@@ -3,29 +3,10 @@
3Enough chit-chat about type-level combinators and representing an API as a
3Enough chit-chat about type-level combinators and representing an API as a
4type. Can we have a webservice already?
4type. Can we have a webservice already?
6−If you want to follow along with the code and run the examples while you read this guide:
9−cabal get servant-examples
10−cd servant-examples-<VERSION>
12−cabal install --dependencies-only
13−cabal configure && cabal build
16−This will produce a `tutorial` executable in the
17−`dist/build/tutorial` directory that just runs the example corresponding
18−to the number specified as a command line argument:
21−$ dist/build/tutorial/tutorial
23− where N is the number of the example you want to run.
26## A first example
6## A first example
28−Equipped with some basic knowledge about the way we represent API, let's now write our first webservice.
8+Equipped with some basic knowledge about the way we represent APIs, let's now
9+write our first webservice.
30The source for this tutorial section is a literate haskell file, so first we
11The source for this tutorial section is a literate haskell file, so first we
31need to have some language extensions and imports:
12need to have some language extensions and imports:
@@ -37,6 +18,7 @@ need to have some language extensions and imports:
37{-# LANGUAGE GeneralizedNewtypeDeriving #-}
18{-# LANGUAGE GeneralizedNewtypeDeriving #-}
38{-# LANGUAGE MultiParamTypeClasses #-}
19{-# LANGUAGE MultiParamTypeClasses #-}
39{-# LANGUAGE OverloadedStrings #-}
20{-# LANGUAGE OverloadedStrings #-}
21+{-# LANGUAGE RankNTypes #-}
40{-# LANGUAGE ScopedTypeVariables #-}
22{-# LANGUAGE ScopedTypeVariables #-}
41{-# LANGUAGE TypeOperators #-}
23{-# LANGUAGE TypeOperators #-}
@@ -45,14 +27,14 @@ module Server where
45import Prelude ()
27import Prelude ()
46import Prelude.Compat
28import Prelude.Compat
48−import Control.Monad.IO.Class
30+import Control.Monad.Except
49import Control.Monad.Reader
31import Control.Monad.Reader
50−import Control.Monad.Trans.Except
51import Data.Aeson
32import Data.Aeson
52import Data.Aeson.Types
33import Data.Aeson.Types
53import Data.Attoparsec.ByteString
34import Data.Attoparsec.ByteString
54import Data.ByteString (ByteString)
35import Data.ByteString (ByteString)
55import Data.List
36import Data.List
56import Data.String.Conversions
38import Data.String.Conversions
57import Data.Time.Calendar
39import Data.Time.Calendar
58import GHC.Generics
40import GHC.Generics
@@ -64,15 +46,18 @@ import Servant
64import System.Directory
46import System.Directory
65import Text.Blaze
47import Text.Blaze
66import Text.Blaze.Html.Renderer.Utf8
48import Text.Blaze.Html.Renderer.Utf8
49+import Servant.Types.SourceT (source)
67import qualified Data.Aeson.Parser
50import qualified Data.Aeson.Parser
68import qualified Text.Blaze.Html
51import qualified Text.Blaze.Html
71−``` haskell ignore
54+**Important**: the `Servant` module comes from the **servant-server** package,
72−{-# LANGUAGE TypeFamilies #-}
55+the one that lets us run webservers that implement a particular API type. It
73−```
56+reexports all the types from the **servant** package that let you declare API
74−
57+types as well as everything you need to turn your request handlers into a
75−**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.
58+fully-fledged webserver. This means that in your applications, you can just add
59+**servant-server** as a dependency, import `Servant` and not worry about anything
77We will write a server that will serve the following API.
62We will write a server that will serve the following API.
@@ -95,7 +80,7 @@ data User = User
95 { name :: String
80 { name :: String
96 , age :: Int
81 , age :: Int
97 , email :: String
82 , email :: String
98− , registrationDate :: Day
83+ , registration_date :: Day
99 } deriving (Eq, Show, Generic)
84 } deriving (Eq, Show, Generic)
101instance ToJSON User
86instance ToJSON User
@@ -111,12 +96,6 @@ users1 =
114−Let's also write our API type.
117−type UserAPI1 = "users" :> Get '[JSON] [User]
120We can now take care of writing the actual webservice that will handle requests
99We can now take care of writing the actual webservice that will handle requests
121to such an API. This one will be very simple, being reduced to just a single
100to such an API. This one will be very simple, being reduced to just a single
122endpoint. The type of the web application is determined by the API type,
101endpoint. The type of the web application is determined by the API type,
@@ -128,11 +107,11 @@ corresponding API type.
128The first thing to know about the `Server` type family is that behind the
107The first thing to know about the `Server` type family is that behind the
129scenes it will drive the routing, letting you focus only on the business
108scenes it will drive the routing, letting you focus only on the business
130logic. The second thing to know is that for each endpoint, your handlers will
109logic. The second thing to know is that for each endpoint, your handlers will
131−by default run in the `ExceptT ServantErr IO` monad. This is overridable very
110+by default run in the `Handler` monad. This is overridable very
132easily, as explained near the end of this guide. Third thing, the type of the
111easily, as explained near the end of this guide. Third thing, the type of the
133value returned in that monad must be the same as the second argument of the
112value returned in that monad must be the same as the second argument of the
134HTTP method combinator used for the corresponding endpoint. In our case, it
113HTTP method combinator used for the corresponding endpoint. In our case, it
135−means we must provide a handler of type `ExceptT ServantErr IO [User]`. Well,
114+means we must provide a handler of type `Handler [User]`. Well,
136we have a monad, let's just `return` our list:
115we have a monad, let's just `return` our list:
138``` haskell
117``` haskell
@@ -140,7 +119,9 @@ server1 :: Server UserAPI1
140server1 = return users1
119server1 = return users1
143−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):
122+That's it. Now we can turn `server` into an actual webserver using
123+[wai](http://hackage.haskell.org/package/wai) and
124+[warp](http://hackage.haskell.org/package/warp):
145``` haskell
126``` haskell
146userAPI :: Proxy UserAPI1
127userAPI :: Proxy UserAPI1
@@ -150,7 +131,7 @@ userAPI = Proxy
150-- which you can think of as an "abstract" web application,
131-- which you can think of as an "abstract" web application,
151-- not yet a webserver.
132-- not yet a webserver.
152app1 :: Application
133app1 :: Application
153−app1 = serve userAPI EmptyConfig server1
134+app1 = serve userAPI server1
156The `userAPI` bit is, alas, boilerplate (we need it to guide type inference).
137The `userAPI` bit is, alas, boilerplate (we need it to guide type inference).
@@ -165,9 +146,8 @@ main = run 8081 app1
166You can put this all into a file or just grab [servant's
147You can put this all into a file or just grab [servant's
167repo](http://github.com/haskell-servant/servant) and look at the
148repo](http://github.com/haskell-servant/servant) and look at the
168−*servant-examples* directory. The code we have just explored is in
149+*doc/tutorial* directory. This code (the source of this web page) is in
169−*tutorial/T1.hs*, runnable with
150+*doc/tutorial/Server.lhs*.
170−`dist/build/tutorial/tutorial 1`.
172If you run it, you can go to `http://localhost:8081/users` in your browser or
152If you run it, you can go to `http://localhost:8081/users` in your browser or
173query it with curl and you see:
153query it with curl and you see:
@@ -179,7 +159,8 @@ $ curl http://localhost:8081/users
180## More endpoints
160## More endpoints
182−What if we want more than one endpoint? Let's add `/albert` and `/isaac` to view the corresponding users encoded in JSON.
162+What if we want more than one endpoint? Let's add `/albert` and `/isaac` to
163+view the corresponding users encoded in JSON.
