# Serving an API Enough chit-chat about type-level combinators and representing an API as a type. Can we have a webservice already? ## A first example Equipped with some basic knowledge about the way we represent APIs, let's now write our first webservice. The source for this tutorial section is a literate haskell file, so first we need to have some language extensions and imports: ``` haskell {-# LANGUAGE DataKinds #-} {-# LANGUAGE DeriveGeneric #-} {-# LANGUAGE FlexibleInstances #-} {-# LANGUAGE GeneralizedNewtypeDeriving #-} {-# LANGUAGE MultiParamTypeClasses #-} {-# LANGUAGE OverloadedStrings #-} {-# LANGUAGE RankNTypes #-} {-# LANGUAGE ScopedTypeVariables #-} {-# LANGUAGE TypeOperators #-} module Server where import Prelude () import Prelude.Compat import Control.Monad.Except import Control.Monad.Reader import Data.Aeson import Data.Aeson.Types import Data.Attoparsec.ByteString import Data.ByteString (ByteString) import Data.List import Data.Maybe import Data.String.Conversions import Data.Time.Calendar import GHC.Generics import Lucid import Network.HTTP.Media ((//), (/:)) import Network.Wai import Network.Wai.Handler.Warp import Servant import System.Directory import Text.Blaze import Text.Blaze.Html.Renderer.Utf8 import Servant.Types.SourceT (source) import qualified Data.Aeson.Parser import qualified Text.Blaze.Html ``` **Important**: the `Servant` module comes from the **servant-server** package, the one that lets us run webservers that implement a particular API type. It reexports all the types from the **servant** package that let you declare API types as well as everything you need to turn your request handlers into a fully-fledged webserver. This means that in your applications, you can just add **servant-server** as a dependency, import `Servant` and not worry about anything else. We will write a server that will serve the following API. ``` haskell type UserAPI1 = "users" :> Get '[JSON] [User] ``` Here's what we would like to see when making a GET request to `/users`. ``` javascript [ {"name": "Isaac Newton", "age": 372, "email": "isaac@newton.co.uk", "registration_date": "1683-03-01"} , {"name": "Albert Einstein", "age": 136, "email": "ae@mc2.org", "registration_date": "1905-12-01"} ] ``` Now let's define our `User` data type and write some instances for it. ``` haskell data User = User { name :: String , age :: Int , email :: String , registration_date :: Day } deriving (Eq, Show, Generic) instance ToJSON User ``` Nothing funny going on here. But we now can define our list of two users. ``` haskell users1 :: [User] users1 = [ User "Isaac Newton" 372 "isaac@newton.co.uk" (fromGregorian 1683 3 1) , User "Albert Einstein" 136 "ae@mc2.org" (fromGregorian 1905 12 1) ] ``` We can now take care of writing the actual webservice that will handle requests to such an API. This one will be very simple, being reduced to just a single endpoint. The type of the web application is determined by the API type, through a *type family* named `Server`. (Type families are just functions that take types as input and return types.) The `Server` type family will compute the right type that a bunch of request handlers should have just from the corresponding API type. The first thing to know about the `Server` type family is that behind the scenes it will drive the routing, letting you focus only on the business logic. The second thing to know is that for each endpoint, your handlers will by default run in the `Handler` monad. This is overridable very easily, as explained near the end of this guide. Third thing, the type of the value returned in that monad must be the same as the second argument of the HTTP method combinator used for the corresponding endpoint. In our case, it means we must provide a handler of type `Handler [User]`. Well, we have a monad, let's just `return` our list: ``` haskell server1 :: Server UserAPI1 server1 = return users1 ``` That's it. Now we can turn `server` into an actual webserver using [wai](http://hackage.haskell.org/package/wai) and [warp](http://hackage.haskell.org/package/warp): ``` haskell userAPI :: Proxy UserAPI1 userAPI = Proxy -- 'serve' comes from servant and hands you a WAI Application, -- which you can think of as an "abstract" web application, -- not yet a webserver. app1 :: Application app1 = serve userAPI server1 ``` The `userAPI` bit is, alas, boilerplate (we need it to guide type inference). But that's about as much boilerplate as you get. And we're done! Let's run our webservice on the port 8081. ``` haskell main :: IO () main = run 8081 app1 ``` You can put this all into a file or just grab [servant's repo](http://github.com/haskell-servant/servant) and look at the *doc/tutorial* directory. This code (the source of this web page) is in *doc/tutorial/Server.lhs*. If you run it, you can go to `http://localhost:8081/users` in