@@ -5,7 +5,7 @@ type. Can we have a webservice already?
6## A first example
6## A first example
8−Equipped with some basic knowledge about the way we represent API, let's now
8+Equipped with some basic knowledge about the way we represent APIs, let's now
9write our first webservice.
9write our first webservice.
11The 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
@@ -18,6 +18,7 @@ need to have some language extensions and imports:
18{-# LANGUAGE GeneralizedNewtypeDeriving #-}
18{-# LANGUAGE GeneralizedNewtypeDeriving #-}
19{-# LANGUAGE MultiParamTypeClasses #-}
19{-# LANGUAGE MultiParamTypeClasses #-}
20{-# LANGUAGE OverloadedStrings #-}
20{-# LANGUAGE OverloadedStrings #-}
21+{-# LANGUAGE RankNTypes #-}
21{-# LANGUAGE ScopedTypeVariables #-}
22{-# LANGUAGE ScopedTypeVariables #-}
22{-# LANGUAGE TypeOperators #-}
23{-# LANGUAGE TypeOperators #-}
@@ -26,14 +27,14 @@ module Server where
26import Prelude ()
27import Prelude ()
27import Prelude.Compat
28import Prelude.Compat
29−import Control.Monad.IO.Class
30+import Control.Monad.Except
30import Control.Monad.Reader
31import Control.Monad.Reader
31−import Control.Monad.Trans.Except
32import Data.Aeson
32import Data.Aeson
33import Data.Aeson.Types
33import Data.Aeson.Types
34import Data.Attoparsec.ByteString
34import Data.Attoparsec.ByteString
35import Data.ByteString (ByteString)
35import Data.ByteString (ByteString)
36import Data.List
36import Data.List
37import Data.String.Conversions
38import Data.String.Conversions
38import Data.Time.Calendar
39import Data.Time.Calendar
39import GHC.Generics
40import GHC.Generics
@@ -45,20 +46,17 @@ import Servant
45import System.Directory
46import System.Directory
46import Text.Blaze
47import Text.Blaze
47import Text.Blaze.Html.Renderer.Utf8
48import Text.Blaze.Html.Renderer.Utf8
49+import Servant.Types.SourceT (source)
48import qualified Data.Aeson.Parser
50import qualified Data.Aeson.Parser
49import qualified Text.Blaze.Html
51import qualified Text.Blaze.Html
53−{-# LANGUAGE TypeFamilies #-}
56−**Important**: the `Servant` module comes from the *servant-server* package,
54+**Important**: the `Servant` module comes from the **servant-server** package,
57the one that lets us run webservers that implement a particular API type. It
55the one that lets us run webservers that implement a particular API type. It
58−reexports all the types from the *servant* package that let you declare API
56+reexports all the types from the **servant** package that let you declare API
59types as well as everything you need to turn your request handlers into a
57types as well as everything you need to turn your request handlers into a
60fully-fledged webserver. This means that in your applications, you can just add
58fully-fledged webserver. This means that in your applications, you can just add
61−*servant-server* as a dependency, import `Servant` and not worry about anything
59+**servant-server** as a dependency, import `Servant` and not worry about anything
64We will write a server that will serve the following API.
62We will write a server that will serve the following API.
@@ -98,12 +96,6 @@ users1 =
101−Let's also write our API type.
104−type UserAPI1 = "users" :> Get '[JSON] [User]
107We 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
108to 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
109endpoint. 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,
@@ -115,11 +107,11 @@ corresponding API type.
115The 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
116scenes 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
117logic. 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
118−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
119easily, 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
120value 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
121HTTP method combinator used for the corresponding endpoint. In our case, it
113HTTP method combinator used for the corresponding endpoint. In our case, it
122−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,
123we have a monad, let's just `return` our list:
115we have a monad, let's just `return` our list:
125``` haskell
117``` haskell
@@ -139,7 +131,7 @@ userAPI = Proxy
139-- which you can think of as an "abstract" web application,
131-- which you can think of as an "abstract" web application,
140-- not yet a webserver.
132-- not yet a webserver.
141app1 :: Application
133app1 :: Application
142−app1 = serve userAPI EmptyConfig server1
134+app1 = serve userAPI server1
145The `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).