184``` haskell
165``` haskell
185type UserAPI2 = "users" :> Get '[JSON] [User]
166type UserAPI2 = "users" :> Get '[JSON] [User]
@@ -202,7 +183,7 @@ users2 = [isaac, albert]
203Now, just like we separate the various endpoints in `UserAPI` with `:<|>`, we
184Now, just like we separate the various endpoints in `UserAPI` with `:<|>`, we
204are going to separate the handlers with `:<|>` too! They must be provided in
185are going to separate the handlers with `:<|>` too! They must be provided in
205−the same order as the one they appear in in the API type.
186+the same order as in the API type.
207``` haskell
188``` haskell
208server2 :: Server UserAPI2
189server2 :: Server UserAPI2
@@ -211,21 +192,21 @@ server2 = return users2
211 :<|> return isaac
192 :<|> return isaac
214−And that's it! You can run this example with
195+And that's it! You can run this example in the same way that we showed for
215−`dist/build/tutorial/tutorial 2` and check out the data available
216−at `/users`, `/albert` and `/isaac`.
196+`server1` and check out the data available at `/users`, `/albert` and `/isaac`.
218## From combinators to handler arguments
198## From combinators to handler arguments
220Fine, we can write trivial webservices easily, but none of the two above use
200Fine, we can write trivial webservices easily, but none of the two above use
221any "fancy" combinator from servant. Let's address this and use `QueryParam`,
201any "fancy" combinator from servant. Let's address this and use `QueryParam`,
222−`Capture` and `ReqBody` right away. You'll see how each occurence of these
202+`Capture` and `ReqBody` right away. You'll see how each occurrence of these
223combinators in an endpoint makes the corresponding handler receive an
203combinators in an endpoint makes the corresponding handler receive an
224argument of the appropriate type automatically. You don't have to worry about
204argument of the appropriate type automatically. You don't have to worry about
225manually looking up URL captures or query string parameters, or
205manually looking up URL captures or query string parameters, or
226decoding/encoding data from/to JSON. Never.
206decoding/encoding data from/to JSON. Never.
228−We are going to use the following data types and functions to implement a server for `API`.
208+We are going to use the following data types and functions to implement a
230``` haskell
211``` haskell
231type API = "position" :> Capture "x" Int :> Capture "y" Int :> Get '[JSON] Position
212type API = "position" :> Capture "x" Int :> Capture "y" Int :> Get '[JSON] Position
@@ -284,15 +265,15 @@ server3 = position
284 :<|> hello
265 :<|> hello
285 :<|> marketing
266 :<|> marketing
287− where position :: Int -> Int -> ExceptT ServantErr IO Position
268+ where position :: Int -> Int -> Handler Position
288 position x y = return (Position x y)
269 position x y = return (Position x y)
290− hello :: Maybe String -> ExceptT ServantErr IO HelloMessage
271+ hello :: Maybe String -> Handler HelloMessage
291 hello mname = return . HelloMessage $ case mname of
272 hello mname = return . HelloMessage $ case mname of
292 Nothing -> "Hello, anonymous coward"
273 Nothing -> "Hello, anonymous coward"
293 Just n -> "Hello, " ++ n
274 Just n -> "Hello, " ++ n
295− marketing :: ClientInfo -> ExceptT ServantErr IO Email
276+ marketing :: ClientInfo -> Handler Email
296 marketing clientinfo = return (emailForClient clientinfo)
277 marketing clientinfo = return (emailForClient clientinfo)
@@ -307,41 +288,40 @@ parameter might not always be there);
308 - a `ReqBody contentTypeList a` becomes an argument of type `a`;
289 - a `ReqBody contentTypeList a` becomes an argument of type `a`;
310−And that's it. You can see this example in action by running `dist/build/tutorial/tutorial 3`.
291+And that's it. Here's the example in action:
313$ curl http://localhost:8081/position/1/2
294$ curl http://localhost:8081/position/1/2
314−{"x":1,"y":2}
295+{"xCoord":1,"yCoord":2}
315$ curl http://localhost:8081/hello
296$ curl http://localhost:8081/hello
316{"msg":"Hello, anonymous coward"}
297{"msg":"Hello, anonymous coward"}
317$ curl http://localhost:8081/hello?name=Alp
298$ curl http://localhost:8081/hello?name=Alp
318{"msg":"Hello, Alp"}
299{"msg":"Hello, Alp"}
319−$ curl -X POST -d '{"name":"Alp Mestanogullari", "email" : "alp@foo.com", "age": 25, "interested_in": ["haskell", "mathematics"]}' -H 'Accept: application/json' -H 'Content-type: application/json' http://localhost:8081/marketing
300+$ 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
320{"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"}
301{"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"}
323−For reference, here's a list of some combinators from *servant* and for those
304+For reference, here's a list of some combinators from **servant**:
324−that get turned into arguments to the handlers, the type of the argument.
326− > - `Delete`, `Get`, `Patch`, `Post`, `Put`: these do not become arguments. They provide the return type of handlers, which usually is `ExceptT ServantErr IO <something>`.
306+ > - `Delete`, `Get`, `Patch`, `Post`, `Put`: these do not become arguments. They provide the return type of handlers, which usually is `Handler <something>`.
327 > - `Capture "something" a` becomes an argument of type `a`.
307 > - `Capture "something" a` becomes an argument of type `a`.
328− > - `QueryParam "something" a`, `MatrixParam "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.
308+ > - `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.
329− > - `QueryFlag "something"` and `MatrixFlag "something"` get turned into arguments of type `Bool`.
309+ > - `QueryFlag "something"` gets turned into an argument of type `Bool`.
330− > - `QueryParams "something" a` and `MatrixParams "something" a` get turned into arguments of type `[a]`.
310+ > - `QueryParams "something" a` gets turned into an argument of type `[a]`.
331 > - `ReqBody contentTypes a` gets turned into an argument of type `a`.
311 > - `ReqBody contentTypes a` gets turned into an argument of type `a`.
333## The `FromHttpApiData`/`ToHttpApiData` classes
313## The `FromHttpApiData`/`ToHttpApiData` classes
335−Wait... How does *servant* know how to decode the `Int`s from the URL? Or how
315+Wait... How does **servant** know how to decode the `Int`s from the URL? Or how
336−to decode a `ClientInfo` value from the request body? This is what this and the
316+to decode a `ClientInfo` value from the request body? The following three sections will
337−following two sections address.
317+help us answer these questions.
339`Capture`s and `QueryParam`s are represented by some textual value in URLs.
319`Capture`s and `QueryParam`s are represented by some textual value in URLs.
340`Header`s are similarly represented by a pair of a header name and a
320`Header`s are similarly represented by a pair of a header name and a
341corresponding (textual) value in the request's "metadata". How types are
321corresponding (textual) value in the request's "metadata". How types are
342decoded from headers, captures, and query params is expressed in a class
322decoded from headers, captures, and query params is expressed in a class
343`FromHttpApiData` (from the package
323`FromHttpApiData` (from the package
344−[*http-api-data*](http://hackage.haskell.org/package/http-api-data)):
324+[**http-api-data**](http://hackage.haskell.org/package/http-api-data)):
346``` haskell ignore
326``` haskell ignore
347class FromHttpApiData a where
327class FromHttpApiData a where
@@ -363,15 +343,15 @@ As you can see, as long as you provide either `parseUrlPiece` (for `Capture`s)
363or `parseQueryParam` (for `QueryParam`s), the other methods will be defined in
343or `parseQueryParam` (for `QueryParam`s), the other methods will be defined in
364terms of this.
344terms of this.
366−*http-api-data* provides a decent number of instances, helpers for defining new
346+**http-api-data** provides a decent number of instances, helpers for defining new
367ones, and wonderful documentation.
347ones, and wonderful documentation.
369There's not much else to say about these classes. You will need instances for
349There's not much else to say about these classes. You will need instances for
370them when using `Capture`, `QueryParam`, `QueryParams`, and `Header` with your
350them when using `Capture`, `QueryParam`, `QueryParams`, and `Header` with your
371types. You will need `FromHttpApiData` instances for server-side request
351types. You will need `FromHttpApiData` instances for server-side request
372handlers and `ToHttpApiData` instances only when using
352handlers and `ToHttpApiData` instances only when using
373−*servant-client*, as described in the [section about deriving haskell
353+**servant-client**, as described in the [section about deriving haskell
374−functions to query an API](/tutorial/client.html).
