++ ": " ++ showRequest req
       oldapp req respond

  where
    showRequest req = C.unpack (requestMethod req) ++ " " ++ C.unpack (rawPathInfo req)

main = do
  cref <- newIORef (0 :: Int)
  run 3000 $ counter cref app
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