@@ -154,9 +146,8 @@ main = run 8081 app1
155You 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
156repo](http://github.com/haskell-servant/servant) and look at the
148repo](http://github.com/haskell-servant/servant) and look at the
157−*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
158−*tutorial/T1.hs*, runnable with
150+*doc/tutorial/Server.lhs*.
159−`dist/build/tutorial/tutorial 1`.
161If 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
162query it with curl and you see:
153query it with curl and you see:
@@ -192,7 +183,7 @@ users2 = [isaac, albert]
193Now, just like we separate the various endpoints in `UserAPI` with `:<|>`, we
184Now, just like we separate the various endpoints in `UserAPI` with `:<|>`, we
194are 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
195−the same order as the one they appear in in the API type.
186+the same order as in the API type.
197``` haskell
188``` haskell
198server2 :: Server UserAPI2
189server2 :: Server UserAPI2
@@ -201,15 +192,14 @@ server2 = return users2
201 :<|> return isaac
192 :<|> return isaac
204−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
205−`dist/build/tutorial/tutorial 2` and check out the data available
206−at `/users`, `/albert` and `/isaac`.
196+`server1` and check out the data available at `/users`, `/albert` and `/isaac`.
208## From combinators to handler arguments
198## From combinators to handler arguments
210Fine, 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
211any "fancy" combinator from servant. Let's address this and use `QueryParam`,
201any "fancy" combinator from servant. Let's address this and use `QueryParam`,
212−`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
213combinators in an endpoint makes the corresponding handler receive an
203combinators in an endpoint makes the corresponding handler receive an
214argument 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
215manually looking up URL captures or query string parameters, or
205manually looking up URL captures or query string parameters, or
@@ -275,15 +265,15 @@ server3 = position
275 :<|> hello
265 :<|> hello
276 :<|> marketing
266 :<|> marketing
278− where position :: Int -> Int -> ExceptT ServantErr IO Position
268+ where position :: Int -> Int -> Handler Position
279 position x y = return (Position x y)
269 position x y = return (Position x y)
281− hello :: Maybe String -> ExceptT ServantErr IO HelloMessage
271+ hello :: Maybe String -> Handler HelloMessage
282 hello mname = return . HelloMessage $ case mname of
272 hello mname = return . HelloMessage $ case mname of
283 Nothing -> "Hello, anonymous coward"
273 Nothing -> "Hello, anonymous coward"
284 Just n -> "Hello, " ++ n
274 Just n -> "Hello, " ++ n
286− marketing :: ClientInfo -> ExceptT ServantErr IO Email
276+ marketing :: ClientInfo -> Handler Email
287 marketing clientinfo = return (emailForClient clientinfo)
277 marketing clientinfo = return (emailForClient clientinfo)
@@ -298,42 +288,40 @@ parameter might not always be there);
299 - a `ReqBody contentTypeList a` becomes an argument of type `a`;
289 - a `ReqBody contentTypeList a` becomes an argument of type `a`;
301−And that's it. You can see this example in action by running
291+And that's it. Here's the example in action:
302−`dist/build/tutorial/tutorial 3`.
305$ curl http://localhost:8081/position/1/2
294$ curl http://localhost:8081/position/1/2
306−{"x":1,"y":2}
295+{"xCoord":1,"yCoord":2}
307$ curl http://localhost:8081/hello
296$ curl http://localhost:8081/hello
308{"msg":"Hello, anonymous coward"}
297{"msg":"Hello, anonymous coward"}
309$ curl http://localhost:8081/hello?name=Alp
298$ curl http://localhost:8081/hello?name=Alp
310{"msg":"Hello, Alp"}
299{"msg":"Hello, Alp"}
311−$ 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
312{"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"}
315−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**:
316−that get turned into arguments to the handlers, the type of the argument.
318− > - `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>`.
319 > - `Capture "something" a` becomes an argument of type `a`.
307 > - `Capture "something" a` becomes an argument of type `a`.
320− > - `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.
321− > - `QueryFlag "something"` and `MatrixFlag "something"` get turned into arguments of type `Bool`.
309+ > - `QueryFlag "something"` gets turned into an argument of type `Bool`.
322− > - `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]`.
