3Enough chit-chat about type-level combinators and representing an API as a
4type. Can we have a webservice already?
8Equipped with some basic knowledge about the way we represent APIs, let's now
9write our first webservice.
11The source for this tutorial section is a literate haskell file, so first we
12need to have some language extensions and imports:
15{-# LANGUAGE DataKinds #-}
16{-# LANGUAGE DeriveGeneric #-}
17{-# LANGUAGE FlexibleInstances #-}
18{-# LANGUAGE GeneralizedNewtypeDeriving #-}
19{-# LANGUAGE MultiParamTypeClasses #-}
20{-# LANGUAGE OverloadedStrings #-}
21{-# LANGUAGE ScopedTypeVariables #-}
22{-# LANGUAGE TypeOperators #-}
29import Control.Monad.Except
30import Control.Monad.Reader
31import Data.Aeson.Compat
32import Data.Aeson.Types
33import Data.Attoparsec.ByteString
34import Data.ByteString (ByteString)
37import Data.String.Conversions
38import Data.Time.Calendar
41import Network.HTTP.Media ((//), (/:))
43import Network.Wai.Handler.Warp
45import System.Directory
47import Text.Blaze.Html.Renderer.Utf8
48import qualified Data.Aeson.Parser
49import qualified Text.Blaze.Html
52**Important**: the `Servant` module comes from the **servant-server** package,
53the one that lets us run webservers that implement a particular API type. It
54reexports all the types from the **servant** package that let you declare API
55types as well as everything you need to turn your request handlers into a
56fully-fledged webserver. This means that in your applications, you can just add
57**servant-server** as a dependency, import `Servant` and not worry about anything
60We will write a server that will serve the following API.
63type UserAPI1 = "users" :> Get '[JSON] [User]
66Here's what we would like to see when making a GET request to `/users`.
69[ {"name": "Isaac Newton", "age": 372, "email": "isaac@newton.co.uk", "registration_date": "1683-03-01"}
70, {"name": "Albert Einstein", "age": 136, "email": "ae@mc2.org", "registration_date": "1905-12-01"}
74Now let's define our `User` data type and write some instances for it.
81 , registration_date :: Day
82 } deriving (Eq, Show, Generic)
87Nothing funny going on here. But we now can define our list of two users.
92 [ User "Isaac Newton" 372 "isaac@newton.co.uk" (fromGregorian 1683 3 1)
93 , User "Albert Einstein" 136 "ae@mc2.org" (fromGregorian 1905 12 1)
97Let's also write our API type.
100type UserAPI1 = "users" :> Get '[JSON] [User]
103We can now take care of writing the actual webservice that will handle requests
104to such an API. This one will be very simple, being reduced to just a single
105endpoint. The type of the web application is determined by the API type,
106through a *type family* named `Server`. (Type families are just functions that
107take types as input and return types.) The `Server` type family will compute
108the right type that a bunch of request handlers should have just from the
109corresponding API type.
111The first thing to know about the `Server` type family is that behind the
112scenes it will drive the routing, letting you focus only on the business
113logic. The second thing to know is that for each endpoint, your handlers will
114by default run in the `Handler` monad. This is overridable very
115easily, as explained near the end of this guide. Third thing, the type of the
116value returned in that monad must be the same as the second argument of the
117HTTP method combinator used for the corresponding endpoint. In our case, it
118means we must provide a handler of type `Handler [User]`. Well,
119we have a monad, let's just `return` our list:
122server1 :: Server UserAPI1
123server1 = return users1
126That's it. Now we can turn `server` into an actual webserver using
127[wai](http://hackage.haskell.org/package/wai) and
128[warp](http://hackage.haskell.org/package/warp):
131userAPI :: Proxy UserAPI1
134-- 'serve' comes from servant and hands you a WAI Application,
135-- which you can think of as an "abstract" web application,
136-- not yet a webserver.
138app1 = serve userAPI server1
141The `userAPI` bit is, alas, boilerplate (we need it to guide type inference).
142But that's about as much boilerplate as you get.
144And we're done! Let's run our webservice on the port 8081.
151You can put this all into a file or just grab [servant's
152repo](http://github.com/haskell-servant/servant) and look at the
153*doc/tutorial* directory. This code (the source of this web page) is in
154*doc/tutorial/Server.lhs*.