354+functions to query an API](Client.html).
376## Using content-types with your data types
356## Using content-types with your data types
@@ -379,14 +359,15 @@ The same principle was operating when decoding request bodies from JSON, and
379responses *into* JSON. (JSON is just the running example - you can do this with
359responses *into* JSON. (JSON is just the running example - you can do this with
380any content-type.)
360any content-type.)
382−This section introduces a couple of typeclasses provided by *servant* that make
362+This section introduces a couple of typeclasses provided by **servant** that make
383all of this work.
363all of this work.
385### The truth behind `JSON`
365### The truth behind `JSON`
368+What exactly is `JSON` (the type as used in `Get '[JSON] User`)? Like the 3
388−What exactly is `JSON`? Like the 3 other content types provided out of the box
369+other content-types provided out of the box by **servant**, it's a really dumb
389−by *servant*, it's a really dumb data type.
370+data type.
391``` haskell ignore
372``` haskell ignore
@@ -396,14 +377,15 @@ data OctetStream
398Obviously, this is not all there is to `JSON`, otherwise it would be quite
379Obviously, this is not all there is to `JSON`, otherwise it would be quite
399−pointless. Like most of the data types in *servant*, `JSON` is mostly there as
380+pointless. Like most of the data types in **servant**, `JSON` is mostly there as
400a special *symbol* that's associated with encoding (resp. decoding) to (resp.
381a special *symbol* that's associated with encoding (resp. decoding) to (resp.
401from) the *JSON* format. The way this association is performed can be
382from) the *JSON* format. The way this association is performed can be
402decomposed into two steps.
383decomposed into two steps.
404The first step is to provide a proper
385The first step is to provide a proper
405−[`MediaType`](https://hackage.haskell.org/package/http-media-0.6.2/docs/Network-HTTP-Media.html)
387+[**http-media**](https://hackage.haskell.org/package/http-media-0.6.2/docs/Network-HTTP-Media.html))
406−representation for `JSON`, or for your own content types. If you look at the
388+representation for `JSON`, or for your own content-types. If you look at the
407haddocks from this link, you can see that we just have to specify
389haddocks from this link, you can see that we just have to specify
408`application/json` using the appropriate functions. In our case, we can just
390`application/json` using the appropriate functions. In our case, we can just
409use `(//) :: ByteString -> ByteString -> MediaType`. The precise way to specify
391use `(//) :: ByteString -> ByteString -> MediaType`. The precise way to specify
@@ -419,14 +401,14 @@ instance Accept JSON where
421The second step is centered around the `MimeRender` and `MimeUnrender` classes.
403The second step is centered around the `MimeRender` and `MimeUnrender` classes.
422−These classes just let you specify a way to respectively encode and decode
404+These classes just let you specify a way to encode and decode
423−values respectively into or from your content-type's representation.
405+values into or from your content-type's representation.
425``` haskell ignore
407``` haskell ignore
426class Accept ctype => MimeRender ctype a where
408class Accept ctype => MimeRender ctype a where
427− mimeRender :: Proxy ctype -> a -> ByteString
409+ mimeRender :: Proxy ctype -> a -> ByteString
428 -- alternatively readable as:
410 -- alternatively readable as:
429− mimeRender :: Proxy ctype -> (a -> ByteString)
411+ mimeRender :: Proxy ctype -> (a -> ByteString)
432Given a content-type and some user type, `MimeRender` provides a function that
414Given a content-type and some user type, `MimeRender` provides a function that
@@ -452,7 +434,7 @@ class Accept ctype => MimeUnrender ctype a where
452We don't have much work to do there either, `Data.Aeson.eitherDecode` is
434We don't have much work to do there either, `Data.Aeson.eitherDecode` is
453precisely what we need. However, it only allows arrays and objects as toplevel
435precisely what we need. However, it only allows arrays and objects as toplevel
454JSON values and this has proven to get in our way more than help us so we wrote
436JSON values and this has proven to get in our way more than help us so we wrote
455−our own little function around *aeson* and *attoparsec* that allows any type of
437+our own little function around **aeson** and **attoparsec** that allows any type of
456JSON value at the toplevel of a "JSON document". Here's the definition in case
438JSON value at the toplevel of a "JSON document". Here's the definition in case
457you are curious.
439you are curious.
@@ -470,20 +452,20 @@ instance FromJSON a => MimeUnrender JSON a where
470 mimeUnrender _ = eitherDecodeLenient
452 mimeUnrender _ = eitherDecodeLenient
473−And this is all the code that lets you use `JSON` for with `ReqBody`, `Get`,
455+And this is all the code that lets you use `JSON` with `ReqBody`, `Get`,
474`Post` and friends. We can check our understanding by implementing support
456`Post` and friends. We can check our understanding by implementing support
475−for an `HTML` content type, so that users of your webservice can access an
457+for an `HTML` content-type, so that users of your webservice can access an
476HTML representation of the data they want, ready to be included in any HTML
458HTML representation of the data they want, ready to be included in any HTML
477−document, e.g. using [jQuery's `load` function](https://api.jquery.com/load/), simply by adding `Accept:
459+document, e.g. using [jQuery's `load` function](https://api.jquery.com/load/),
478−text/html` to their request headers.
460+simply by adding `Accept: text/html` to their request headers.
480−### Case-studies: *servant-blaze* and *servant-lucid*
462+### Case-studies: **servant-blaze** and **servant-lucid**
482These days, most of the haskellers who write their HTML UIs directly from
464These days, most of the haskellers who write their HTML UIs directly from
483−Haskell use either [blaze-html](http://hackage.haskell.org/package/blaze-html)
465+Haskell use either [**blaze-html**](http://hackage.haskell.org/package/blaze-html)
484−or [lucid](http://hackage.haskell.org/package/lucid). The best option for
466+or [**lucid**](http://hackage.haskell.org/package/lucid). The best option for
485−*servant* is obviously to support both (and hopefully other templating
467+**servant** is obviously to support both (and hopefully other templating
486−solutions!).
468+solutions!). We're first going to look at **lucid**:
488``` haskell
470``` haskell
489data HTMLLucid
471data HTMLLucid
@@ -491,24 +473,20 @@ data HTMLLucid
492Once again, the data type is just there as a symbol for the encoding/decoding
474Once again, the data type is just there as a symbol for the encoding/decoding
493functions, except that this time we will only worry about encoding since
475functions, except that this time we will only worry about encoding since
494−*blaze-html* and *lucid* don't provide a way to extract data from HTML.
476+**lucid** doesn't provide a way to extract data from HTML.
496−Both packages also have the same `Accept` instance for their `HTMLLucid` type.
498``` haskell
478``` haskell
499instance Accept HTMLLucid where
479instance Accept HTMLLucid where
500 contentType _ = "text" // "html" /: ("charset", "utf-8")
480 contentType _ = "text" // "html" /: ("charset", "utf-8")
503−Note that this instance uses the `(/:)` operator from *http-media* which lets
483+Note that this instance uses the `(/:)` operator from **http-media** which lets
504us specify additional information about a content-type, like the charset here.
484us specify additional information about a content-type, like the charset here.
506−The rendering instances for both packages both call similar functions that take
486+The rendering instances call similar functions that take
507types with an appropriate instance to an "abstract" HTML representation and
487types with an appropriate instance to an "abstract" HTML representation and
508then write that to a `ByteString`.
488then write that to a `ByteString`.
