# NamedRoutes - Using records to define APIs *Available in Servant 0.19 or higher* Servant offers a very natural way of constructing APIs with nested records, called `NamedRoutes`. This cookbook explains how to implement such nested-record-APIs using `NamedRoutes` through the example of a Movie Catalog. First, we start by constructing the domain types of our Movie Catalog. After, we show you how to implement the API type with the NamedRoutes records. Lastly, we make a Server and a Client out of the API type. However, it should be understood that this cookbook does _not_ dwell on the built-in servant combinators as the [ cookbook ]() already covers that angle. ## Why would I want to use `NamedRoutes` over the alternative `:<|>` operator? With `NamedRoutes`, we don’t need to care about the declaration order of the endpoints. For example, with the `:<|>` operator there’s room for error when the order of the API type ```haskell,ignore type API1 = "version" :> Get '[JSON] Version :<|> "movies" :> Get '[JSON] [Movie] ``` does not follow the `Handler` implementation order ```haskell,ignore apiHandler :: ServerT API1 Handler apiHandler = getMovies :<|> getVersion ``` GHC could scold you with a very tedious message such as : ```console • Couldn't match type 'Handler NoContent' with 'Movie -> Handler NoContent' Expected type: ServerT MovieCatalogAPI Handler Actual type: Handler Version :<|> ((Maybe SortBy -> Handler [Movie]) :<|> ((MovieId -> Handler (Maybe Movie)) :<|> ((MovieId -> Movie -> Handler NoContent) :<|> (MovieId -> Handler NoContent)))) • In the expression: versionHandler :<|> movieListHandler :<|> getMovieHandler :<|> updateMovieHandler :<|> deleteMovieHandler In an equation for 'server': server = versionHandler :<|> movieListHandler :<|> getMovieHandler :<|> updateMovieHandler :<|> deleteMovieHandler | 226 | server = versionHandler ``` On the contrary, with the `NamedRoutes` technique, we refer to the routes by their name: ```haskell,ignore data API mode = API { list :: "list" :> ... , delete :: "delete" :> ... } ``` and GHC follows the lead : ```console • Couldn't match type 'NoContent' with 'Movie' Expected type: AsServerT Handler :- Delete '[JSON] Movie Actual type: Handler NoContent • In the 'delete' field of a record In the expression: MovieAPI {get = getMovieHandler movieId, update = updateMovieHandler movieId, delete = deleteMovieHandler movieId} In an equation for 'movieHandler': movieHandler movieId = MovieAPI {get = getMovieHandler movieId, update = updateMovieHandler movieId, delete = deleteMovieHandler movieId} | 252 | , delete = deleteMovieHandler movieId ``` So, NamedRoutes is more readable for a human, and GHC gives you more accurate error messages. What are we waiting for? ## Boilerplate time! First, let’s get rid of the the extensions and imports boilerplate in order to focus on our new technique: ```haskell {-# LANGUAGE DataKinds #-} {-# LANGUAGE DeriveAnyClass #-} {-# LANGUAGE DerivingStrategies #-} {-# LANGUAGE DeriveGeneric #-} {-# LANGUAGE OverloadedStrings #-} {-# LANGUAGE TypeOperators #-} import GHC.Generics ( Generic ) import Data.Aeson ( FromJSON, ToJSON ) import Data.Proxy ( Proxy(..) ) import Network.Wai.Handler.Warp ( run ) import Servant ( NamedRoutes , Handler, serve ) import Servant.API (Capture, Delete, Get, Put, QueryParam, ReqBody , JSON, NoContent (..) , FromHttpApiData (..),ToHttpApiData(..) , (:>) ) import Servant.API.Generic ( (:-) ) import Servant.Client ( AsClientT, ClientM, client , (//), (/:) ) import Servant.Client.Generic () import Servant.Server ( Application, ServerT ) import Servant.Server.Generic ( AsServerT ) ``` ## Domain context Now that we’ve handled the boilerplate, we can dive into our Movie Catalog domain. Consider a `Movie` constructed from a `Title` and a `Year` of publication. ``` haskell data Movie = Movie { movieId :: MovieId , title :: Title , year :: Year } deriving stock Generic deriving anyclass (FromJSON, ToJSON) type MovieId = String type Title = String type Year = Int ``` Let’s forget about the deriving stuff for now and think about the API that we want to make. ``` "version" -> Get Version / api "list" -> Get [Movie] ?sortBy= Title | Year (sort by the Title or the Year) \ / "movies" Get Movie \ / Capture MovieId - Put Movie \ Delete MovieId ``` In this example, we create a very simple endpoint for the Version, and several complex endpoints that use nested records for the CRUD part of the movie. So, the URLs would look like - GET …/version - GET …/movies/list?sortby=Title - GET …/movies// - PUT …/movies// - DELETE …/movies/ ### API Type Now that we have a very clear idea of the API we want to make, we need to transform it into usable Haskell code: ``` haskell data API mode = API { version :: mode :- "version" :> Get '[JSON] Version , movies :: mode :- "movies" :> NamedRoutes MoviesAPI } deriving stock Generic type Version = String -- This will do for the sake of example. ``` Here, we see the first node of our tree. It contains the two branches “version” and “movies” respectively: The “version” branch is very simple and self-explanatory. The “movies” branch will contain another node, represented by another record (see above). That is why we need the `NameRoutes` helper. Note: The `mode` type parameter indicates into which implementation the record’s `Generic` representation will be transformed—as a client or as a server. We will discuss that later. Let's jump into the "movies" subtree node: ``` haskell data MoviesAPI mode = MoviesAPI { list :: mode :- "list" :> QueryParam "SortBy" SortBy :> Get '[JSON] [Movie] , movie :: mode :- Capture "movieId" MovieId :> NamedRoutes MovieAPI } deriving stock Generic data SortBy = Year | Title instance ToHttpApiData SortBy where toQueryParam Year = "year" toQueryParam Title = "title" instance FromHttpApiData SortBy where parseQueryParam "year" = Right Year parseQueryParam "title" = Right Title parseQueryParam param = Left $ param <> " is not a valid value" ``` So, remember, this type represents the `MoviesAPI` node that we’ve connected earlier to the main `API` tree. In this subtree, we illustrated both an endpoint with a **query param** and also, a **capture** with a subtree underneath it. So, let's go deeper into our API tree. ``` haskell data MovieAPI mode = MovieAPI { get :: mode :- Get '[JSON] (Maybe Movie) , update :: mode :- ReqBody '[JSON] Movie :> Put '[JSON] NoContent , delete :: mode :- Delete '[JSON] NoContent } deriving stock Generic ``` As you can see, we end up implementing the deepest routes of our API. Small detail: as our main API tree is also a record, we need the `NamedRoutes` helper. To improve readability, we suggest you create a type alias: ``` haskell type MovieCatalogAPI = NamedRoutes API ``` That's it, we have our `MovieCatalogAPI` type! Let's make a server and a client out of it! ### The Server As you know, we can’t talk about a server, without addressing the handlers. First, we take our handlers… ```haskell versionHandler :: Handler Version versionHandler = pure "0.0.1" movieListHandler :: Maybe SortBy -> Handler [Movie] movieListHandler _ = pure moviesDB moviesDB :: [Movie] moviesDB = [ Movie "1" "Se7en" 1995 , Movie "2" "Minority Report" 2002 , Movie "3" "The Godfather" 1972 ] getMovieHandler :: MovieId -> Handler (Maybe Movie) getMovieHandler requestMovieId = go moviesDB where go [] = pure Nothing go (movie:ms) | movieId movie == requestMovieId = pure $ Just movie go (m:ms) = go ms updateMovieHandler :: MovieId -> Movie -> Handler NoContent updateMovieHandler requestedMovieId newMovie = -- update the movie list in the database... pure NoContent deleteMovieHandler :: MovieId -> Handler NoContent deleteMovieHandler _ = -- delete the movie from the database... pure NoContent ``` And assemble them together with the record structure, which is the glue here. ```haskell server :: ServerT MovieCatalogAPI Handler server = API { version = versionHandler , movies = moviesHandler } moviesHandler :: MoviesAPI (AsServerT Handler) moviesHandler = MoviesAPI { list = movieListHandler , movie = movieHandler } movieHandler :: MovieId -> MovieAPI (AsServerT Handler) movieHandler movieId = MovieAPI { get = getMovieHandler movieId , update = updateMovieHandler movieId , delete = deleteMovieHandler movieId } ``` As you might have noticed, we build our handlers out of the same record types we used to define our API: `MoviesAPI` and `MovieAPI`. What kind of magic is this ? Remember the `mode` type parameter we saw earlier? Since we need to transform our API type into a _server_, we need to provide a server `mode`, which is `AsServerT Handler` here. Finally, we can run the server and connect the API routes to the handlers as usual: ``` haskell api :: Proxy MovieCatalogAPI api = Proxy main :: IO () main = run 8081 app app :: Application app = serve api server ``` Yay! That’s done and we’ve got our server! ## The Client The client, so to speak, is very easy to implement: ``` haskell movieCatalogClient :: API (AsClientT ClientM) movieCatalogClient = client api -- remember: api: Proxy MovieCatalogAPI ``` Have you noticed the `mode` `AsClient ClientM`? We’ve also introduced some operators that help navigate through the nested records. `(//)` is used to jump from one record to another. `(/:)` is used to provide a parameter, whether it be a query param or a capture. Let’s use those nice helpers for our movie catalog: ```haskell listMovies :: Maybe SortBy -> ClientM [Movie] listMovies sortBy = movieCatalogClient // movies // list /: sortBy getMovie :: MovieId -> ClientM (Maybe Movie) getMovie movieId = movieCatalogClient // movies // movie /: movieId // get updateMovie :: MovieId -> Movie -> ClientM NoContent updateMovie movieId newMovie = movieCatalogClient // movies // movie /: movieId // update /: newMovie deleteMovie :: MovieId -> ClientM NoContent deleteMovie movieId = movieCatalogClient // movies // movie /: movieId // delete ``` Done! We’ve got our client! ## Conclusion We hope that you found this workbook helpful, and that you now feel more confident using the `NamedRoutes` technique. If you are interested in further understanding the built-in Servant combinators, see [Structuring APIs](https://docs.servant.dev/en/stable/cookbook/structuring-apis/StructuringApis.html). Since `NamedRoutes` is based on the Generic mechanism, you might want to have a look at [Sandy Maguire’s _Thinking with Types_ book](https://doku.pub/download/sandy-maguire-thinking-with-typesz-liborgpdf-4lo5ne7kdj0x).