156If you run it, you can go to `http://localhost:8081/users` in your browser or
157query it with curl and you see:
160$ curl http://localhost:8081/users
161[{"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"}]
166What if we want more than one endpoint? Let's add `/albert` and `/isaac` to
167view the corresponding users encoded in JSON.
170type UserAPI2 = "users" :> Get '[JSON] [User]
171 :<|> "albert" :> Get '[JSON] User
172 :<|> "isaac" :> Get '[JSON] User
175And let's adapt our code a bit.
179isaac = User "Isaac Newton" 372 "isaac@newton.co.uk" (fromGregorian 1683 3 1)
182albert = User "Albert Einstein" 136 "ae@mc2.org" (fromGregorian 1905 12 1)
185users2 = [isaac, albert]
188Now, just like we separate the various endpoints in `UserAPI` with `:<|>`, we
189are going to separate the handlers with `:<|>` too! They must be provided in
190the same order as in in the API type.
193server2 :: Server UserAPI2
194server2 = return users2
199And that's it! You can run this example in the same way that we showed for
200`server1` and check out the data available at `/users`, `/albert` and `/isaac`.
202## From combinators to handler arguments
204Fine, we can write trivial webservices easily, but none of the two above use
205any "fancy" combinator from servant. Let's address this and use `QueryParam`,
206`Capture` and `ReqBody` right away. You'll see how each occurence of these
207combinators in an endpoint makes the corresponding handler receive an
208argument of the appropriate type automatically. You don't have to worry about
209manually looking up URL captures or query string parameters, or
210decoding/encoding data from/to JSON. Never.
212We are going to use the following data types and functions to implement a
216type API = "position" :> Capture "x" Int :> Capture "y" Int :> Get '[JSON] Position
217 :<|> "hello" :> QueryParam "name" String :> Get '[JSON] HelloMessage
218 :<|> "marketing" :> ReqBody '[JSON] ClientInfo :> Post '[JSON] Email
220data Position = Position
225instance ToJSON Position
227newtype HelloMessage = HelloMessage { msg :: String }
230instance ToJSON HelloMessage
232data ClientInfo = ClientInfo
233 { clientName :: String
234 , clientEmail :: String
236 , clientInterestedIn :: [String]
239instance FromJSON ClientInfo
240instance ToJSON ClientInfo
251emailForClient :: ClientInfo -> Email
252emailForClient c = Email from' to' subject' body'
254 where from' = "great@company.com"
256 subject' = "Hey " ++ clientName c ++ ", we miss you!"
257 body' = "Hi " ++ clientName c ++ ",\n\n"
258 ++ "Since you've recently turned " ++ show (clientAge c)
259 ++ ", have you checked out our latest "
260 ++ intercalate ", " (clientInterestedIn c)
261 ++ " products? Give us a visit!"
264We can implement handlers for the three endpoints:
272 where position :: Int -> Int -> Handler Position
273 position x y = return (Position x y)
275 hello :: Maybe String -> Handler HelloMessage
276 hello mname = return . HelloMessage $ case mname of
277 Nothing -> "Hello, anonymous coward"
278 Just n -> "Hello, " ++ n
280 marketing :: ClientInfo -> Handler Email
281 marketing clientinfo = return (emailForClient clientinfo)
284Did you see that? The types for your handlers changed to be just what we
285needed! In particular:
287 - a `Capture "something" a` becomes an argument of type `a` (for `position`);
288 - a `QueryParam "something" a` becomes an argument of type `Maybe a` (because
289an endpoint can technically be accessed without specifying any query
290string parameter, we decided to "force" handlers to be aware that the
291parameter might not always be there);
293 - a `ReqBody contentTypeList a` becomes an argument of type `a`;
295And that's it. Here's the example in action:
298$ curl http://localhost:8081/position/1/2
299{"xCoord":1,"yCoord":2}
300$ curl http://localhost:8081/hello
301{"msg":"Hello, anonymous coward"}
302$ curl http://localhost:8081/hello?name=Alp
304$ 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
305{"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"}
308For reference, here's a list of some combinators from **servant**:
310 > - `Delete`, `Get`, `Patch`, `Post`, `Put`: these do not become arguments. They provide the return type of handlers, which usually is `Handler <something>`.
311 > - `Capture "something" a` becomes an argument of type `a`.