323 > - `ReqBody contentTypes a` gets turned into an argument of type `a`.
311 > - `ReqBody contentTypes a` gets turned into an argument of type `a`.
325## The `FromHttpApiData`/`ToHttpApiData` classes
313## The `FromHttpApiData`/`ToHttpApiData` classes
327−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
328−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
329−following two sections address.
317+help us answer these questions.
331`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.
332`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
333corresponding (textual) value in the request's "metadata". How types are
321corresponding (textual) value in the request's "metadata". How types are
334decoded from headers, captures, and query params is expressed in a class
322decoded from headers, captures, and query params is expressed in a class
335`FromHttpApiData` (from the package
323`FromHttpApiData` (from the package
336−[*http-api-data*](http://hackage.haskell.org/package/http-api-data)):
324+[**http-api-data**](http://hackage.haskell.org/package/http-api-data)):
338``` haskell ignore
326``` haskell ignore
339class FromHttpApiData a where
327class FromHttpApiData a where
@@ -355,15 +343,15 @@ As you can see, as long as you provide either `parseUrlPiece` (for `Capture`s)
355or `parseQueryParam` (for `QueryParam`s), the other methods will be defined in
343or `parseQueryParam` (for `QueryParam`s), the other methods will be defined in
356terms of this.
344terms of this.
358−*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
359ones, and wonderful documentation.
347ones, and wonderful documentation.
361There'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
362them when using `Capture`, `QueryParam`, `QueryParams`, and `Header` with your
350them when using `Capture`, `QueryParam`, `QueryParams`, and `Header` with your
363types. You will need `FromHttpApiData` instances for server-side request
351types. You will need `FromHttpApiData` instances for server-side request
364handlers and `ToHttpApiData` instances only when using
352handlers and `ToHttpApiData` instances only when using
365−*servant-client*, as described in the [section about deriving haskell
353+**servant-client**, as described in the [section about deriving haskell
366−functions to query an API](/tutorial/client.html).
354+functions to query an API](Client.html).
368## Using content-types with your data types
356## Using content-types with your data types
@@ -371,14 +359,15 @@ The same principle was operating when decoding request bodies from JSON, and
371responses *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
372any content-type.)
360any content-type.)
374−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
375all of this work.
363all of this work.
377### 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
380−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
381−by *servant*, it's a really dumb data type.
370+data type.
383``` haskell ignore
372``` haskell ignore
@@ -388,14 +377,15 @@ data OctetStream
390Obviously, 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
391−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
392a special *symbol* that's associated with encoding (resp. decoding) to (resp.
381a special *symbol* that's associated with encoding (resp. decoding) to (resp.
393from) the *JSON* format. The way this association is performed can be
382from) the *JSON* format. The way this association is performed can be
394decomposed into two steps.
383decomposed into two steps.
396The first step is to provide a proper
385The first step is to provide a proper
397−[`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))
398−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
399haddocks 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
400`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
401use `(//) :: ByteString -> ByteString -> MediaType`. The precise way to specify
391use `(//) :: ByteString -> ByteString -> MediaType`. The precise way to specify
@@ -411,14 +401,14 @@ instance Accept JSON where
413The second step is centered around the `MimeRender` and `MimeUnrender` classes.
403The second step is centered around the `MimeRender` and `MimeUnrender` classes.
414−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
415−values respectively into or from your content-type's representation.
405+values into or from your content-type's representation.
417``` haskell ignore
407``` haskell ignore
418class Accept ctype => MimeRender ctype a where
408class Accept ctype => MimeRender ctype a where
419− mimeRender :: Proxy ctype -> a -> ByteString
409+ mimeRender :: Proxy ctype -> a -> ByteString
420 -- alternatively readable as:
410 -- alternatively readable as:
421− mimeRender :: Proxy ctype -> (a -> ByteString)
411+ mimeRender :: Proxy ctype -> (a -> ByteString)
424Given 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
@@ -444,7 +434,7 @@ class Accept ctype => MimeUnrender ctype a where
444We 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
445precisely 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
446JSON 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
447−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
448JSON 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
449you are curious.
439you are curious.