your browser or query it with curl and you see: ``` bash $ curl http://localhost:8081/users [{"email":"isaac@newton.co.uk","registration_date":"1683-03-01","age":372,"name":"Isaac Newton"},{"email":"ae@mc2.org","registration_date":"1905-12-01","age":136,"name":"Albert Einstein"}] ``` ## More endpoints What if we want more than one endpoint? Let's add `/albert` and `/isaac` to view the corresponding users encoded in JSON. ``` haskell type UserAPI2 = "users" :> Get '[JSON] [User] :<|> "albert" :> Get '[JSON] User :<|> "isaac" :> Get '[JSON] User ``` And let's adapt our code a bit. ``` haskell isaac :: User isaac = User "Isaac Newton" 372 "isaac@newton.co.uk" (fromGregorian 1683 3 1) albert :: User albert = User "Albert Einstein" 136 "ae@mc2.org" (fromGregorian 1905 12 1) users2 :: [User] users2 = [isaac, albert] ``` Now, just like we separate the various endpoints in `UserAPI` with `:<|>`, we are going to separate the handlers with `:<|>` too! They must be provided in the same order as in the API type. ``` haskell server2 :: Server UserAPI2 server2 = return users2 :<|> return albert :<|> return isaac ``` And that's it! You can run this example in the same way that we showed for `server1` and check out the data available at `/users`, `/albert` and `/isaac`. ## From combinators to handler arguments Fine, we can write trivial webservices easily, but none of the two above use any "fancy" combinator from servant. Let's address this and use `QueryParam`, `Capture` and `ReqBody` right away. You'll see how each occurrence of these combinators in an endpoint makes the corresponding handler receive an argument of the appropriate type automatically. You don't have to worry about manually looking up URL captures or query string parameters, or decoding/encoding data from/to JSON. Never. We are going to use the following data types and functions to implement a server for `API`. ``` haskell type API = "position" :> Capture "x" Int :> Capture "y" Int :> Get '[JSON] Position :<|> "hello" :> QueryParam "name" String :> Get '[JSON] HelloMessage :<|> "marketing" :> ReqBody '[JSON] ClientInfo :> Post '[JSON] Email data Position = Position { xCoord :: Int , yCoord :: Int } deriving Generic instance ToJSON Position newtype HelloMessage = HelloMessage { msg :: String } deriving Generic instance ToJSON HelloMessage data ClientInfo = ClientInfo { clientName :: String , clientEmail :: String , clientAge :: Int , clientInterestedIn :: [String] } deriving Generic instance FromJSON ClientInfo instance ToJSON ClientInfo data Email = Email { from :: String , to :: String , subject :: String , body :: String } deriving Generic instance ToJSON Email emailForClient :: ClientInfo -> Email emailForClient c = Email from' to' subject' body' where from' = "great@company.com" to' = clientEmail c subject' = "Hey " ++ clientName c ++ ", we miss you!" body' = "Hi " ++ clientName c ++ ",\n\n" ++ "Since you've recently turned " ++ show (clientAge c) ++ ", have you checked out our latest " ++ intercalate ", " (clientInterestedIn c) ++ " products? Give us a visit!" ``` We can implement handlers for the three endpoints: ``` haskell server3 :: Server API server3 = position :<|> hello :<|> marketing where position :: Int -> Int -> Handler Position position x y = return (Position x y) hello :: Maybe String -> Handler HelloMessage hello mname = return . HelloMessage $ case mname of Nothing -> "Hello, anonymous coward" Just n -> "Hello, " ++ n marketing :: ClientInfo -> Handler Email marketing clientinfo = return (emailForClient clientinfo) ``` Did you see that? The types for your handlers changed to be just what we needed! In particular: - a `Capture "something" a` becomes an argument of type `a` (for `position`); - a `QueryParam "something" a` becomes an argument of type `Maybe a` (because an endpoint can technically be accessed without specifying any query string parameter, we decided to "force" handlers to be aware that the parameter might not always be there); - a `ReqBody contentTypeList a` becomes an argument of type `a`; And that's it. Here's the example in action: ``` bash $ curl http://localhost:8081/position/1/2 {"xCoord":1,"yCoord":2} $ curl http://localhost:8081/hello {"msg":"Hello, anonymous coward"} $ curl http://localhost:8081/hello?name=Alp {"msg":"Hello, Alp"} $ curl -X POST -d '{"clientName":"Alp Mestanogullari", "clientEmail" : "alp@foo.com", "clientAge": 25, "clientInterestedIn": ["haskell", "mathematics"]}' -H 'Accept: application/json' -H 'Content-type: application/json' http://localhost:8081/marketing {"subject":"Hey Alp Mestanogullari, we miss you!","body":"Hi Alp Mestanogullari,\n\nSince you've recently turned 25, have you checked out our latest haskell, mathematics products? Give us a visit!","to":"alp@foo.com","from":"great@company.com"} ``` For reference, here's a