312 > - `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.
313 > - `QueryFlag "something"` gets turned into an argument of type `Bool`.
314 > - `QueryParams "something" a` gets turned into an argument of type `[a]`.
315 > - `ReqBody contentTypes a` gets turned into an argument of type `a`.
317## The `FromHttpApiData`/`ToHttpApiData` classes
319Wait... How does **servant** know how to decode the `Int`s from the URL? Or how
320to decode a `ClientInfo` value from the request body? This is what this and the
321following two sections address.
323`Capture`s and `QueryParam`s are represented by some textual value in URLs.
324`Header`s are similarly represented by a pair of a header name and a
325corresponding (textual) value in the request's "metadata". How types are
326decoded from headers, captures, and query params is expressed in a class
327`FromHttpApiData` (from the package
328[**http-api-data**](http://hackage.haskell.org/package/http-api-data)):
331class FromHttpApiData a where
332 {-# MINIMAL parseUrlPiece | parseQueryParam #-}
333 -- | Parse URL path piece.
334 parseUrlPiece :: Text -> Either Text a
335 parseUrlPiece = parseQueryParam
337 -- | Parse HTTP header value.
338 parseHeader :: ByteString -> Either Text a
339 parseHeader = parseUrlPiece . decodeUtf8
341 -- | Parse query param value.
342 parseQueryParam :: Text -> Either Text a
343 parseQueryParam = parseUrlPiece
346As you can see, as long as you provide either `parseUrlPiece` (for `Capture`s)
347or `parseQueryParam` (for `QueryParam`s), the other methods will be defined in
350**http-api-data** provides a decent number of instances, helpers for defining new
351ones, and wonderful documentation.
353There's not much else to say about these classes. You will need instances for
354them when using `Capture`, `QueryParam`, `QueryParams`, and `Header` with your
355types. You will need `FromHttpApiData` instances for server-side request
356handlers and `ToHttpApiData` instances only when using
357**servant-client**, as described in the [section about deriving haskell
358functions to query an API](Client.html).
360## Using content-types with your data types
362The same principle was operating when decoding request bodies from JSON, and
363responses *into* JSON. (JSON is just the running example - you can do this with
366This section introduces a couple of typeclasses provided by **servant** that make
369### The truth behind `JSON`
372What exactly is `JSON` (the type as used in `Get '[JSON] User`)? Like the 3
373other content-types provided out of the box by **servant**, it's a really dumb
383Obviously, this is not all there is to `JSON`, otherwise it would be quite
384pointless. Like most of the data types in **servant**, `JSON` is mostly there as
385a special *symbol* that's associated with encoding (resp. decoding) to (resp.
386from) the *JSON* format. The way this association is performed can be
387decomposed into two steps.
389The first step is to provide a proper
391[**http-media**](https://hackage.haskell.org/package/http-media-0.6.2/docs/Network-HTTP-Media.html))
392representation for `JSON`, or for your own content-types. If you look at the
393haddocks from this link, you can see that we just have to specify
394`application/json` using the appropriate functions. In our case, we can just
395use `(//) :: ByteString -> ByteString -> MediaType`. The precise way to specify
396the `MediaType` is to write an instance for the `Accept` class:
400class Accept ctype where
401 contentType :: Proxy ctype -> MediaType
403instance Accept JSON where
404 contentType _ = "application" // "json"
407The second step is centered around the `MimeRender` and `MimeUnrender` classes.
408These classes just let you specify a way to encode and decode
409values into or from your content-type's representation.
412class Accept ctype => MimeRender ctype a where
413 mimeRender :: Proxy ctype -> a -> ByteString
414 -- alternatively readable as:
415 mimeRender :: Proxy ctype -> (a -> ByteString)
418Given a content-type and some user type, `MimeRender` provides a function that
419encodes values of type `a` to lazy `ByteString`s.
421In the case of `JSON`, this is easily dealt with! For any type `a` with a
422`ToJSON` instance, we can render values of that type to JSON using
426instance ToJSON a => MimeRender JSON a where
427 mimeRender _ = encode
430And now the `MimeUnrender` class, which lets us extract values from lazy
431`ByteString`s, alternatively failing with an error string.