512``` haskell
490``` haskell
513instance ToHtml a => MimeRender HTMLLucid a where
491instance ToHtml a => MimeRender HTMLLucid a where
514 mimeRender _ = renderBS . toHtml
492 mimeRender _ = renderBS . toHtml
@@ -519,7 +497,7 @@ instance MimeRender HTMLLucid (Html a) where
519 mimeRender _ = renderBS
497 mimeRender _ = renderBS
522−For *blaze-html*:
500+For **blaze-html** everything works very similarly:
524``` haskell
502``` haskell
525-- For this tutorial to compile 'HTMLLucid' and 'HTMLBlaze' have to be
503-- For this tutorial to compile 'HTMLLucid' and 'HTMLBlaze' have to be
@@ -539,15 +517,13 @@ instance MimeRender HTMLBlaze Text.Blaze.Html.Html where
539 mimeRender _ = renderHtml
517 mimeRender _ = renderHtml
542−Both [servant-blaze](http://hackage.haskell.org/package/servant-blaze) and
520+Both [**servant-blaze**](http://hackage.haskell.org/package/servant-blaze) and
543−[servant-lucid](http://hackage.haskell.org/package/servant-lucid) let you use
521+[**servant-lucid**](http://hackage.haskell.org/package/servant-lucid) let you use
544−`HTMLLucid` in any content type list as long as you provide an instance of the
522+`HTMLLucid` and `HTMLBlaze` in any content-type list as long as you provide an instance of the
545−appropriate class (`ToMarkup` for *blaze-html*, `ToHtml` for *lucid*).
523+appropriate class (`ToMarkup` for **blaze-html**, `ToHtml` for **lucid**).
547−We can now write webservice that uses *servant-lucid* to show the `HTMLLucid`
525+We can now write a webservice that uses **servant-lucid** to show the `HTMLLucid`
548−content type in action. First off, imports and pragmas as usual.
550−We will be serving the following API:
526+content-type in action. We will be serving the following API:
552``` haskell
528``` haskell
553type PersonAPI = "persons" :> Get '[JSON, HTMLLucid] [Person]
529type PersonAPI = "persons" :> Get '[JSON, HTMLLucid] [Person]
@@ -564,7 +540,7 @@ data Person = Person
564instance ToJSON Person
540instance ToJSON Person
567−Now, let's teach *lucid* how to render a `Person` as a row in a table, and then
543+Now, let's teach **lucid** how to render a `Person` as a row in a table, and then
568a list of `Person`s as a table with a row per person.
544a list of `Person`s as a table with a row per person.
570``` haskell
546``` haskell
@@ -608,48 +584,45 @@ server4 :: Server PersonAPI
608server4 = return people
584server4 = return people
610app2 :: Application
586app2 :: Application
611−app2 = serve personAPI EmptyConfig server4
587+app2 = serve personAPI server4
614−And we're good to go. You can run this example with `dist/build/tutorial/tutorial 4`.
590+And we're good to go:
617− $ curl http://localhost:8081/persons
593+$ curl http://localhost:8081/persons
618− [{"lastName":"Newton","firstName":"Isaac"},{"lastName":"Einstein","firstName":"Albert"}]
594+[{"lastName":"Newton","firstName":"Isaac"},{"lastName":"Einstein","firstName":"Albert"}]
619− $ curl -H 'Accept: text/html' http://localhost:8081/persons
595+$ curl -H 'Accept: text/html' http://localhost:8081/persons
620− <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>
596+<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>
621− # or just point your browser to http://localhost:8081/persons
597+# or just point your browser to http://localhost:8081/persons
624−## The `ExceptT ServantErr IO` monad
600+## The `Handler` monad
626−At the heart of the handlers is the monad they run in, namely `ExceptT
602+At the heart of the handlers is the monad they run in, namely a newtype `Handler` around `ExceptT ServerError IO`
603+([haddock documentation for `ExceptT`](http://hackage.haskell.org/package/mtl-2.2.1/docs/Control-Monad-Except.html#t:ExceptT)).
627−ServantErr IO`. One might wonder: why this monad? The answer is that it is the
604+One might wonder: why this monad? The answer is that it is the
628simplest monad with the following properties:
605simplest monad with the following properties:
630−- it lets us both return a successful result (with the `Right` branch of
607+- it lets us both return a successful result (using `return`)
631−`Either`) or "fail" with a descriptive error (with the `Left` branch of
608+or "fail" with a descriptive error (using `throwError`);
633- it lets us perform IO, which is absolutely vital since most webservices exist
609- it lets us perform IO, which is absolutely vital since most webservices exist
634−as interfaces to databases that we interact with in `IO`;
610+as interfaces to databases that we interact with in `IO`.
636Let's recall some definitions.
612Let's recall some definitions.
638``` haskell ignore
614``` haskell ignore
640−data Either e a = Left e | Right a
642-- from the 'mtl' package at
615-- from the 'mtl' package at
643−newtype ExceptT e m a = ExceptT ( m (Either e a) )
616+newtype ExceptT e m a = ExceptT (m (Either e a))
646−In short, this means that a handler of type `ExceptT ServantErr IO a` is simply
619+In short, this means that a handler of type `Handler a` is simply
647−equivalent to a computation of type `IO (Either ServantErr a)`, that is, an IO
620+equivalent to a computation of type `IO (Either ServerError a)`, that is, an IO
648action that either returns an error or a result.
621action that either returns an error or a result.
650−The module [`Control.Monad.Except`](https://hackage.haskell.org/package/mtl-2.2.1/docs/Control-Monad-Except.html#t:ExceptT)
623+The module [`Control.Monad.Except`](https://hackage.haskell.org/package/mtl/docs/Control-Monad-Except.html#t:ExceptT)
651from which `ExceptT` comes is worth looking at.
624from which `ExceptT` comes is worth looking at.
652−Perhaps most importantly, `ExceptT` is an instance of `MonadError`, so
625+Perhaps most importantly, `ExceptT` and `Handler` are instances of `MonadError`, so
653`throwError` can be used to return an error from your handler (whereas `return`
626`throwError` can be used to return an error from your handler (whereas `return`
654 is enough to return a success).
627 is enough to return a success).
@@ -659,18 +632,17 @@ kind and abort early. The next two sections cover how to do just that.
660### Performing IO
633### Performing IO
662−Another important instance from the list above is `MonadIO m => MonadIO
635+Other important instances from the list above are `MonadIO m => MonadIO
663−(ExceptT e m)`.
636+(ExceptT e m)`, and therefore also `MonadIO Handler` as there is a `MonadIO IO` instance.
664−[`MonadIO`](http://hackage.haskell.org/package/transformers-0.4.3.0/docs/Control-Monad-IO-Class.html)
637+[`MonadIO`](http://hackage.haskell.org/package/base/docs/Control-Monad-IO-Class.html#t:MonadIO)
665−is a class from the *transformers* package defined as:
638+is a class from the **transformers** package defined as:
667``` haskell ignore
640``` haskell ignore
668class Monad m => MonadIO m where
641class Monad m => MonadIO m where
669 liftIO :: IO a -> m a
642 liftIO :: IO a -> m a
672−Obviously, the `IO` monad provides a `MonadIO` instance. Hence for any type
673−`e`, `ExceptT e IO` has a `MonadIO` instance. So if you want to run any kind of
645+So if you want to run any kind of
674IO computation in your handlers, just use `liftIO`:
646IO computation in your handlers, just use `liftIO`:
676``` haskell
648``` haskell
@@ -688,16 +660,16 @@ server5 = do
688 return (FileContent filecontent)
660 return (FileContent filecontent)
691−### Failing, through `ServantErr`
663+### Failing, through `ServerError`
693If you want to explicitly fail at providing the result promised by an endpoint
665If you want to explicitly fail at providing the result promised by an endpoint
694using the appropriate HTTP status code (not found, unauthorized, etc) and some
666using the appropriate HTTP status code (not found, unauthorized, etc) and some
695−error message, all you have to do is use the `left` function mentioned above
667+error message, all you have to do is use the `throwError` function mentioned above
696−and provide it with the appropriate value of type `ServantErr`, which is
668+and provide it with the appropriate value of type `ServerError`, which is
697defined as:
669defined as:
699``` haskell ignore
671``` haskell ignore
700−data ServantErr = ServantErr
672+data ServerError = ServerError
701 { errHTTPCode :: Int
673 { errHTTPCode :: Int
702 , errReasonPhrase :: String
674 , errReasonPhrase :: String
703 , errBody :: ByteString -- lazy bytestring
675 , errBody :: ByteString -- lazy bytestring
@@ -710,10 +682,10 @@ module. If you want to use these values but add a body or some headers, just
710use record update syntax:
682use record update syntax:
712``` haskell
684``` haskell
713−failingHandler :: ExceptT ServantErr IO ()
685+failingHandler :: Handler ()
714−failingHandler = throwE myerr
686+failingHandler = throwError myerr
716− where myerr :: ServantErr
688+ where myerr :: ServerError
717 myerr = err503 { errBody = "Sorry dear user." }
689 myerr = err503 { errBody = "Sorry dear user." }
@@ -726,47 +698,47 @@ server6 = do
726 exists <- liftIO (doesFileExist "myfile.txt")
698 exists <- liftIO (doesFileExist "myfile.txt")
727 if exists
699 if exists
728 then liftIO (readFile "myfile.txt") >>= return . FileContent
700 then liftIO (readFile "myfile.txt") >>= return . FileContent
729− else throwE custom404Err
701+ else throwError custom404Err
731 where custom404Err = err404 { errBody = "myfile.txt just isn't there, please leave this server alone." }
703 where custom404Err = err404 { errBody = "myfile.txt just isn't there, please leave this server alone." }
734−Let's run this server (`dist/build/tutorial/tutorial 5`) and
706+Here's how that server looks in action:
735−query it, first without the file and then with the file.