@@ -462,20 +452,20 @@ instance FromJSON a => MimeUnrender JSON a where
462 mimeUnrender _ = eitherDecodeLenient
452 mimeUnrender _ = eitherDecodeLenient
465−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`,
466`Post` and friends. We can check our understanding by implementing support
456`Post` and friends. We can check our understanding by implementing support
467−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
468HTML 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
469document, e.g. using [jQuery's `load` function](https://api.jquery.com/load/),
459document, e.g. using [jQuery's `load` function](https://api.jquery.com/load/),
470simply by adding `Accept: text/html` to their request headers.
460simply by adding `Accept: text/html` to their request headers.
472−### Case-studies: *servant-blaze* and *servant-lucid*
462+### Case-studies: **servant-blaze** and **servant-lucid**
474These 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
475−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)
476−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
477−*servant* is obviously to support both (and hopefully other templating
467+**servant** is obviously to support both (and hopefully other templating
478−solutions!).
468+solutions!). We're first going to look at **lucid**:
480``` haskell
470``` haskell
481data HTMLLucid
471data HTMLLucid
@@ -483,24 +473,20 @@ data HTMLLucid
484Once 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
485functions, except that this time we will only worry about encoding since
475functions, except that this time we will only worry about encoding since
486−*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.
488−Both packages also have the same `Accept` instance for their `HTMLLucid` type.
490``` haskell
478``` haskell
491instance Accept HTMLLucid where
479instance Accept HTMLLucid where
492 contentType _ = "text" // "html" /: ("charset", "utf-8")
480 contentType _ = "text" // "html" /: ("charset", "utf-8")
495−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
496us specify additional information about a content-type, like the charset here.
484us specify additional information about a content-type, like the charset here.
498−The rendering instances for both packages both call similar functions that take
486+The rendering instances call similar functions that take
499types with an appropriate instance to an "abstract" HTML representation and
487types with an appropriate instance to an "abstract" HTML representation and
500then write that to a `ByteString`.
488then write that to a `ByteString`.
504``` haskell
490``` haskell
505instance ToHtml a => MimeRender HTMLLucid a where
491instance ToHtml a => MimeRender HTMLLucid a where
506 mimeRender _ = renderBS . toHtml
492 mimeRender _ = renderBS . toHtml
@@ -511,7 +497,7 @@ instance MimeRender HTMLLucid (Html a) where
511 mimeRender _ = renderBS
497 mimeRender _ = renderBS
514−For *blaze-html*:
500+For **blaze-html** everything works very similarly:
516``` haskell
502``` haskell
517-- For this tutorial to compile 'HTMLLucid' and 'HTMLBlaze' have to be
503-- For this tutorial to compile 'HTMLLucid' and 'HTMLBlaze' have to be
@@ -531,15 +517,13 @@ instance MimeRender HTMLBlaze Text.Blaze.Html.Html where
531 mimeRender _ = renderHtml
517 mimeRender _ = renderHtml
534−Both [servant-blaze](http://hackage.haskell.org/package/servant-blaze) and
520+Both [**servant-blaze**](http://hackage.haskell.org/package/servant-blaze) and
535−[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
536−`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
537−appropriate class (`ToMarkup` for *blaze-html*, `ToHtml` for *lucid*).
523+appropriate class (`ToMarkup` for **blaze-html**, `ToHtml` for **lucid**).
539−We can now write webservice that uses *servant-lucid* to show the `HTMLLucid`
540−content type in action. First off, imports and pragmas as usual.
525+We can now write a webservice that uses **servant-lucid** to show the `HTMLLucid`
542−We will be serving the following API:
526+content-type in action. We will be serving the following API:
544``` haskell
528``` haskell
545type PersonAPI = "persons" :> Get '[JSON, HTMLLucid] [Person]
529type PersonAPI = "persons" :> Get '[JSON, HTMLLucid] [Person]
@@ -556,7 +540,7 @@ data Person = Person
556instance ToJSON Person
540instance ToJSON Person
559−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
560a 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.
562``` haskell
546``` haskell
@@ -600,48 +584,45 @@ server4 :: Server PersonAPI
600server4 = return people
584server4 = return people
602app2 :: Application
586app2 :: Application
603−app2 = serve personAPI EmptyConfig server4
587+app2 = serve personAPI server4
606−And we're good to go. You can run this example with `dist/build/tutorial/tutorial 4`.