434class Accept ctype => MimeUnrender ctype a where
435 mimeUnrender :: Proxy ctype -> ByteString -> Either String a
438We don't have much work to do there either, `Data.Aeson.eitherDecode` is
439precisely what we need. However, it only allows arrays and objects as toplevel
440JSON values and this has proven to get in our way more than help us so we wrote
441our own little function around **aeson** and **attoparsec** that allows any type of
442JSON value at the toplevel of a "JSON document". Here's the definition in case
446eitherDecodeLenient :: FromJSON a => ByteString -> Either String a
447eitherDecodeLenient input = do
448 v :: Value <- parseOnly (Data.Aeson.Parser.value <* endOfInput) (cs input)
449 parseEither parseJSON v
452This function is exactly what we need for our `MimeUnrender` instance.
455instance FromJSON a => MimeUnrender JSON a where
456 mimeUnrender _ = eitherDecodeLenient
459And this is all the code that lets you use `JSON` with `ReqBody`, `Get`,
460`Post` and friends. We can check our understanding by implementing support
461for an `HTML` content-type, so that users of your webservice can access an
462HTML representation of the data they want, ready to be included in any HTML
463document, e.g. using [jQuery's `load` function](https://api.jquery.com/load/),
464simply by adding `Accept: text/html` to their request headers.
466### Case-studies: **servant-blaze** and **servant-lucid**
468These days, most of the haskellers who write their HTML UIs directly from
469Haskell use either [**blaze-html**](http://hackage.haskell.org/package/blaze-html)
470or [**lucid**](http://hackage.haskell.org/package/lucid). The best option for
471**servant** is obviously to support both (and hopefully other templating
472solutions!). We're first going to look at **lucid**:
478Once again, the data type is just there as a symbol for the encoding/decoding
479functions, except that this time we will only worry about encoding since
480**lucid** doesn't provide a way to extract data from HTML.
483instance Accept HTMLLucid where
484 contentType _ = "text" // "html" /: ("charset", "utf-8")
487Note that this instance uses the `(/:)` operator from **http-media** which lets
488us specify additional information about a content-type, like the charset here.
490The rendering instances call similar functions that take
491types with an appropriate instance to an "abstract" HTML representation and
492then write that to a `ByteString`.
495instance ToHtml a => MimeRender HTMLLucid a where
496 mimeRender _ = renderBS . toHtml
498-- let's also provide an instance for lucid's
500instance MimeRender HTMLLucid (Html a) where
501 mimeRender _ = renderBS
504For **blaze-html** everything works very similarly:
507-- For this tutorial to compile 'HTMLLucid' and 'HTMLBlaze' have to be
508-- distinct. Usually you would stick to one html rendering library and then
509-- you can go with one 'HTML' type.
512instance Accept HTMLBlaze where
513 contentType _ = "text" // "html" /: ("charset", "utf-8")
515instance ToMarkup a => MimeRender HTMLBlaze a where
516 mimeRender _ = renderHtml . Text.Blaze.Html.toHtml
518-- while we're at it, just like for lucid we can
519-- provide an instance for rendering blaze's 'Html' type
520instance MimeRender HTMLBlaze Text.Blaze.Html.Html where
521 mimeRender _ = renderHtml
524Both [**servant-blaze**](http://hackage.haskell.org/package/servant-blaze) and
525[**servant-lucid**](http://hackage.haskell.org/package/servant-lucid) let you use
526`HTMLLucid` and `HTMLBlaze` in any content-type list as long as you provide an instance of the
527appropriate class (`ToMarkup` for **blaze-html**, `ToHtml` for **lucid**).
529We can now write a webservice that uses **servant-lucid** to show the `HTMLLucid`
530content-type in action. We will be serving the following API:
533type PersonAPI = "persons" :> Get '[JSON, HTMLLucid] [Person]
536where `Person` is defined as follows:
540 { firstName :: String
542 } deriving Generic -- for the JSON instance
544instance ToJSON Person
547Now, let's teach **lucid** how to render a `Person` as a row in a table, and then
548a list of `Person`s as a table with a row per person.