738− $ curl --verbose http://localhost:8081/myfile.txt
709+$ curl --verbose http://localhost:8081/myfile.txt
740− * Connected to localhost (127.0.0.1) port 8081 (#0)
711+* Connected to localhost (127.0.0.1) port 8081 (#0)
741− > GET /myfile.txt HTTP/1.1
712+> GET /myfile.txt HTTP/1.1
742− > User-Agent: curl/7.30.0
713+> User-Agent: curl/7.30.0
743− > Host: localhost:8081
714+> Host: localhost:8081
744− > Accept: */*
715+> Accept: */*
746− < HTTP/1.1 404 Not Found
717+< HTTP/1.1 404 Not Found
748− myfile.txt just isnt there, please leave this server alone.
719+myfile.txt just isn't there, please leave this server alone.
750− $ echo Hello > myfile.txt
721+$ echo Hello > myfile.txt
752− $ curl --verbose http://localhost:8081/myfile.txt
723+$ curl --verbose http://localhost:8081/myfile.txt
754− * Connected to localhost (127.0.0.1) port 8081 (#0)
725+* Connected to localhost (127.0.0.1) port 8081 (#0)
755− > GET /myfile.txt HTTP/1.1
726+> GET /myfile.txt HTTP/1.1
756− > User-Agent: curl/7.30.0
727+> User-Agent: curl/7.30.0
757− > Host: localhost:8081
728+> Host: localhost:8081
758− > Accept: */*
729+> Accept: */*
760− < HTTP/1.1 200 OK
731+< HTTP/1.1 200 OK
762− < Content-Type: application/json
733+< Content-Type: application/json
764− {"content":"Hello\n"}
735+{"content":"Hello\n"}
767## Response headers
738## Response headers
740+To add headers to your response, use
769−To add headers to your response, use [addHeader](http://hackage.haskell.org/package/servant-0.4.4/docs/Servant-API-ResponseHeaders.html).
741+[addHeader](http://hackage.haskell.org/package/servant/docs/Servant-API-ResponseHeaders.html).
770Note that this changes the type of your API, as we can see in the following example:
742Note that this changes the type of your API, as we can see in the following example:
772``` haskell
744``` haskell
@@ -776,159 +748,89 @@ myHandler :: Server MyHandler
776myHandler = return $ addHeader 1797 albert
748myHandler = return $ addHeader 1797 albert
779−Note that the type of `addHeader x` is different than the type of `x`!
751+Note that the type of `addHeader header x` is different than the type of `x`!
752+And if you add more headers, more headers will appear in the header list:
755+type MyHeadfulHandler = Get '[JSON] (Headers '[Header "X-A-Bool" Bool, Header "X-An-Int" Int] User)
757+myHeadfulHandler :: Server MyHeadfulHandler
758+myHeadfulHandler = return $ addHeader True $ addHeader 1797 albert
761+But what if your handler only *sometimes* adds a header? If you declare that
762+your handler adds headers, and you don't add one, the return type of your
763+handler will be different than expected. To solve this, you have to explicitly
764+*not* add a header by using `noHeader`:
767+type MyMaybeHeaderHandler
768+ = Capture "withHeader" Bool :> Get '[JSON] (Headers '[Header "X-An-Int" Int] User)
770+myMaybeHeaderHandler :: Server MyMaybeHeaderHandler
771+myMaybeHeaderHandler x = return $ if x then addHeader 1797 albert
772+ else noHeader albert
781## Serving static files
775## Serving static files
783−*servant-server* also provides a way to just serve the content of a directory
777+**servant-server** also provides a way to just serve the content of a directory
784under some path in your web API. As mentioned earlier in this document, the
778under some path in your web API. As mentioned earlier in this document, the
785`Raw` combinator can be used in your APIs to mean "plug here any WAI
779`Raw` combinator can be used in your APIs to mean "plug here any WAI
786−application". Well, servant-server provides a function to get a file and
780+application". Well, **servant-server** provides a function to get a file and
787directory serving WAI application, namely:
781directory serving WAI application, namely:
789``` haskell ignore
783``` haskell ignore
790-- exported by Servant and Servant.Server
784-- exported by Servant and Servant.Server
791−serveDirectory :: FilePath -> Server Raw
785+serveDirectoryWebApp :: FilePath -> Server Raw
794−`serveDirectory`'s argument must be a path to a valid directory. You can see an
788+`serveDirectoryWebApp`'s argument must be a path to a valid directory.
795−example below, runnable with `dist/build/tutorial/tutorial 6`
796−(you **must** run it from within the *servant-examples/* directory!), which is
797−a webserver that serves the various bits of code covered in this
800−The API type will be the following.
790+Here's an example API that will serve some static files:
802``` haskell
792``` haskell
803−type CodeAPI = "code" :> Raw
793+type StaticAPI = "static" :> Raw
806And the server:
796And the server:
808``` haskell
798``` haskell
809−codeAPI :: Proxy CodeAPI
799+staticAPI :: Proxy StaticAPI
810−codeAPI = Proxy
800+staticAPI = Proxy
813``` haskell
803``` haskell
814−server7 :: Server CodeAPI
804+server7 :: Server StaticAPI
815−server7 = serveDirectory "tutorial"
805+server7 = serveDirectoryWebApp "static-files"
817app3 :: Application
807app3 :: Application
818−app3 = serve codeAPI EmptyConfig server7
808+app3 = serve staticAPI server7
821−This server will match any request whose path starts with `/code` and will look
811+This server will match any request whose path starts with `/static` and will look
822for a file at the path described by the rest of the request path, inside the
812for a file at the path described by the rest of the request path, inside the
823− *tutorial/* directory of the path you run the program from.
813+ *static-files/* directory of the path you run the program from.
827−- If a client requests `/code/foo.txt`, the server will look for a file at `./tutorial/foo.txt` (and fail)
828−- If a client requests `/code/T1.hs`, the server will look for a file at `./tutorial/T1.hs` (and succeed)
829−- If a client requests `/code/foo/bar/baz/movie.mp4`, the server will look for a file at `./tutorial/foo/bar/baz/movie.mp4` (and fail)
831−Here is our little server in action.
834−$ curl http://localhost:8081/code/T1.hs
835−{-# LANGUAGE DataKinds #-}
836−{-# LANGUAGE TypeFamilies #-}
837−{-# LANGUAGE DeriveGeneric #-}
838−{-# LANGUAGE TypeOperators #-}
841−import Data.Aeson
815+In other words: If a client requests `/static/foo.txt`, the server will look for a file at
842−import Data.Time.Calendar
816+`./static-files/foo.txt`. If that file exists it'll succeed and serve the file.