590+And we're good to go:
609− $ curl http://localhost:8081/persons
593+$ curl http://localhost:8081/persons
610− [{"lastName":"Newton","firstName":"Isaac"},{"lastName":"Einstein","firstName":"Albert"}]
594+[{"lastName":"Newton","firstName":"Isaac"},{"lastName":"Einstein","firstName":"Albert"}]
611− $ curl -H 'Accept: text/html' http://localhost:8081/persons
595+$ curl -H 'Accept: text/html' http://localhost:8081/persons
612− <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>
613− # or just point your browser to http://localhost:8081/persons
597+# or just point your browser to http://localhost:8081/persons
616−## The `ExceptT ServantErr IO` monad
600+## The `Handler` monad
618−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)).
619−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
620simplest monad with the following properties:
605simplest monad with the following properties:
622−- it lets us both return a successful result (with the `Right` branch of
607+- it lets us both return a successful result (using `return`)
623−`Either`) or "fail" with a descriptive error (with the `Left` branch of
608+or "fail" with a descriptive error (using `throwError`);
625- 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
626−as interfaces to databases that we interact with in `IO`;
610+as interfaces to databases that we interact with in `IO`.
628Let's recall some definitions.
612Let's recall some definitions.
630``` haskell ignore
614``` haskell ignore
632−data Either e a = Left e | Right a
634-- from the 'mtl' package at
615-- from the 'mtl' package at
635−newtype ExceptT e m a = ExceptT ( m (Either e a) )
616+newtype ExceptT e m a = ExceptT (m (Either e a))
638−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
639−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
640action that either returns an error or a result.
621action that either returns an error or a result.
642−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)
643from which `ExceptT` comes is worth looking at.
624from which `ExceptT` comes is worth looking at.
644−Perhaps most importantly, `ExceptT` is an instance of `MonadError`, so
625+Perhaps most importantly, `ExceptT` and `Handler` are instances of `MonadError`, so
645`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`
646 is enough to return a success).
627 is enough to return a success).
@@ -651,18 +632,17 @@ kind and abort early. The next two sections cover how to do just that.
652### Performing IO
633### Performing IO
654−Another important instance from the list above is `MonadIO m => MonadIO
635+Other important instances from the list above are `MonadIO m => MonadIO
655−(ExceptT e m)`.
636+(ExceptT e m)`, and therefore also `MonadIO Handler` as there is a `MonadIO IO` instance.
656−[`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)
657−is a class from the *transformers* package defined as:
638+is a class from the **transformers** package defined as:
659``` haskell ignore
640``` haskell ignore
660class Monad m => MonadIO m where
641class Monad m => MonadIO m where
661 liftIO :: IO a -> m a
642 liftIO :: IO a -> m a
664−Obviously, the `IO` monad provides a `MonadIO` instance. Hence for any type
665−`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
666IO computation in your handlers, just use `liftIO`:
646IO computation in your handlers, just use `liftIO`:
668``` haskell
648``` haskell
@@ -680,16 +660,16 @@ server5 = do
680 return (FileContent filecontent)
660 return (FileContent filecontent)
683−### Failing, through `ServantErr`
663+### Failing, through `ServerError`
685If 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
686using the appropriate HTTP status code (not found, unauthorized, etc) and some
666using the appropriate HTTP status code (not found, unauthorized, etc) and some
687−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
688−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
689defined as:
669defined as:
691``` haskell ignore
671``` haskell ignore
692−data ServantErr = ServantErr
672+data ServerError = ServerError
693 { errHTTPCode :: Int
673 { errHTTPCode :: Int
694 , errReasonPhrase :: String
674 , errReasonPhrase :: String
695 , errBody :: ByteString -- lazy bytestring
675 , errBody :: ByteString -- lazy bytestring
@@ -702,10 +682,10 @@ module. If you want to use these values but add a body or some headers, just
702use record update syntax:
682use record update syntax:
704``` haskell
684``` haskell
705−failingHandler :: ExceptT ServantErr IO ()
685+failingHandler :: Handler ()
706−failingHandler = throwE myerr
686+failingHandler = throwError myerr
708− where myerr :: ServantErr
688+ where myerr :: ServerError
709 myerr = err503 { errBody = "Sorry dear user." }
689 myerr = err503 { errBody = "Sorry dear user." }
@@ -718,42 +698,41 @@ server6 = do
718 exists <- liftIO (doesFileExist "myfile.txt")
698 exists <- liftIO (doesFileExist "myfile.txt")
719 if exists
699 if exists
720 then liftIO (readFile "myfile.txt") >>= return . FileContent
700 then liftIO (readFile "myfile.txt") >>= return . FileContent
721− else throwE custom404Err
701+ else throwError custom404Err
723 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." }
726−Let's run this server (`dist/build/tutorial/tutorial 5`) and
706+Here's how that server looks in action:
727−query it, first without the file and then with the file.