551-- HTML serialization of a single person
552instance ToHtml Person where
555 td_ (toHtml $ firstName person)
556 td_ (toHtml $ lastName person)
558 -- do not worry too much about this
561-- HTML serialization of a list of persons
562instance ToHtml [Person] where
563 toHtml persons = table_ $ do
568 -- this just calls toHtml on each person of the list
569 -- and concatenates the resulting pieces of HTML together
570 foldMap toHtml persons
575We create some `Person` values and serve them as a list:
580 [ Person "Isaac" "Newton"
581 , Person "Albert" "Einstein"
584personAPI :: Proxy PersonAPI
587server4 :: Server PersonAPI
588server4 = return people
591app2 = serve personAPI server4
597$ curl http://localhost:8081/persons
598[{"lastName":"Newton","firstName":"Isaac"},{"lastName":"Einstein","firstName":"Albert"}]
599$ curl -H 'Accept: text/html' http://localhost:8081/persons
600<table><tr><td>first name</td><td>last name</td></tr><tr><td>Isaac</td><td>Newton</td></tr><tr><td>Albert</td><td>Einstein</td></tr></table>
601# or just point your browser to http://localhost:8081/persons
604## The `Handler` monad
606At the heart of the handlers is the monad they run in, namely `ExceptT ServantErr IO`
607([haddock documentation for `ExceptT`](http://hackage.haskell.org/package/mtl-2.2.1/docs/Control-Monad-Except.html#t:ExceptT)), which is aliased as `Handler`.
608One might wonder: why this monad? The answer is that it is the
609simplest monad with the following properties:
611- it lets us both return a successful result (using `return`)
612or "fail" with a descriptive error (using `throwError`);
613- it lets us perform IO, which is absolutely vital since most webservices exist
614as interfaces to databases that we interact with in `IO`.
616Let's recall some definitions.
619-- from the 'mtl' package at
620newtype ExceptT e m a = ExceptT (m (Either e a))
623In short, this means that a handler of type `Handler a` is simply
624equivalent to a computation of type `IO (Either ServantErr a)`, that is, an IO
625action that either returns an error or a result.
627The module [`Control.Monad.Except`](https://hackage.haskell.org/package/mtl-2.2.1/docs/Control-Monad-Except.html#t:ExceptT)
628from which `ExceptT` comes is worth looking at.
629Perhaps most importantly, `ExceptT` is an instance of `MonadError`, so
630`throwError` can be used to return an error from your handler (whereas `return`
631 is enough to return a success).
633Most of what you'll be doing in your handlers is running some IO and,
634depending on the result, you might sometimes want to throw an error of some
635kind and abort early. The next two sections cover how to do just that.
639Another important instance from the list above is `MonadIO m => MonadIO
641[`MonadIO`](http://hackage.haskell.org/package/transformers-0.4.3.0/docs/Control-Monad-IO-Class.html)
642is a class from the **transformers** package defined as:
645class Monad m => MonadIO m where
646 liftIO :: IO a -> m a
649The `IO` monad provides a `MonadIO` instance. Hence for any type
650`e`, `ExceptT e IO` has a `MonadIO` instance. So if you want to run any kind of
651IO computation in your handlers, just use `liftIO`:
654type IOAPI1 = "myfile.txt" :> Get '[JSON] FileContent
656newtype FileContent = FileContent
657 { content :: String }
660instance ToJSON FileContent
662server5 :: Server IOAPI1
664 filecontent <- liftIO (readFile "myfile.txt")
665 return (FileContent filecontent)
668### Failing, through `ServantErr`
670If you want to explicitly fail at providing the result promised by an endpoint
671using the appropriate HTTP status code (not found, unauthorized, etc) and some
672error message, all you have to do is use the `throwError` function mentioned above
673and provide it with the appropriate value of type `ServantErr`, which is
677data ServantErr = ServantErr
679 , errReasonPhrase :: String
680 , errBody :: ByteString -- lazy bytestring
681 , errHeaders :: [Header]
685Many standard values are provided out of the box by the `Servant.Server`
686module. If you want to use these values but add a body or some headers, just
687use record update syntax:
690failingHandler :: Handler ()
691failingHandler = throwError myerr
693 where myerr :: ServantErr
694 myerr = err503 { errBody = "Sorry dear user." }
697Here's an example where we return a customised 404-Not-Found error message in
698the response body if "myfile.txt" isn't there:
701server6 :: Server IOAPI1
703 exists <- liftIO (doesFileExist "myfile.txt")
705 then liftIO (readFile "myfile.txt") >>= return . FileContent
706 else throwError custom404Err
708 where custom404Err = err404 { errBody = "myfile.txt just isn't there, please leave this server alone." }
711Here's how that server looks in action:
714$ curl --verbose http://localhost:8081/myfile.txt
716* Connected to localhost (127.0.0.1) port 8081 (#0)
717> GET /myfile.txt HTTP/1.1
718> User-Agent: curl/7.30.0
719> Host: localhost:8081
722< HTTP/1.1 404 Not Found
724myfile.txt just isnt there, please leave this server alone.