list of some combinators from **servant**: > - `Delete`, `Get`, `Patch`, `Post`, `Put`: these do not become arguments. They provide the return type of handlers, which usually is `Handler `. > - `Capture "something" a` becomes an argument of type `a`. > - `QueryParam "something" a`, `Header "something" a` all become arguments of type `Maybe a`, because there might be no value at all specified by the client for these. > - `QueryFlag "something"` gets turned into an argument of type `Bool`. > - `QueryParams "something" a` gets turned into an argument of type `[a]`. > - `ReqBody contentTypes a` gets turned into an argument of type `a`. ## The `FromHttpApiData`/`ToHttpApiData` classes Wait... How does **servant** know how to decode the `Int`s from the URL? Or how to decode a `ClientInfo` value from the request body? The following three sections will help us answer these questions. `Capture`s and `QueryParam`s are represented by some textual value in URLs. `Header`s are similarly represented by a pair of a header name and a corresponding (textual) value in the request's "metadata". How types are decoded from headers, captures, and query params is expressed in a class `FromHttpApiData` (from the package [**http-api-data**](http://hackage.haskell.org/package/http-api-data)): ``` haskell ignore class FromHttpApiData a where {-# MINIMAL parseUrlPiece | parseQueryParam #-} -- | Parse URL path piece. parseUrlPiece :: Text -> Either Text a parseUrlPiece = parseQueryParam -- | Parse HTTP header value. parseHeader :: ByteString -> Either Text a parseHeader = parseUrlPiece . decodeUtf8 -- | Parse query param value. parseQueryParam :: Text -> Either Text a parseQueryParam = parseUrlPiece ``` As you can see, as long as you provide either `parseUrlPiece` (for `Capture`s) or `parseQueryParam` (for `QueryParam`s), the other methods will be defined in terms of this. **http-api-data** provides a decent number of instances, helpers for defining new ones, and wonderful documentation. There's not much else to say about these classes. You will need instances for them when using `Capture`, `QueryParam`, `QueryParams`, and `Header` with your types. You will need `FromHttpApiData` instances for server-side request handlers and `ToHttpApiData` instances only when using **servant-client**, as described in the [section about deriving haskell functions to query an API](Client.html). ## Using content-types with your data types The same principle was operating when decoding request bodies from JSON, and responses *into* JSON. (JSON is just the running example - you can do this with any content-type.) This section introduces a couple of typeclasses provided by **servant** that make all of this work. ### The truth behind `JSON` What exactly is `JSON` (the type as used in `Get '[JSON] User`)? Like the 3 other content-types provided out of the box by **servant**, it's a really dumb data type. ``` haskell ignore data JSON data PlainText data FormUrlEncoded data OctetStream ``` Obviously, this is not all there is to `JSON`, otherwise it would be quite pointless. Like most of the data types in **servant**, `JSON` is mostly there as a special *symbol* that's associated with encoding (resp. decoding) to (resp. from) the *JSON* format. The way this association is performed can be decomposed into two steps. The first step is to provide a proper `MediaType` (from [**http-media**](https://hackage.haskell.org/package/http-media-0.6.2/docs/Network-HTTP-Media.html)) representation for `JSON`, or for your own content-types. If you look at the haddocks from this link, you can see that we just have to specify `application/json` using the appropriate functions. In our case, we can just use `(//) :: ByteString -> ByteString -> MediaType`. The precise way to specify the `MediaType` is to write an instance for the `Accept` class: ``` haskell ignore -- for reference: class Accept ctype where contentType :: Proxy ctype -> MediaType instance Accept JSON where contentType _ = "application" // "json" ``` The second step is centered around the `MimeRender` and `MimeUnrender` classes. These classes just let you specify a way to encode and decode values into or from your content-type's representation. ``` haskell ignore class Accept ctype => MimeRender ctype a where mimeRender :: Proxy ctype -> a -> ByteString -- alternatively readable as: mimeRender :: Proxy ctype -> (a -> ByteString) ``` Given a content-type and some user type, `MimeRender` provides a function that encodes values of type `a` to lazy `ByteString`s. In the case of `JSON`, this is easily dealt with! For any type `a` with a `ToJSON` instance, we can render values of that type to JSON using `Data.Aeson.encode`. ``` haskell ignore instance ToJSON a => MimeRender JSON a where mimeRender _ = encode ``` And now the `MimeUnrender` class, which lets us extract