843−import GHC.Generics
817+If it doesn't exist, the handler will fail with a `404` status code.
851− , registration_date :: Day
852− } deriving (Eq, Show, Generic)
854−-- orphan ToJSON instance for Day. necessary to derive one for User
819+`serveDirectoryWebApp` uses some standard settings that fit the use case of
855−instance ToJSON Day where
820+serving static files for most web apps. You can find out about the other
856− -- display a day in YYYY-mm-dd format
821+options in the documentation of the `Servant.Server.StaticFiles` module.
857− toJSON d = toJSON (showGregorian d)
861−type UserAPI = "users" :> Get '[JSON] [User]
865− [ User "Isaac Newton" 372 "isaac@newton.co.uk" (fromGregorian 1683 3 1)
866− , User "Albert Einstein" 136 "ae@mc2.org" (fromGregorian 1905 12 1)
869−userAPI :: Proxy UserAPI
872−server :: Server UserAPI
873−server = return users
876−app = serve userAPI server
877−$ curl http://localhost:8081/code/tutorial.hs
879−import Network.Wai.Handler.Warp
880−import System.Environment
892−app :: String -> (Application -> IO ()) -> IO ()
902− "9" -> T9.writeJSFiles >> f T9.app
910− [n] -> app n (run 8081)
915− putStrLn "Usage:\t tutorial N"
916− putStrLn "\t\twhere N is the number of the example you want to run."
918−$ curl http://localhost:8081/foo
922## Nested APIs
823## Nested APIs
924−Let's see how you can define APIs in a modular way, while avoiding repetition. Consider this simple example:
825+Let's see how you can define APIs in a modular way, while avoiding repetition.
826+Consider this simple example:
926``` haskell
828``` haskell
927type UserAPI3 = -- view the user with given userid, in JSON
829type UserAPI3 = -- view the user with given userid, in JSON
928 Capture "userid" Int :> Get '[JSON] User
830 Capture "userid" Int :> Get '[JSON] User
930 :<|> -- delete the user with given userid. empty response
832 :<|> -- delete the user with given userid. empty response
931− Capture "userid" Int :> Delete '[] ()
833+ Capture "userid" Int :> DeleteNoContent
934We can instead factor out the `userid`:
836We can instead factor out the `userid`:
@@ -936,32 +838,34 @@ We can instead factor out the `userid`:
936``` haskell
838``` haskell
937type UserAPI4 = Capture "userid" Int :>
839type UserAPI4 = Capture "userid" Int :>
938 ( Get '[JSON] User
840 ( Get '[JSON] User
939− :<|> Delete '[] ()
841+ :<|> DeleteNoContent
943−However, you have to be aware that this has an effect on the type of the corresponding `Server`:
845+However, you have to be aware that this has an effect on the type of the
846+corresponding `Server`:
945``` haskell ignore
848``` haskell ignore
946−Server UserAPI3 = (Int -> ExceptT ServantErr IO User)
849+Server UserAPI3 = (Int -> Handler User)
947− :<|> (Int -> ExceptT ServantErr IO ())
850+ :<|> (Int -> Handler NoContent)
949−Server UserAPI4 = Int -> ( ExceptT ServantErr IO User
852+Server UserAPI4 = Int -> ( Handler User
950− :<|> ExceptT ServantErr IO ()
853+ :<|> Handler NoContent
954In the first case, each handler receives the *userid* argument. In the latter,
857In the first case, each handler receives the *userid* argument. In the latter,
955−the whole `Server` takes the *userid* and has handlers that are just computations in `ExceptT`, with no arguments. In other words:
858+the whole `Server` takes the *userid* and has handlers that are just
859+computations in `Handler`, with no arguments. In other words:
957``` haskell
861``` haskell
958server8 :: Server UserAPI3
862server8 :: Server UserAPI3
959server8 = getUser :<|> deleteUser
863server8 = getUser :<|> deleteUser
961− where getUser :: Int -> ExceptT ServantErr IO User
865+ where getUser :: Int -> Handler User
962 getUser _userid = error "..."
866 getUser _userid = error "..."
964− deleteUser :: Int -> ExceptT ServantErr IO ()
868+ deleteUser :: Int -> Handler NoContent
965 deleteUser _userid = error "..."
869 deleteUser _userid = error "..."
967-- notice how getUser and deleteUser
871-- notice how getUser and deleteUser
@@ -970,14 +874,17 @@ server8 = getUser :<|> deleteUser
970server9 :: Server UserAPI4
874server9 :: Server UserAPI4
971server9 userid = getUser userid :<|> deleteUser userid
875server9 userid = getUser userid :<|> deleteUser userid
973− where getUser :: Int -> ExceptT ServantErr IO User
877+ where getUser :: Int -> Handler User
974 getUser = error "..."
878 getUser = error "..."
976− deleteUser :: Int -> ExceptT ServantErr IO ()
880+ deleteUser :: Int -> Handler NoContent
977 deleteUser = error "..."
881 deleteUser = error "..."
980−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`.
884+Note that there's nothing special about `Capture` that lets you "factor it
885+out": this can be done with any combinator. Here are a few examples of APIs
886+with a combinator factored out for which we can write a perfectly valid
982``` haskell
889``` haskell
983-- we just factor out the "users" path fragment
890-- we just factor out the "users" path fragment
@@ -989,62 +896,63 @@ type API1 = "users" :>
989-- we factor out the Request Body
896-- we factor out the Request Body
990type API2 = ReqBody '[JSON] User :>
897type API2 = ReqBody '[JSON] User :>
991 ( Get '[JSON] User -- just display the same user back, don't register it
898 ( Get '[JSON] User -- just display the same user back, don't register it
992− :<|> Post '[JSON] () -- register the user. empty response
899+ :<|> PostNoContent -- register the user. empty response
995-- we factor out a Header
902-- we factor out a Header
996type API3 = Header "Authorization" Token :>
903type API3 = Header "Authorization" Token :>
997 ( Get '[JSON] SecretData -- get some secret data, if authorized
904 ( Get '[JSON] SecretData -- get some secret data, if authorized
998− :<|> ReqBody '[JSON] SecretData :> Post '[] () -- add some secret data, if authorized
905+ :<|> ReqBody '[JSON] SecretData :> PostNoContent -- add some secret data, if authorized
1001newtype Token = Token ByteString
908newtype Token = Token ByteString
1002newtype SecretData = SecretData ByteString
909newtype SecretData = SecretData ByteString
1005−This approach lets you define APIs modularly and assemble them all into one big API type only at the end.
912+This approach lets you define APIs modularly and assemble them all into one big
913+API type only at the end.
1007``` haskell
915``` haskell
1008type UsersAPI =
916type UsersAPI =
1009 Get '[JSON] [User] -- list users
917 Get '[JSON] [User] -- list users
1010− :<|> ReqBody '[JSON] User :> Post '[] () -- add a user
918+ :<|> ReqBody '[JSON] User :> PostNoContent -- add a user
1011 :<|> Capture "userid" Int :>
919 :<|> Capture "userid" Int :>
1012 ( Get '[JSON] User -- view a user
920 ( Get '[JSON] User -- view a user
1013− :<|> ReqBody '[JSON] User :> Put '[] () -- update a user
921+ :<|> ReqBody '[JSON] User :> PutNoContent -- update a user
1014− :<|> Delete '[] () -- delete a user
922+ :<|> DeleteNoContent -- delete a user
1017usersServer :: Server UsersAPI
925usersServer :: Server UsersAPI
1018usersServer = getUsers :<|> newUser :<|> userOperations
926usersServer = getUsers :<|> newUser :<|> userOperations
1020− where getUsers :: ExceptT ServantErr IO [User]
928+ where getUsers :: Handler [User]
1021 getUsers = error "..."
929 getUsers = error "..."
1023− newUser :: User -> ExceptT ServantErr IO ()
931+ newUser :: User -> Handler NoContent
1024 newUser = error "..."
932 newUser = error "..."