726$ echo Hello > myfile.txt
728$ curl --verbose http://localhost:8081/myfile.txt
730* Connected to localhost (127.0.0.1) port 8081 (#0)
731> GET /myfile.txt HTTP/1.1
732> User-Agent: curl/7.30.0
733> Host: localhost:8081
738< Content-Type: application/json
745To add headers to your response, use
746[addHeader](http://hackage.haskell.org/package/servant/docs/Servant-API-ResponseHeaders.html).
747Note that this changes the type of your API, as we can see in the following example:
750type MyHandler = Get '[JSON] (Headers '[Header "X-An-Int" Int] User)
752myHandler :: Server MyHandler
753myHandler = return $ addHeader 1797 albert
756Note that the type of `addHeader header x` is different than the type of `x`!
757And if you add more headers, more headers will appear in the header list:
760type MyHeadfulHandler = Get '[JSON] (Headers '[Header "X-A-Bool" Bool, Header "X-An-Int" Int] User)
762myHeadfulHandler :: Server MyHeadfulHandler
763myHeadfulHandler = return $ addHeader True $ addHeader 1797 albert
766But what if your handler only *sometimes* adds a header? If you declare that
767your handler adds headers, and you don't add one, the return type of your
768handler will be different than expected. To solve this, you have to explicitly
769*not* add a header by using `noHeader`:
772type MyMaybeHeaderHandler
773 = Capture "withHeader" Bool :> Get '[JSON] (Headers '[Header "X-An-Int" Int] User)
775myMaybeHeaderHandler :: Server MyMaybeHeaderHandler
776myMaybeHeaderHandler x = return $ if x then addHeader 1797 albert
780## Serving static files
782**servant-server** also provides a way to just serve the content of a directory
783under some path in your web API. As mentioned earlier in this document, the
784`Raw` combinator can be used in your APIs to mean "plug here any WAI
785application". Well, **servant-server** provides a function to get a file and
786directory serving WAI application, namely:
789-- exported by Servant and Servant.Server
790serveDirectory :: FilePath -> Server Raw
793`serveDirectory`'s argument must be a path to a valid directory.
795Here's an example API that will serve some static files:
798type StaticAPI = "static" :> Raw
804staticAPI :: Proxy StaticAPI
809server7 :: Server StaticAPI
810server7 = serveDirectory "static-files"
813app3 = serve staticAPI server7
816This server will match any request whose path starts with `/static` and will look
817for a file at the path described by the rest of the request path, inside the
818 *static-files/* directory of the path you run the program from.
820In other words: If a client requests `/static/foo.txt`, the server will look for a file at
821`./static-files/foo.txt`. If that file exists it'll succeed and serve the file.
822If it doesn't exist, the handler will fail with a `404` status code.
826Let's see how you can define APIs in a modular way, while avoiding repetition.
827Consider this simple example:
830type UserAPI3 = -- view the user with given userid, in JSON
831 Capture "userid" Int :> Get '[JSON] User
833 :<|> -- delete the user with given userid. empty response
834 Capture "userid" Int :> DeleteNoContent '[JSON] NoContent
837We can instead factor out the `userid`:
840type UserAPI4 = Capture "userid" Int :>
842 :<|> DeleteNoContent '[JSON] NoContent
846However, you have to be aware that this has an effect on the type of the
847corresponding `Server`:
850Server UserAPI3 = (Int -> Handler User)
851 :<|> (Int -> Handler NoContent)
853Server UserAPI4 = Int -> ( Handler User
854 :<|> Handler NoContent
858In the first case, each handler receives the *userid* argument. In the latter,
859the whole `Server` takes the *userid* and has handlers that are just
860computations in `ExceptT`, with no arguments. In other words:
863server8 :: Server UserAPI3
864server8 = getUser :<|> deleteUser
866 where getUser :: Int -> Handler User
867 getUser _userid = error "..."