730− $ curl --verbose http://localhost:8081/myfile.txt
709+$ curl --verbose http://localhost:8081/myfile.txt
732− * Connected to localhost (127.0.0.1) port 8081 (#0)
711+* Connected to localhost (127.0.0.1) port 8081 (#0)
733− > GET /myfile.txt HTTP/1.1
712+> GET /myfile.txt HTTP/1.1
734− > User-Agent: curl/7.30.0
713+> User-Agent: curl/7.30.0
735− > Host: localhost:8081
714+> Host: localhost:8081
736− > Accept: */*
715+> Accept: */*
738− < HTTP/1.1 404 Not Found
717+< HTTP/1.1 404 Not Found
740− myfile.txt just isnt there, please leave this server alone.
719+myfile.txt just isn't there, please leave this server alone.
742− $ echo Hello > myfile.txt
721+$ echo Hello > myfile.txt
744− $ curl --verbose http://localhost:8081/myfile.txt
723+$ curl --verbose http://localhost:8081/myfile.txt
746− * Connected to localhost (127.0.0.1) port 8081 (#0)
725+* Connected to localhost (127.0.0.1) port 8081 (#0)
747− > GET /myfile.txt HTTP/1.1
726+> GET /myfile.txt HTTP/1.1
748− > User-Agent: curl/7.30.0
727+> User-Agent: curl/7.30.0
749− > Host: localhost:8081
728+> Host: localhost:8081
750− > Accept: */*
729+> Accept: */*
752− < HTTP/1.1 200 OK
731+< HTTP/1.1 200 OK
754− < Content-Type: application/json
733+< Content-Type: application/json
756− {"content":"Hello\n"}
735+{"content":"Hello\n"}
759## Response headers
738## Response headers
@@ -769,151 +748,77 @@ myHandler :: Server MyHandler
769myHandler = return $ addHeader 1797 albert
748myHandler = return $ addHeader 1797 albert
772−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
774## Serving static files
775## Serving static files
776−*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
777under 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
778`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
779−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
780directory serving WAI application, namely:
781directory serving WAI application, namely:
782``` haskell ignore
783``` haskell ignore
783-- exported by Servant and Servant.Server
784-- exported by Servant and Servant.Server
784−serveDirectory :: FilePath -> Server Raw
785+serveDirectoryWebApp :: FilePath -> Server Raw
787−`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.
788−example below, runnable with `dist/build/tutorial/tutorial 6`
789−(you **must** run it from within the *servant-examples/* directory!), which is
790−a webserver that serves the various bits of code covered in this
793−The API type will be the following.
790+Here's an example API that will serve some static files:
795``` haskell
792``` haskell
796−type CodeAPI = "code" :> Raw
793+type StaticAPI = "static" :> Raw
799And the server:
796And the server:
801``` haskell
798``` haskell
802−codeAPI :: Proxy CodeAPI
799+staticAPI :: Proxy StaticAPI
803−codeAPI = Proxy
800+staticAPI = Proxy
806``` haskell
803``` haskell
807−server7 :: Server CodeAPI
804+server7 :: Server StaticAPI
808−server7 = serveDirectory "tutorial"
805+server7 = serveDirectoryWebApp "static-files"
810app3 :: Application
807app3 :: Application
811−app3 = serve codeAPI EmptyConfig server7
808+app3 = serve staticAPI server7
814−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
815for 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
816− *tutorial/* directory of the path you run the program from.
813+ *static-files/* directory of the path you run the program from.