values from lazy `ByteString`s, alternatively failing with an error string. ``` haskell ignore class Accept ctype => MimeUnrender ctype a where mimeUnrender :: Proxy ctype -> ByteString -> Either String a ``` We don't have much work to do there either, `Data.Aeson.eitherDecode` is precisely what we need. However, it only allows arrays and objects as toplevel JSON values and this has proven to get in our way more than help us so we wrote our own little function around **aeson** and **attoparsec** that allows any type of JSON value at the toplevel of a "JSON document". Here's the definition in case you are curious. ``` haskell eitherDecodeLenient :: FromJSON a => ByteString -> Either String a eitherDecodeLenient input = do v :: Value <- parseOnly (Data.Aeson.Parser.value <* endOfInput) (cs input) parseEither parseJSON v ``` This function is exactly what we need for our `MimeUnrender` instance. ``` haskell ignore instance FromJSON a => MimeUnrender JSON a where mimeUnrender _ = eitherDecodeLenient ``` And this is all the code that lets you use `JSON` with `ReqBody`, `Get`, `Post` and friends. We can check our understanding by implementing support for an `HTML` content-type, so that users of your webservice can access an HTML representation of the data they want, ready to be included in any HTML document, e.g. using [jQuery's `load` function](https://api.jquery.com/load/), simply by adding `Accept: text/html` to their request headers. ### Case-studies: **servant-blaze** and **servant-lucid** These days, most of the haskellers who write their HTML UIs directly from Haskell use either [**blaze-html**](http://hackage.haskell.org/package/blaze-html) or [**lucid**](http://hackage.haskell.org/package/lucid). The best option for **servant** is obviously to support both (and hopefully other templating solutions!). We're first going to look at **lucid**: ``` haskell data HTMLLucid ``` Once again, the data type is just there as a symbol for the encoding/decoding functions, except that this time we will only worry about encoding since **lucid** doesn't provide a way to extract data from HTML. ``` haskell instance Accept HTMLLucid where contentType _ = "text" // "html" /: ("charset", "utf-8") ``` Note that this instance uses the `(/:)` operator from **http-media** which lets us specify additional information about a content-type, like the charset here. The rendering instances call similar functions that take types with an appropriate instance to an "abstract" HTML representation and then write that to a `ByteString`. ``` haskell instance ToHtml a => MimeRender HTMLLucid a where mimeRender _ = renderBS . toHtml -- let's also provide an instance for lucid's -- 'Html' wrapper. instance MimeRender HTMLLucid (Html a) where mimeRender _ = renderBS ``` For **blaze-html** everything works very similarly: ``` haskell -- For this tutorial to compile 'HTMLLucid' and 'HTMLBlaze' have to be -- distinct. Usually you would stick to one html rendering library and then -- you can go with one 'HTML' type. data HTMLBlaze instance Accept HTMLBlaze where contentType _ = "text" // "html" /: ("charset", "utf-8") instance ToMarkup a => MimeRender HTMLBlaze a where mimeRender _ = renderHtml . Text.Blaze.Html.toHtml -- while we're at it, just like for lucid we can -- provide an instance for rendering blaze's 'Html' type instance MimeRender HTMLBlaze Text.Blaze.Html.Html where mimeRender _ = renderHtml ``` Both [**servant-blaze**](http://hackage.haskell.org/package/servant-blaze) and [**servant-lucid**](http://hackage.haskell.org/package/servant-lucid) let you use `HTMLLucid` and `HTMLBlaze` in any content-type list as long as you provide an instance of the appropriate class (`ToMarkup` for **blaze-html**, `ToHtml` for **lucid**). We can now write a webservice that uses **servant-lucid** to show the `HTMLLucid` content-type in action. We will be serving the following API: ``` haskell type PersonAPI = "persons" :> Get '[JSON, HTMLLucid] [Person] ``` where `Person` is defined as follows: ``` haskell data Person = Person { firstName :: String , lastName :: String } deriving Generic -- for the JSON instance instance ToJSON Person ``` Now, let's teach **lucid** how to render a `Person` as a row in a table, and then a list of `Person`s as a table with a row per person. ``` haskell -- HTML serialization of a single person instance ToHtml Person where toHtml person = tr_ $ do td_ (toHtml $ firstName person) td_ (toHtml $ lastName person) -- do not worry too much about this toHtmlRaw = toHtml -- HTML serialization of a list of persons instance ToHtml [Person] where toHtml persons = table_ $ do tr_ $ do th_ "first name" th_ "last name" -- this just calls toHtml on each person of the list -- and concatenates the resulting pieces of HTML together foldMap toHtml persons toHtmlRaw = toHtml ``` We create some `Person` values and serve them as a list: ``` haskell people :: [Person] people = [ Person "Isaac" "Newton" , Person "Albert" "Einstein" ] personAPI :: Proxy PersonAPI personAPI = Proxy server4 :: Server PersonAPI server4 = return people app2 :: Application app2 = serve personAPI server4 ``` And we're good to go: ``` bash $ curl http://localhost:8081/persons [{"lastName":"Newton","firstName":"Isaac"},{"lastName":"Einstein","firstName":"Albert"}] $ curl -H 'Accept: text/html' http://localhost:8081/persons
first namelast name