869 deleteUser :: Int -> Handler NoContent
870 deleteUser _userid = error "..."
872-- notice how getUser and deleteUser
873-- have a different type! no argument anymore,
874-- the argument directly goes to the whole Server
875server9 :: Server UserAPI4
876server9 userid = getUser userid :<|> deleteUser userid
878 where getUser :: Int -> Handler User
879 getUser = error "..."
881 deleteUser :: Int -> Handler NoContent
882 deleteUser = error "..."
885Note that there's nothing special about `Capture` that lets you "factor it
886out": this can be done with any combinator. Here are a few examples of APIs
887with a combinator factored out for which we can write a perfectly valid
891-- we just factor out the "users" path fragment
892type API1 = "users" :>
893 ( Get '[JSON] [User] -- user listing
894 :<|> Capture "userid" Int :> Get '[JSON] User -- view a particular user
897-- we factor out the Request Body
898type API2 = ReqBody '[JSON] User :>
899 ( Get '[JSON] User -- just display the same user back, don't register it
900 :<|> PostNoContent '[JSON] NoContent -- register the user. empty response
903-- we factor out a Header
904type API3 = Header "Authorization" Token :>
905 ( Get '[JSON] SecretData -- get some secret data, if authorized
906 :<|> ReqBody '[JSON] SecretData :> PostNoContent '[JSON] NoContent -- add some secret data, if authorized
909newtype Token = Token ByteString
910newtype SecretData = SecretData ByteString
913This approach lets you define APIs modularly and assemble them all into one big
914API type only at the end.
918 Get '[JSON] [User] -- list users
919 :<|> ReqBody '[JSON] User :> PostNoContent '[JSON] NoContent -- add a user
920 :<|> Capture "userid" Int :>
921 ( Get '[JSON] User -- view a user
922 :<|> ReqBody '[JSON] User :> PutNoContent '[JSON] NoContent -- update a user
923 :<|> DeleteNoContent '[JSON] NoContent -- delete a user
926usersServer :: Server UsersAPI
927usersServer = getUsers :<|> newUser :<|> userOperations
929 where getUsers :: Handler [User]
930 getUsers = error "..."
932 newUser :: User -> Handler NoContent
933 newUser = error "..."
935 userOperations userid =
936 viewUser userid :<|> updateUser userid :<|> deleteUser userid
939 viewUser :: Int -> Handler User
940 viewUser = error "..."
942 updateUser :: Int -> User -> Handler NoContent
943 updateUser = error "..."
945 deleteUser :: Int -> Handler NoContent
946 deleteUser = error "..."
951 Get '[JSON] [Product] -- list products
952 :<|> ReqBody '[JSON] Product :> PostNoContent '[JSON] NoContent -- add a product
953 :<|> Capture "productid" Int :>
954 ( Get '[JSON] Product -- view a product
955 :<|> ReqBody '[JSON] Product :> PutNoContent '[JSON] NoContent -- update a product
956 :<|> DeleteNoContent '[JSON] NoContent -- delete a product
959data Product = Product { productId :: Int }
961productsServer :: Server ProductsAPI
962productsServer = getProducts :<|> newProduct :<|> productOperations
964 where getProducts :: Handler [Product]
965 getProducts = error "..."
967 newProduct :: Product -> Handler NoContent
968 newProduct = error "..."
970 productOperations productid =
971 viewProduct productid :<|> updateProduct productid :<|> deleteProduct productid
974 viewProduct :: Int -> Handler Product
975 viewProduct = error "..."
977 updateProduct :: Int -> Product -> Handler NoContent
978 updateProduct = error "..."
980 deleteProduct :: Int -> Handler NoContent
981 deleteProduct = error "..."
985type CombinedAPI = "users" :> UsersAPI
986 :<|> "products" :> ProductsAPI
988server10 :: Server CombinedAPI
989server10 = usersServer :<|> productsServer
992Finally, we can realize the user and product APIs are quite similar and
996-- API for values of type 'a'
997-- indexed by values of type 'i'
999 Get '[JSON] [a] -- list 'a's
1000 :<|> ReqBody '[JSON] a :> PostNoContent '[JSON] NoContent -- add an 'a'
1001 :<|> Capture "id" i :>
1002 ( Get '[JSON] a -- view an 'a' given its "identifier" of type 'i'
1003 :<|> ReqBody '[JSON] a :> PutNoContent '[JSON] NoContent -- update an 'a'
1004 :<|> DeleteNoContent '[JSON] NoContent -- delete an 'a'
1007-- Build the appropriate 'Server'
1008-- given the handlers of the right type.