1026 userOperations userid =
934 userOperations userid =
1027 viewUser userid :<|> updateUser userid :<|> deleteUser userid
935 viewUser userid :<|> updateUser userid :<|> deleteUser userid
1030− viewUser :: Int -> ExceptT ServantErr IO User
938+ viewUser :: Int -> Handler User
1031 viewUser = error "..."
939 viewUser = error "..."
1033− updateUser :: Int -> User -> ExceptT ServantErr IO ()
941+ updateUser :: Int -> User -> Handler NoContent
1034 updateUser = error "..."
942 updateUser = error "..."
1036− deleteUser :: Int -> ExceptT ServantErr IO ()
944+ deleteUser :: Int -> Handler NoContent
1037 deleteUser = error "..."
945 deleteUser = error "..."
1040``` haskell
948``` haskell
1041type ProductsAPI =
949type ProductsAPI =
1042 Get '[JSON] [Product] -- list products
950 Get '[JSON] [Product] -- list products
1043− :<|> ReqBody '[JSON] Product :> Post '[] () -- add a product
951+ :<|> ReqBody '[JSON] Product :> PostNoContent -- add a product
1044 :<|> Capture "productid" Int :>
952 :<|> Capture "productid" Int :>
1045 ( Get '[JSON] Product -- view a product
953 ( Get '[JSON] Product -- view a product
1046− :<|> ReqBody '[JSON] Product :> Put '[] () -- update a product
954+ :<|> ReqBody '[JSON] Product :> PutNoContent -- update a product
1047− :<|> Delete '[] () -- delete a product
955+ :<|> DeleteNoContent -- delete a product
1050data Product = Product { productId :: Int }
958data Product = Product { productId :: Int }
@@ -1052,23 +960,23 @@ data Product = Product { productId :: Int }
1052productsServer :: Server ProductsAPI
960productsServer :: Server ProductsAPI
1053productsServer = getProducts :<|> newProduct :<|> productOperations
961productsServer = getProducts :<|> newProduct :<|> productOperations
1055− where getProducts :: ExceptT ServantErr IO [Product]
963+ where getProducts :: Handler [Product]
1056 getProducts = error "..."
964 getProducts = error "..."
1058− newProduct :: Product -> ExceptT ServantErr IO ()
966+ newProduct :: Product -> Handler NoContent
1059 newProduct = error "..."
967 newProduct = error "..."
1061 productOperations productid =
969 productOperations productid =
1062 viewProduct productid :<|> updateProduct productid :<|> deleteProduct productid
970 viewProduct productid :<|> updateProduct productid :<|> deleteProduct productid
1065− viewProduct :: Int -> ExceptT ServantErr IO Product
973+ viewProduct :: Int -> Handler Product
1066 viewProduct = error "..."
974 viewProduct = error "..."
1068− updateProduct :: Int -> Product -> ExceptT ServantErr IO ()
976+ updateProduct :: Int -> Product -> Handler NoContent
1069 updateProduct = error "..."
977 updateProduct = error "..."
1071− deleteProduct :: Int -> ExceptT ServantErr IO ()
979+ deleteProduct :: Int -> Handler NoContent
1072 deleteProduct = error "..."
980 deleteProduct = error "..."
@@ -1080,135 +988,213 @@ server10 :: Server CombinedAPI
1080server10 = usersServer :<|> productsServer
988server10 = usersServer :<|> productsServer
1083−Finally, we can realize the user and product APIs are quite similar and abstract that away:
991+Finally, we can realize the user and product APIs are quite similar and
1085``` haskell
994``` haskell
1086-- API for values of type 'a'
995-- API for values of type 'a'
1087-- indexed by values of type 'i'
996-- indexed by values of type 'i'
1088type APIFor a i =
997type APIFor a i =
1089 Get '[JSON] [a] -- list 'a's
998 Get '[JSON] [a] -- list 'a's
1090− :<|> ReqBody '[JSON] a :> Post '[] () -- add an 'a'
999+ :<|> ReqBody '[JSON] a :> PostNoContent -- add an 'a'
1091 :<|> Capture "id" i :>
1000 :<|> Capture "id" i :>
1092 ( Get '[JSON] a -- view an 'a' given its "identifier" of type 'i'
1001 ( Get '[JSON] a -- view an 'a' given its "identifier" of type 'i'
1093− :<|> ReqBody '[JSON] a :> Put '[] () -- update an 'a'
1002+ :<|> ReqBody '[JSON] a :> PutNoContent -- update an 'a'
1094− :<|> Delete '[] () -- delete an 'a'
1003+ :<|> DeleteNoContent -- delete an 'a'
1097-- Build the appropriate 'Server'
1006-- Build the appropriate 'Server'
1098-- given the handlers of the right type.
1007-- given the handlers of the right type.
1099−serverFor :: ExceptT ServantErr IO [a] -- handler for listing of 'a's
1008+serverFor :: Handler [a] -- handler for listing of 'a's
1100− -> (a -> ExceptT ServantErr IO ()) -- handler for adding an 'a'
1009+ -> (a -> Handler NoContent) -- handler for adding an 'a'
1101− -> (i -> ExceptT ServantErr IO a) -- handler for viewing an 'a' given its identifier of type 'i'
1010+ -> (i -> Handler a) -- handler for viewing an 'a' given its identifier of type 'i'
1102− -> (i -> a -> ExceptT ServantErr IO ()) -- updating an 'a' with given id
1011+ -> (i -> a -> Handler NoContent) -- updating an 'a' with given id
1103− -> (i -> ExceptT ServantErr IO ()) -- deleting an 'a' given its id
1012+ -> (i -> Handler NoContent) -- deleting an 'a' given its id
1104 -> Server (APIFor a i)
1013 -> Server (APIFor a i)
1105serverFor = error "..."
1014serverFor = error "..."
1106-- implementation left as an exercise. contact us on IRC
1015-- implementation left as an exercise. contact us on IRC
1107-- or the mailing list if you get stuck!
1016-- or the mailing list if you get stuck!
1019+When your API contains the `EmptyAPI` combinator, you'll want to use
1020+`emptyServer` in the corresponding slot for your server, which will simply fail
1021+with 404 whenever a request reaches it:
1024+type CombinedAPI2 = API :<|> "empty" :> EmptyAPI
1026+server11 :: Server CombinedAPI2
1027+server11 = server3 :<|> emptyServer
1110## Using another monad for your handlers
1030## Using another monad for your handlers
1112−Remember how `Server` turns combinators for HTTP methods into `ExceptT ServantErr IO`? Well, actually, there's more to that. `Server` is actually a simple type synonym.
1032+Remember how `Server` turns combinators for HTTP methods into `Handler`? Well, actually, there's more to that. `Server` is actually a
1033+simple type synonym.
1114``` haskell ignore
1035``` haskell ignore
1115−type Server api = ServerT api (ExceptT ServantErr IO)
1036+type Server api = ServerT api Handler
1118−`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 a third 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`.
1039+`ServerT` is the actual type family that computes the required types for the
1040+handlers that's part of the `HasServer` class. It's like `Server` except that
1041+it takes another parameter which is the monad you want your handlers to run in,
1042+or more generally the return types of your handlers. This third parameter is
1043+used for specifying the return type of the handler for an endpoint, e.g when
1044+computing `ServerT (Get '[JSON] Person) SomeMonad`. The result would be
1120−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?
1047+The first and main question one might have then is: how do we write handlers
1048+that run in another monad? How can we "bring back" the value from a given monad
1049+into something **servant** can understand?
1122### Natural transformations
1051### Natural transformations
1124If we have a function that gets us from an `m a` to an `n a`, for any `a`, what
1053If we have a function that gets us from an `m a` to an `n a`, for any `a`, what
1125do we have?
1054do we have?
1127−``` haskell ignore
1056+``` haskell
1128−newtype m :~> n = Nat { unNat :: forall a. m a -> n a}
1057+type (~>) m n = forall a. m a -> n a
1130−-- For example
1062+``` haskell
1131−-- listToMaybeNat ::`[] :~> Maybe`
1063+listToMaybe' :: [] ~> Maybe
1132−-- listToMaybeNat = Nat listToMaybe -- from Data.Maybe
1064+listToMaybe' = listToMaybe -- from Data.Maybe
1134−(`Nat` comes from "natural transformation", in case you're wondering.)