IsaacNewton
AlbertEinstein
# or just point your browser to http://localhost:8081/persons ``` ## The `Handler` monad At the heart of the handlers is the monad they run in, namely a newtype `Handler` around `ExceptT ServerError IO` ([haddock documentation for `ExceptT`](http://hackage.haskell.org/package/mtl-2.2.1/docs/Control-Monad-Except.html#t:ExceptT)). One might wonder: why this monad? The answer is that it is the simplest monad with the following properties: - it lets us both return a successful result (using `return`) or "fail" with a descriptive error (using `throwError`); - it lets us perform IO, which is absolutely vital since most webservices exist as interfaces to databases that we interact with in `IO`. Let's recall some definitions. ``` haskell ignore -- from the 'mtl' package at newtype ExceptT e m a = ExceptT (m (Either e a)) ``` In short, this means that a handler of type `Handler a` is simply equivalent to a computation of type `IO (Either ServerError a)`, that is, an IO action that either returns an error or a result. The module [`Control.Monad.Except`](https://hackage.haskell.org/package/mtl/docs/Control-Monad-Except.html#t:ExceptT) from which `ExceptT` comes is worth looking at. Perhaps most importantly, `ExceptT` and `Handler` are instances of `MonadError`, so `throwError` can be used to return an error from your handler (whereas `return` is enough to return a success). Most of what you'll be doing in your handlers is running some IO and, depending on the result, you might sometimes want to throw an error of some kind and abort early. The next two sections cover how to do just that. ### Performing IO Other important instances from the list above are `MonadIO m => MonadIO (ExceptT e m)`, and therefore also `MonadIO Handler` as there is a `MonadIO IO` instance. [`MonadIO`](http://hackage.haskell.org/package/base/docs/Control-Monad-IO-Class.html#t:MonadIO) is a class from the **transformers** package defined as: ``` haskell ignore class Monad m => MonadIO m where liftIO :: IO a -> m a ``` So if you want to run any kind of IO computation in your handlers, just use `liftIO`: ``` haskell type IOAPI1 = "myfile.txt" :> Get '[JSON] FileContent newtype FileContent = FileContent { content :: String } deriving Generic instance ToJSON FileContent server5 :: Server IOAPI1 server5 = do filecontent <- liftIO (readFile "myfile.txt") return (FileContent filecontent) ``` ### Failing, through `ServerError` If you want to explicitly fail at providing the result promised by an endpoint using the appropriate HTTP status code (not found, unauthorized, etc) and some error message, all you have to do is use the `throwError` function mentioned above and provide it with the appropriate value of type `ServerError`, which is defined as: ``` haskell ignore data ServerError = ServerError { errHTTPCode :: Int , errReasonPhrase :: String , errBody :: ByteString -- lazy bytestring , errHeaders :: [Header] } ``` Many standard values are provided out of the box by the `Servant.Server` module. If you want to use these values but add a body or some headers, just use record update syntax: ``` haskell failingHandler :: Handler () failingHandler = throwError myerr where myerr :: ServerError myerr = err503 { errBody = "Sorry dear user." } ``` Here's an example where we return a customised 404-Not-Found error message in the response body if "myfile.txt" isn't there: ``` haskell server6 :: Server IOAPI1 server6 = do exists <- liftIO (doesFileExist "myfile.txt") if exists then liftIO (readFile "myfile.txt") >>= return . FileContent else throwError custom404Err where custom404Err = err404 { errBody = "myfile.txt just isn't there, please leave this server alone." } ``` Here's how that server looks in action: ``` bash $ curl --verbose http://localhost:8081/myfile.txt [snip] * Connected to localhost (127.0.0.1) port 8081 (#0) > GET /myfile.txt HTTP/1.1 > User-Agent: curl/7.30.0 > Host: localhost:8081 > Accept: */* > < HTTP/1.1 404 Not Found [snip] myfile.txt just isn't there, please leave this server alone. $ echo Hello > myfile.txt $ curl --verbose http://localhost:8081/myfile.txt [snip] * Connected to localhost (127.0.0.1) port 8081 (#0) > GET /myfile.txt HTTP/1.1 > User-Agent: curl/7.30.0 > Host: localhost:8081 > Accept: */* > < HTTP/1.1 200 OK [snip] < Content-Type: application/json [snip] {"content":"Hello\n"} ``` ## Response headers To add headers to your response, use [addHeader](http://hackage.haskell.org/package/servant/docs/Servant-API-ResponseHeaders.html). Note that this changes the type of your API, as we can see in the following example: ``` haskell type MyHandler = Get '[JSON] (Headers '[Header "X-An-Int" Int] User) myHandler :: Server MyHandler myHandler = return $ addHeader 1797 albert ``` Note that the type of `addHeader header x` is different than the type of `x`! And if you add more headers, more headers will appear in the header list: ``` haskell type MyHeadfulHandler = Get '[JSON] (Headers '[Header "X-A-Bool" Bool, Header "X-An-Int" Int] User) myHeadfulHandler :: Server MyHeadfulHandler myHeadfulHandler = return $ addHeader True $ addHeader 1797 albert ``` But what if your handler only *sometimes* adds a header? If you declare that your handler adds headers, and you don't add one, the return type of your handler will be different than expected. To solve this, you have to explicitly *not* add a header by using `noHeader`: ``` haskell type MyMaybeHeaderHandler = Capture "withHeader" Bool :> Get '[JSON] (Headers '[Header "X-An-Int" Int] User) myMaybeHeaderHandler :: Server MyMaybeHeaderHandler myMaybeHeaderHandler x = return $ if x then addHeader 1797 albert else noHeader albert ``` ## Serving static files **servant-server** also provides a way to just serve the content of a directory under some path in your web API. As mentioned earlier in this document, the `Raw` combinator can be used in your APIs to mean "plug here any WAI application". Well, **servant-server** provides a function to get a file and directory serving WAI application, namely: ``` haskell ignore -- exported by Servant and Servant.Server serveDirectoryWebApp :: FilePath -> Server Raw ``` `serveDirectoryWebApp`'s argument must be a path to a valid directory. Here's an example API that will serve some static files: ``` haskell type StaticAPI = "static" :> Raw ``` And the server: ``` haskell staticAPI :: Proxy StaticAPI staticAPI = Proxy ``` ``` haskell server7 :: Server StaticAPI server7 = serveDirectoryWebApp "static-files" app3 :: Application app3 = serve staticAPI server7 ``` This server will match any request whose path starts with `/static` and will look for a file at the path described by the rest of the request path, inside the *static-files/* directory of the path you run the program from. In other words: If a client requests `/static/foo.txt`, the server will look for a file at `./static-files/foo.txt`. If that file exists it'll succeed and serve the file. If it doesn't exist, the handler will fail with a `404` status code. `serveDirectoryWebApp` uses some standard settings that fit the use case of serving static files for most web apps. You can find out about the other options in the documentation of the `Servant.Server.StaticFiles` module. ## Nested APIs Let's see how you can define APIs in a modular way, while avoiding repetition. Consider this simple example: ``` haskell type UserAPI3 = -- view the user with given userid, in JSON Capture "userid" Int :> Get '[JSON] User :<|> -- delete the user with given userid. empty response Capture "userid" Int :> DeleteNoContent ``` We can instead factor out the `userid`: ``` haskell type UserAPI4 = Capture "userid" Int :> ( Get '[JSON] User :<|> DeleteNoContent ) ``` However, you have to be aware that this has an effect on the type of the corresponding `Server`: ``` haskell ignore Server UserAPI3 = (Int -> Handler User) :<|> (Int -> Handler NoContent) Server UserAPI4 = Int -> ( Handler User :<|> Handler NoContent ) ``` In the first case, each handler receives the *userid* argument. In the latter, the whole `Server` takes the *userid* and has handlers that are just computations in `Handler`, with no arguments. In other words: ``` haskell server8 :: Server UserAPI3 server8 = getUser :<|> deleteUser where getUser :: Int -> Handler User getUser _userid = error "..." deleteUser :: Int -> Handler NoContent deleteUser _userid = error "..." -- notice how getUser and deleteUser -- have a different type! no argument anymore, -- the argument directly goes to the whole Server server9 :: Server UserAPI4 server9 userid = getUser userid :<|> deleteUser userid where getUser :: Int -> Handler User getUser = error "..." deleteUser :: Int -> Handler NoContent deleteUser = error "..." ``` Note that there's nothing special about `Capture` that lets you "factor it out": this can be done with any combinator. Here are a few examples of APIs with a combinator factored out for which we can write a perfectly valid `Server`. ``` haskell -- we just factor out the "users" path fragment type API1 = "users" :> ( Get '[JSON] [User] -- user listing :<|> Capture "userid" Int :> Get '[JSON] User -- view a particular