1009serverFor :: Handler [a] -- handler for listing of 'a's
1010 -> (a -> Handler NoContent) -- handler for adding an 'a'
1011 -> (i -> Handler a) -- handler for viewing an 'a' given its identifier of type 'i'
1012 -> (i -> a -> Handler NoContent) -- updating an 'a' with given id
1013 -> (i -> Handler NoContent) -- deleting an 'a' given its id
1014 -> Server (APIFor a i)
1015serverFor = error "..."
1016-- implementation left as an exercise. contact us on IRC
1017-- or the mailing list if you get stuck!
1020## Using another monad for your handlers
1022Remember how `Server` turns combinators for HTTP methods into `Handler`? Well, actually, there's more to that. `Server` is actually a
1026type Server api = ServerT api Handler
1029`ServerT` is the actual type family that computes the required types for the
1030handlers that's part of the `HasServer` class. It's like `Server` except that
1031it takes another parameter which is the monad you want your handlers to run in,
1032or more generally the return types of your handlers. This third parameter is
1033used for specifying the return type of the handler for an endpoint, e.g when
1034computing `ServerT (Get '[JSON] Person) SomeMonad`. The result would be
1037The first and main question one might have then is: how do we write handlers
1038that run in another monad? How can we "bring back" the value from a given monad
1039into something **servant** can understand?
1041### Natural transformations
1043If we have a function that gets us from an `m a` to an `n a`, for any `a`, what
1047newtype m :~> n = NT { ($$) :: forall a. m a -> n a}
1053listToMaybeNT :: [] :~> Maybe
1054listToMaybeNT = NT listToMaybe -- from Data.Maybe
1057(`NT` comes from "natural transformation", in case you're wondering.)
1059So if you want to write handlers using another monad/type than `Handler`, say the `Reader String` monad, the first thing you have to
1060prepare is a function:
1063readerToHandler :: Reader String :~> Handler
1066Let's start with `readerToHandler'`. We obviously have to run the `Reader`
1067computation by supplying it with a `String`, like `"hi"`. We get an `a` out
1068from that and can then just `return` it into `ExceptT`. We can then just wrap
1069that function with the `NT` constructor to make it have the fancier type.
1072readerToHandler' :: forall a. Reader String a -> Handler a
1073readerToHandler' r = return (runReader r "hi")
1075readerToHandler :: Reader String :~> Handler
1076readerToHandler = NT readerToHandler'
1079We can write some simple webservice with the handlers running in `Reader String`.
1082type ReaderAPI = "a" :> Get '[JSON] Int
1083 :<|> "b" :> Get '[JSON] String
1085readerAPI :: Proxy ReaderAPI
1088readerServerT :: ServerT ReaderAPI (Reader String)
1089readerServerT = a :<|> b
1091 where a :: Reader String Int
1094 b :: Reader String String
1098We unfortunately can't use `readerServerT` as an argument of `serve`, because
1099`serve` wants a `Server ReaderAPI`, i.e., with handlers running in `Handler`. But there's a simple solution to this.
1103That's right. We have just written `readerToHandler`, which is exactly what we
1104would need to apply to all handlers to make the handlers have the
1105right type for `serve`. Being cumbersome to do by hand, we provide a function
1106`enter` which takes a natural transformation between two parametrized types `m`
1107and `n` and a `ServerT someapi m`, and returns a `ServerT someapi n`.
1109In our case, we can wrap up our little webservice by using `enter
1110readerToHandler` on our handlers.
1113readerServer :: Server ReaderAPI
1114readerServer = enter readerToHandler readerServerT
1117app4 = serve readerAPI readerServer
1120This is the webservice in action:
1123$ curl http://localhost:8081/a
1125$ curl http://localhost:8081/b
1131You're now equipped to write webservices/web-applications using
1132**servant**. The rest of this document focuses on **servant-client**,
1133**servant-js** and **servant-docs**.
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