Generate reflex client functions for querying a servant API
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servant-reflex lets you share your servant APIs with the frontend

Keeping your frontend in sync with your API server can be difficult - when the API changes its input parameters or return type, XHR requests from the frontend will fail at runtime. If your API is defined by servant combinators, you can use servant-reflex to share the API between the server and frontend. Syncronization between is checked at compile time, and rather than building XHR requests by hand, API endpoints are available behind reflex's FRP semantics.


We have a webservice API defined in a module where both the server (compiled with ghc) and the frontend (compiled with ghcjs) can see it:

type API = "getint"  :> Get '[JSON] Int
      :<|> "sayhi"   :> QueryParam  "username" Text
                     :> QueryParams "greetings" Text
                     :> QueryFlag   "gusto"
                     :> Get '[JSON] Text
      :<|> "double" :> ReqBody '[JSON] Double
                    :> Post '[JSON] Double
      :<|> Raw

servant-reflex then computes client functions that can query the API through an XhrRequest.

 runGUI :: forall t m.MonadWidget t m => do

  -- servant-reflex computes FRP functions for each API endpoint
  let (getint :<|> sayhi :<|> doubleit :<|> _) = client (Proxy :: Proxy API)
                                                        (Proxy :: Proxy m)
                                                        (Proxy :: Proxy ())
                                                        (constDyn (BasePath "/"))

These client functions are computed from your API type. They manage serialization, XhrRequest generation, and deserialization for you. a parameters used in URL captures become Dynamic t (Either Text a) parameters in the client functions. QueryFlag, QueryParams and QueryParam API parameters map to Dynamic t Bool, Dynamic t [a] and Dynamic t (QParam a) respectively. These parameters to the client function are wrapped with failure possibility to allow you to indicate at any time whether input validation for that parameter has failed and no valid XHR request can be generated. The final parameter is a trigger event for the XHR request. The return value Event t (ReqResult a) contains responses from the API server.

   -- No need to write these functions. servant-reflex creates them for you!
   getint :: MonadWidget t m
          => Event t ()  -- ^ Trigger the XHR Request
          -> m (Event t (ReqResult () Int)) -- ^ Consume the answer

   sayhi :: MonadWidget t m
         => Dynamic t (QParam Text) 
            -- ^ One input parameter - the 'name', wrapped in 'QParam'
         -> Dynamic t [Text]
            -- ^ Another input: list of preferred greetings
         -> Dynamic t Bool
            -- ^ Flag for capitalizing the response
         -> Event t ()
            -- ^ Trigger the XHR Request
         -> m (Event t (ReqResult () Text))

   doubleit :: MonadWidget t m
            => Dynamic t (Either Text Double)
            -> Event t ()
            -> m (Event t (ReqResult () Double))

ReqResult tag a is defined in Servant.Common.Req and reports whether or not your request was sent (if validation fails, the request won't be sent), and how decoding of the response went. You can pattern match on these explicitly, but usually you'll want to use fmapMaybe :: (a -> Maybe b) -> Event t a -> Event t b and one of the elimination functions to filter the result type you care about, like this:

  -- ... continued ...
  res :: Event t (ReqResult Double) <- doubleIt xs triggers
  let ys   = fmapMaybe reqSuccess res
      errs = fmapMaybe reqFailure res
  -- Green <p> tag showing the last good result 
  elAttr "p" ("style" =: "color:green") $ do
    text "Last good result: "
    dynText =<< holdDyn "" (fmap show ys)
  -- Red <p> tag showing the last error, cleared by a new good value
  elAttr "p" ("style" =: "color:red") $
    dynText =<< holdDyn "" (leftmost [errs, const "" <$> ys])

This example builds some input fields to enter API parameters, buttons to trigger the API calls, and text elements to show the results:

  elClass "div" "int-demo" $ do
    intButton  <- button "Get Int"
    serverInts <- fmapMaybe resSuccess <$> getint intButton
    display =<< holdDyn (Just 0) serverInts

  elClass "div" "hello-demo" $ do
    nameText <- QParamSome . value <$> textInput def
    greetings <- (fmap words . value) <$> textInput def
    withGusto <- checkbox def
    helloButton <- button "Say hi"
    hellos <- fmapMaybe resResult <$> sayhi nameText greetings withGusto helloButton
    display =<< holdDyn Nothing hellos

  elClass "div" "demo-double" $ do
    inputDouble  <- (fmapMaybe readMaybe) <$> textInput def
    doubleButton <- button "Double it"
    outputDouble <- fmapMaybe resSuccess <$> doubleit inputDouble doubleButton
    display =<< holdDyn Nothing outputDouble

For a great introduction to recative DOM building, see the README for the reflex-platform. For more information about servant, see their documentation. Thanks to the respective authors of these fabulous libraries.

Building the library and test server

This repository comes with a small example of an API shared between a ghcjs-compiled frontend (exec/) and a ghc-compiled backend (testserver/. To build these components, we will use the nix packages for the testserver. It manages the frontend build as a dependency.

nix build -f travis.nix testserver -o server

Point it at the static directory and run it:

STATIC_DIR=server/static server/back -p 8000

Browse to http://localhost:8000 and click around. Open the firefox or chrome developer tools and examine the XHR activity under the Network tab.

For a larger example of a project that shares types between backend and frontend, see hsnippet.

Tagging requests

The input and the return type of a client function like getDouble are both event streams. The individual input events and responses occur at different times and aren't automatically paired up, but you can recover the relationship by tagging the requests.

So far we have used an Event t () to trigger sending a request. If we choose e.g. Double for the third Proxy argument to client, then Event t Double will be used to trigger requests, and each ReqResult will carry the tag of its request. Imagine we wanted to display not just the last "double" from doubleIt, but a whole table of valid inputs and their doubled responses:

  inp <- textInput def

  -- Convert the raw text into an `Either Text Double`
  let inpNum = maybe (Left "No Parse") Right . readMaybe . T.unpack <$> value inp
  go  <- button "Double It"

  -- Call 'doubleIt' with triggers that coincide with good input parses
  rs  <- doubleIt inpNum (fforMaybe (tagPromptlyDyn inpNum go) $ \case
                                 Left  _ -> Nothing
                                 Right a -> Just a

  -- Accumulate good responses in a map
  doubleTable <- foldDyn (<>) mempty $ ffor rs $ \case
    ResponseSuccess tag v _ -> tag =: v
    _                       -> mempty

  el "table" $ do
    listWithKey doubleTable $ \k dv -> el "tr" $ do
      el "td" $ text (T.pack $ show k)
      el "td" $ dynText (T.pack . show <$> dv)

Simultaneous requests

Servant.Reflex.Multi provides an alternative client-generation function called clientA (client applicative). Choose a container type that has both Applicative and Traversable instances, and pass it to clientA through another Proxy. Our sayHi client function will then have this type:

  :: Dynamic t (f (QParam Text))
  -> Dynamic t (f [Text])
  -> Dynamic t (f Bool)
  -> Event t tag
  -> m (Event t (f (ReqResult tag Text)))

The dynamic params are each wrapped in f. For every firing of the trigger event tag, all of these parameters will be combined according to f's Applicative instance (when f is [], you will get all combinations of all parameters taken together as a request; when f is ZipList, the Nth elemens of each parameters list will be taken together as a request). Using this interface, you can trigger many XHR's from a single event occurence, and expect the responses to be structured the same way as the requests.