user ) -- we factor out the Request Body type API2 = ReqBody '[JSON] User :> ( Get '[JSON] User -- just display the same user back, don't register it :<|> PostNoContent -- register the user. empty response ) -- we factor out a Header type API3 = Header "Authorization" Token :> ( Get '[JSON] SecretData -- get some secret data, if authorized :<|> ReqBody '[JSON] SecretData :> PostNoContent -- add some secret data, if authorized ) newtype Token = Token ByteString newtype SecretData = SecretData ByteString ``` This approach lets you define APIs modularly and assemble them all into one big API type only at the end. ``` haskell type UsersAPI = Get '[JSON] [User] -- list users :<|> ReqBody '[JSON] User :> PostNoContent -- add a user :<|> Capture "userid" Int :> ( Get '[JSON] User -- view a user :<|> ReqBody '[JSON] User :> PutNoContent -- update a user :<|> DeleteNoContent -- delete a user ) usersServer :: Server UsersAPI usersServer = getUsers :<|> newUser :<|> userOperations where getUsers :: Handler [User] getUsers = error "..." newUser :: User -> Handler NoContent newUser = error "..." userOperations userid = viewUser userid :<|> updateUser userid :<|> deleteUser userid where viewUser :: Int -> Handler User viewUser = error "..." updateUser :: Int -> User -> Handler NoContent updateUser = error "..." deleteUser :: Int -> Handler NoContent deleteUser = error "..." ``` ``` haskell type ProductsAPI = Get '[JSON] [Product] -- list products :<|> ReqBody '[JSON] Product :> PostNoContent -- add a product :<|> Capture "productid" Int :> ( Get '[JSON] Product -- view a product :<|> ReqBody '[JSON] Product :> PutNoContent -- update a product :<|> DeleteNoContent -- delete a product ) data Product = Product { productId :: Int } productsServer :: Server ProductsAPI productsServer = getProducts :<|> newProduct :<|> productOperations where getProducts :: Handler [Product] getProducts = error "..." newProduct :: Product -> Handler NoContent newProduct = error "..." productOperations productid = viewProduct productid :<|> updateProduct productid :<|> deleteProduct productid where viewProduct :: Int -> Handler Product viewProduct = error "..." updateProduct :: Int -> Product -> Handler NoContent updateProduct = error "..." deleteProduct :: Int -> Handler NoContent deleteProduct = error "..." ``` ``` haskell type CombinedAPI = "users" :> UsersAPI :<|> "products" :> ProductsAPI server10 :: Server CombinedAPI server10 = usersServer :<|> productsServer ``` Finally, we can realize the user and product APIs are quite similar and abstract that away: ``` haskell -- API for values of type 'a' -- indexed by values of type 'i' type APIFor a i = Get '[JSON] [a] -- list 'a's :<|> ReqBody '[JSON] a :> PostNoContent -- add an 'a' :<|> Capture "id" i :> ( Get '[JSON] a -- view an 'a' given its "identifier" of type 'i' :<|> ReqBody '[JSON] a :> PutNoContent -- update an 'a' :<|> DeleteNoContent -- delete an 'a' ) -- Build the appropriate 'Server' -- given the handlers of the right type. serverFor :: Handler [a] -- handler for listing of 'a's -> (a -> Handler NoContent) -- handler for adding an 'a' -> (i -> Handler a) -- handler for viewing an 'a' given its identifier of type 'i' -> (i -> a -> Handler NoContent) -- updating an 'a' with given id -> (i -> Handler NoContent) -- deleting an 'a' given its id -> Server (APIFor a i) serverFor = error "..." -- implementation left as an exercise. contact us on IRC -- or the mailing list if you get stuck! ``` When your API contains the `EmptyAPI` combinator, you'll want to use `emptyServer` in the corresponding slot for your server, which will simply fail with 404 whenever a request reaches it: ``` haskell type CombinedAPI2 = API :<|> "empty" :> EmptyAPI server11 :: Server CombinedAPI2 server11 = server3 :<|> emptyServer ``` ## Using another monad for your handlers Remember how `Server` turns combinators for HTTP methods into `Handler`? Well, actually, there's more to that. `Server` is actually a simple type synonym. ``` haskell ignore type Server api = ServerT api Handler ``` `ServerT` is the actual type family that computes the required types for the handlers that's part of the `HasServer` class. It's like `Server` except that it takes another parameter which is the monad you want your handlers to run in, or more generally the return types of your handlers. This third parameter is used for specifying the return type of the handler for an endpoint, e.g when computing `ServerT (Get '[JSON] Person) SomeMonad`. The result would be `SomeMonad Person`. The first and main question one might have then is: how do we write handlers that run in another monad? How can we "bring back" the value from a given monad into something **servant** can understand? ### Natural transformations If we have a function that gets us from an `m a` to an `n a`, for any `a`, what do we have? ``` haskell type (~>) m n = forall a. m a -> n a ``` For example: ``` haskell listToMaybe' :: [] ~> Maybe listToMaybe' = listToMaybe -- from Data.Maybe ``` Note that `servant` doesn't declare the `~>` type-alias, as the unfolded variant isn't much longer to write, as we'll see shortly. So if you want to write handlers using another monad/type than `Handler`, say the `Reader String` monad, the first thing you have to prepare is a function: ``` haskell ignore readerToHandler :: Reader String a -> Handler a ``` We obviously have to run the `Reader` computation by supplying it with a `String`, like `"hi"`. We get an `a` out from that and can then just `return` it into `Handler`. ``` haskell readerToHandler :: Reader String a -> Handler a readerToHandler r = return (runReader r "hi") ``` We can write some simple webservice with the handlers running in `Reader String`. ``` haskell type ReaderAPI = "a" :> Get '[JSON] Int :<|> "b" :> ReqBody '[JSON] Double :> Get '[JSON] Bool readerAPI :: Proxy ReaderAPI readerAPI = Proxy readerServerT :: ServerT ReaderAPI (Reader String) readerServerT = a :<|> b where a :: Reader String Int a = return 1797 b :: Double -> Reader String Bool b _ = asks (== "hi") ``` We unfortunately can't use `readerServerT` as an argument of `serve`, because `serve` wants a `Server ReaderAPI`, i.e., with handlers running in `Handler`. But there's a simple solution to this. ### Welcome `hoistServer` That's right. We have just written `readerToHandler`, which is exactly what we would need to apply to all handlers to make the handlers have the right type for `serve`. Being cumbersome to do by hand, we provide a function `hoistServer` which takes a natural transformation between two parameterized types `m` and `n` and a `ServerT someapi m`, and returns a `ServerT someapi n`. In our case, we can wrap up our little webservice by using `hoistServer readerAPI readerToHandler` on our handlers. ``` haskell readerServer :: Server ReaderAPI readerServer = hoistServer readerAPI readerToHandler readerServerT app4 :: Application app4 = serve readerAPI readerServer ``` This is the webservice in action: ``` bash $ curl http://localhost:8081/a 1797 $ curl http://localhost:8081/b -X GET -d '42.0' -H 'Content-Type: application/json' true ``` ### An arrow is a reader too. In previous versions of `servant` we had an `enter` to do what `hoistServer` does now. `enter` had an ambitious design goals, but was problematic in practice. One problematic situation was when the source monad was `(->) r`, yet it's handy in practice, because `(->) r` is isomorphic to `Reader r`. We can rewrite the previous example without `Reader`: ```haskell funServerT :: ServerT ReaderAPI ((->) String) funServerT = a :<|> b where a :: String -> Int a _ = 1797 -- unfortunately, we cannot make `String` the first argument. b :: Double -> String -> Bool b _ s = s == "hi" funToHandler :: (String -> a) -> Handler a funToHandler f = return (f "hi") app5 :: Application app5 = serve readerAPI (hoistServer readerAPI funToHandler funServerT) ``` ## Streaming endpoints We can create endpoints that don't just give back a single result, but give back a *stream* of results, served one at a time. Stream endpoints only provide a single content type, and also specify what framing strategy is used to delineate the results. To serve these results, we need to give back a stream producer. Adapters can be written to *Pipes*, *Conduit* and the like, or written directly as `SourceIO`s. SourceIO builds upon servant's own `SourceT` stream type (it's simpler than *Pipes* or *Conduit*). The API of a streaming endpoint needs to explicitly specify which sort of generator it produces. Note that the generator itself is returned by a `Handler` action, so that additional IO may be done in the creation of one. ``` haskell type StreamAPI = "userStream" :> StreamGet NewlineFraming JSON (SourceIO User) streamAPI :: Proxy StreamAPI streamAPI = Proxy streamUsers :: SourceIO User streamUsers = source [isaac, albert, albert] app6 :: Application app6 = serve streamAPI (return streamUsers) ``` This simple application returns a stream of `User` values encoded in JSON format, with each value separated by a newline. In this case, the stream will consist of the value of `isaac`, followed by the value of `albert`, then the value of `albert` a second time. Importantly, the stream is written back as results are produced, rather than all at once. This means first that results are delivered when they are available, and second, that if an exception interrupts production of the full stream, nonetheless partial results have already been written back. ## Conclusion You're now equipped to write webservices/web-applications using **servant**. The rest of this document focuses on **servant-client**, **servant-js** and **servant-docs**.