7edd35c · servant · git.iko.soy

git.iko.soy/servant

on master help sign in

docs for streaming (#852)

7edd35cgbaz9 years ago

doc/tutorial/ApiType.lhs+13

compare with master

unifiedsplit

@@ -127,6 +127,19 @@ type UserAPI4 = "users" :> Get '[JSON] [User]

127 :<|> "admins" :> Get '[JSON] [User]

128```

129

130+### `StreamGet` and `StreamPost`

131+

132+The `StreamGet` and `StreamPost` combinators are defined in terms of the more general `Stream`

133+

134+``` haskell ignore

135+data Stream (method :: k1) (framing :: *) (contentType :: *) a

136+type StreamGet = Stream 'GET

137+type StreamPost = Stream 'POST

138+```

139+

140+These describe endpoints that return a stream of values rather than just a single value. They not only take a single content type as a paremeter, but also a framing strategy -- this specifies how the individual results are deliniated from one another in the stream. The two standard strategies given with Servant are `NewlineFraming` and `NetstringFraming`, but others can be written to match other protocols.

141+

142+

130### `Capture`

143### `Capture`

131

144

132URL captures are segments of the path of a URL that are variable and whose actual value is

145URL captures are segments of the path of a URL that are variable and whose actual value is

doc/tutorial/Client.lhs+45−1

@@ -153,4 +153,48 @@ HelloMessage {msg = "Hello, servant"}

153Email {from = "great@company.com", to = "alp@foo.com", subject = "Hey Alp, we miss you!", body = "Hi Alp,\n\nSince you've recently turned 26, have you checked out our latest haskell, mathematics products? Give us a visit!"}

154```

155

156−The types of the arguments for the functions are the same as for (server-side) request handlers. You now know how to use **servant-client**!

156+The types of the arguments for the functions are the same as for (server-side) request handlers.

157+

158+## Querying Streaming APIs.

159+

160+Consider the following streaming API type:

161+

162+``` haskell

163+type StreamAPI = "positionStream" :> StreamGet NewlineFraming JSON (ResultStream Position)

164+```

165+

166+Note that when we declared an API to serve, we specified a `StreamGenerator` as a producer of streams. Now we specify our result type as a `ResultStream`. With types that can be used both ways, if appropriate adaptors are written (in the form of `ToStreamGenerator` and `BuildFromStream` instances), then this asymmetry isn't necessary. Otherwise, if you want to share the same API across clients and servers, you can parameterize it like so:

167+

168+``` haskell ignore

169+type StreamAPI f = "positionStream" :> StreamGet NewlineFraming JSON (f Position)

170+type ServerStreamAPI = StreamAPI StreamGenerator

171+type ClientStreamAPI = StreamAPI ResultStream

172+```

173+

174+In any case, here's how we write a function to query our API:

175+

176+``` haskell

177+streamAPI :: Proxy StreamAPI

178+streamAPI = Proxy

179+

180+posStream :: ClientM (ResultStream Position)

181+

182+posStream = client streamAPI

183+```

184+

185+And here's how to just print out all elements from a `ResultStream`, to give some idea of how to work with them.

186+

187+``` haskell

188+printResultStream :: Show a => ResultStream a -> IO ()

189+printResultStream (ResultStream k) = k $ \getResult ->

190+ let loop = do

191+ r <- getResult

192+ case r of

193+ Nothing -> return ()

194+ Just x -> print x >> loop

195+ in loop

196+```

197+

198+The stream is parsed and provided incrementally. So the above loop prints out each result as soon as it is received on the stream, rather than waiting until they are all available to print them at once.

199+

200+You now know how to use **servant-client**!

doc/tutorial/Server.lhs+24−3

@@ -204,7 +204,7 @@ And that's it! You can run this example in the same way that we showed for

204

205Fine, we can write trivial webservices easily, but none of the two above use

206any "fancy" combinator from servant. Let's address this and use `QueryParam`,

207−`Capture` and `ReqBody` right away. You'll see how each occurence of these

207+`Capture` and `ReqBody` right away. You'll see how each occurrence of these

208combinators in an endpoint makes the corresponding handler receive an

209argument of the appropriate type automatically. You don't have to worry about

210manually looking up URL captures or query string parameters, or

@@ -1092,7 +1092,7 @@ We can write some simple webservice with the handlers running in `Reader String`

1092

1093``` haskell

1094type ReaderAPI = "a" :> Get '[JSON] Int

1095− :<|> "b" :> ReqBody '[JSON] Double :> Get '[JSON] Bool

1095+ :<|> "b" :> ReqBody '[JSON] Double :> Get '[JSON] Bool

1096

1097readerAPI :: Proxy ReaderAPI

1098readerAPI = Proxy

@@ -1114,7 +1114,7 @@ We unfortunately can't use `readerServerT` as an argument of `serve`, because

1114That's right. We have just written `readerToHandler`, which is exactly what we

1115would need to apply to all handlers to make the handlers have the

1116right type for `serve`. Being cumbersome to do by hand, we provide a function

1117−`hoistServer` which takes a natural transformation between two parametrized types `m`

1117+`hoistServer` which takes a natural transformation between two parameterized types `m`

1118and `n` and a `ServerT someapi m`, and returns a `ServerT someapi n`.

1119

1120In our case, we can wrap up our little webservice by using

@@ -1164,6 +1164,27 @@ app5 :: Application

1164app5 = serve readerAPI (hoistServer readerAPI funToHandler funServerT)

1165```

1166

1167+## Streaming endpoints

1168+

1169+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 `StreamGenerator`s. StreamGenerators are IO-based continuations that are handed two functions -- the first to write the first result back, and the second to write all subsequent results back. (This is to allow handling of situations where the entire stream is prefixed by a header, or where a boundary is written between elements, but not prior to the first element). 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.

1170+

1171+``` haskell

1172+type StreamAPI = "userStream" :> StreamGet NewlineFraming JSON (StreamGenerator User)

1173+streamAPI :: Proxy StreamAPI

1174+streamAPI = Proxy

1175+

1176+streamUsers :: StreamGenerator User

1177+streamUsers = StreamGenerator $ \sendFirst sendRest -> do

1178+ sendFirst isaac

1179+ sendRest albert

1180+ sendRest albert

1181+

1182+app6 :: Application

1183+app6 = serve streamAPI (return streamUsers)

1184+```

1185+

1186+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 third 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.

1187+

1167## Conclusion

1188## Conclusion

1168

1189

1169You're now equipped to write webservices/web-applications using

1190You're now equipped to write webservices/web-applications using