1067+Note that `servant` doesn't declare the `~>` type-alias, as the unfolded
1068+variant isn't much longer to write, as we'll see shortly.
1136−So if you want to write handlers using another monad/type than `ExceptT
1070+So if you want to write handlers using another monad/type than `Handler`, say the `Reader String` monad, the first thing you have to
1137−ServantErr IO`, say the `Reader String` monad, the first thing you have to
1138prepare is a function:
1071prepare is a function:
1140``` haskell ignore
1073``` haskell ignore
1141−readerToEither :: Reader String :~> ExceptT ServantErr IO
1074+readerToHandler :: Reader String a -> Handler a
1144−Let's start with `readerToEither'`. We obviously have to run the `Reader`
1145−computation by supplying it with a `String`, like `"hi"`. We get an `a` out
1077+We obviously have to run the `Reader` computation by supplying it with a
1146−from that and can then just `return` it into `ExceptT`. We can then just wrap
1078+`String`, like `"hi"`. We get an `a` out from that and can then just `return`
1147−that function with the `Nat` constructor to make it have the fancier type.
1079+it into `Handler`.
1149``` haskell
1081``` haskell
1150−readerToEither' :: forall a. Reader String a -> ExceptT ServantErr IO a
1082+readerToHandler :: Reader String a -> Handler a
1151−readerToEither' r = return (runReader r "hi")
1083+readerToHandler r = return (runReader r "hi")
1153−readerToEither :: Reader String :~> ExceptT ServantErr IO
1154−readerToEither = Nat readerToEither'
1157We can write some simple webservice with the handlers running in `Reader String`.
1086We can write some simple webservice with the handlers running in `Reader String`.
1159``` haskell
1088``` haskell
1160type ReaderAPI = "a" :> Get '[JSON] Int
1089type ReaderAPI = "a" :> Get '[JSON] Int
1161− :<|> "b" :> Get '[JSON] String
1090+ :<|> "b" :> ReqBody '[JSON] Double :> Get '[JSON] Bool
1163readerAPI :: Proxy ReaderAPI
1092readerAPI :: Proxy ReaderAPI
1164readerAPI = Proxy
1093readerAPI = Proxy
1166readerServerT :: ServerT ReaderAPI (Reader String)
1095readerServerT :: ServerT ReaderAPI (Reader String)
1167−readerServerT = a :<|> b
1096+readerServerT = a :<|> b where
1169− where a :: Reader String Int
1097+ a :: Reader String Int
1170− a = return 1797
1098+ a = return 1797
1172− b :: Reader String String
1100+ b :: Double -> Reader String Bool
1173− b = ask
1101+ b _ = asks (== "hi")
1176We unfortunately can't use `readerServerT` as an argument of `serve`, because
1104We unfortunately can't use `readerServerT` as an argument of `serve`, because
1177−`serve` wants a `Server ReaderAPI`, i.e., with handlers running in `ExceptT
1105+`serve` wants a `Server ReaderAPI`, i.e., with handlers running in `Handler`. But there's a simple solution to this.
1178−ServantErr IO`. But there's a simple solution to this.
1180−### Enter `enter`
1107+### Welcome `hoistServer`
1182−That's right. We have just written `readerToEither`, which is exactly what we
1109+That's right. We have just written `readerToHandler`, which is exactly what we
1183−would need to apply to the results of all handlers to make the handlers have the
1110+would need to apply to all handlers to make the handlers have the
1184right type for `serve`. Being cumbersome to do by hand, we provide a function
1111right type for `serve`. Being cumbersome to do by hand, we provide a function
1185−`enter` which takes a natural transformation between two parametrized types `m`
1112+`hoistServer` which takes a natural transformation between two parameterized types `m`
1186and `n` and a `ServerT someapi m`, and returns a `ServerT someapi n`.
1113and `n` and a `ServerT someapi m`, and returns a `ServerT someapi n`.
1188−In our case, we can wrap up our little webservice by using `enter readerToEither` on our handlers.
1115+In our case, we can wrap up our little webservice by using
1116+`hoistServer readerAPI readerToHandler` on our handlers.
1190``` haskell
1118``` haskell
1191readerServer :: Server ReaderAPI
1119readerServer :: Server ReaderAPI
1192−readerServer = enter readerToEither readerServerT
1120+readerServer = hoistServer readerAPI readerToHandler readerServerT
1194app4 :: Application
1122app4 :: Application
1195−app4 = serve readerAPI EmptyConfig readerServer
1123+app4 = serve readerAPI readerServer
1198−And we can indeed see this webservice in action by running `dist/build/tutorial/tutorial 7`.
1126+This is the webservice in action:
1201$ curl http://localhost:8081/a
1129$ curl http://localhost:8081/a
1203−$ curl http://localhost:8081/b
1131+$ curl http://localhost:8081/b -X GET -d '42.0' -H 'Content-Type: application/json'
1207−## Conclusion
1135+### An arrow is a reader too.
1137+In previous versions of `servant` we had an `enter` to do what `hoistServer`
1138+does now. `enter` had an ambitious design goals, but was problematic in practice.
1140+One problematic situation was when the source monad was `(->) r`, yet it's
1141+handy in practice, because `(->) r` is isomorphic to `Reader r`.
1143+We can rewrite the previous example without `Reader`:
1146+funServerT :: ServerT ReaderAPI ((->) String)
1147+funServerT = a :<|> b where
1151+ -- unfortunately, we cannot make `String` the first argument.
1152+ b :: Double -> String -> Bool
1209−You're now equipped to write any kind of webservice/web-application using *servant*. One thing not covered here is how to incorporate your own combinators and will be the topic of a page on the website. The rest of this document focuses on *servant-client*, *servant-jquery* and *servant-docs*.
1155+funToHandler :: (String -> a) -> Handler a
1156+funToHandler f = return (f "hi")
1159+app5 = serve readerAPI (hoistServer readerAPI funToHandler funServerT)
1162+## Streaming endpoints
1164+We can create endpoints that don't just give back a single result, but give
1165+back a *stream* of results, served one at a time. Stream endpoints only provide
1166+a single content type, and also specify what framing strategy is used to
1167+delineate the results. To serve these results, we need to give back a stream
1168+producer. Adapters can be written to *Pipes*, *Conduit* and the like, or
1169+written directly as `SourceIO`s. SourceIO builds upon servant's own `SourceT`
1170+stream type (it's simpler than *Pipes* or *Conduit*).
1171+The API of a streaming endpoint needs to explicitly specify which sort of
1172+generator it produces. Note that the generator itself is returned by a
1173+`Handler` action, so that additional IO may be done in the creation of one.
1176+type StreamAPI = "userStream" :> StreamGet NewlineFraming JSON (SourceIO User)
1177+streamAPI :: Proxy StreamAPI
1180+streamUsers :: SourceIO User
1181+streamUsers = source [isaac, albert, albert]
1184+app6 = serve streamAPI (return streamUsers)
1187+This simple application returns a stream of `User` values encoded in JSON
1188+format, with each value separated by a newline. In this case, the stream will
1189+consist of the value of `isaac`, followed by the value of `albert`, then the
1190+value of `albert` a second time. Importantly, the stream is written back as
1191+results are produced, rather than all at once. This means first that results
1192+are delivered when they are available, and second, that if an exception
1193+interrupts production of the full stream, nonetheless partial results have
1194+already been written back.
1211−<div style="text-align: center;">
1198+You're now equipped to write webservices/web-applications using
1212− <p><a href="/tutorial/api-type.html">Previous page: A web API as a type</a></p>
1199+**servant**. The rest of this document focuses on **servant-client**,
1213− <p><a href="/tutorial/client.html">Next page: Deriving Haskell functions to query an API</a></p>
1200+**servant-js** and **servant-docs**.