818−In other words:
815+In other words: If a client requests `/static/foo.txt`, the server will look for a file at
816+`./static-files/foo.txt`. If that file exists it'll succeed and serve the file.
817+If it doesn't exist, the handler will fail with a `404` status code.
820−- If a client requests `/code/foo.txt`, the server will look for a file at
819+`serveDirectoryWebApp` uses some standard settings that fit the use case of
821− `./tutorial/foo.txt` (and fail)
820+serving static files for most web apps. You can find out about the other
822−- If a client requests `/code/T1.hs`, the server will look for a file at
821+options in the documentation of the `Servant.Server.StaticFiles` module.
823− `./tutorial/T1.hs` (and succeed)
824−- If a client requests `/code/foo/bar/baz/movie.mp4`, the server will look for
825− a file at `./tutorial/foo/bar/baz/movie.mp4` (and fail)
827−Here is our little server in action.
830−$ curl http://localhost:8081/code/T1.hs
831−{-# LANGUAGE DataKinds #-}
832−{-# LANGUAGE TypeFamilies #-}
833−{-# LANGUAGE DeriveGeneric #-}
834−{-# LANGUAGE TypeOperators #-}
838−import Data.Time.Calendar
847− , registration_date :: Day
848− } deriving (Eq, Show, Generic)
850−-- orphan ToJSON instance for Day. necessary to derive one for User
851−instance ToJSON Day where
852− -- display a day in YYYY-mm-dd format
853− toJSON d = toJSON (showGregorian d)
857−type UserAPI = "users" :> Get '[JSON] [User]
861− [ User "Isaac Newton" 372 "isaac@newton.co.uk" (fromGregorian 1683 3 1)
862− , User "Albert Einstein" 136 "ae@mc2.org" (fromGregorian 1905 12 1)
865−userAPI :: Proxy UserAPI
868−server :: Server UserAPI
869−server = return users
872−app = serve userAPI server
873−$ curl http://localhost:8081/code/tutorial.hs
875−import Network.Wai.Handler.Warp
876−import System.Environment
888−app :: String -> (Application -> IO ()) -> IO ()
898− "9" -> T9.writeJSFiles >> f T9.app
906− [n] -> app n (run 8081)
911− putStrLn "Usage:\t tutorial N"
912− putStrLn "\t\twhere N is the number of the example you want to run."
914−$ curl http://localhost:8081/foo
918## Nested APIs
823## Nested APIs
@@ -925,7 +830,7 @@ type UserAPI3 = -- view the user with given userid, in JSON
925 Capture "userid" Int :> Get '[JSON] User
830 Capture "userid" Int :> Get '[JSON] User
927 :<|> -- delete the user with given userid. empty response
832 :<|> -- delete the user with given userid. empty response
928− Capture "userid" Int :> Delete '[] ()
833+ Capture "userid" Int :> DeleteNoContent
931We can instead factor out the `userid`:
836We can instead factor out the `userid`:
@@ -933,7 +838,7 @@ We can instead factor out the `userid`:
933``` haskell
838``` haskell
934type UserAPI4 = Capture "userid" Int :>
839type UserAPI4 = Capture "userid" Int :>
935 ( Get '[JSON] User
840 ( Get '[JSON] User
936− :<|> Delete '[] ()
841+ :<|> DeleteNoContent
@@ -941,26 +846,26 @@ However, you have to be aware that this has an effect on the type of the
941corresponding `Server`:
846corresponding `Server`:
943``` haskell ignore
848``` haskell ignore
944−Server UserAPI3 = (Int -> ExceptT ServantErr IO User)
849+Server UserAPI3 = (Int -> Handler User)
945− :<|> (Int -> ExceptT ServantErr IO ())
850+ :<|> (Int -> Handler NoContent)
947−Server UserAPI4 = Int -> ( ExceptT ServantErr IO User
852+Server UserAPI4 = Int -> ( Handler User
948− :<|> ExceptT ServantErr IO ()
853+ :<|> Handler NoContent
952In 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,
953the whole `Server` takes the *userid* and has handlers that are just
858the whole `Server` takes the *userid* and has handlers that are just
954−computations in `ExceptT`, with no arguments. In other words:
859+computations in `Handler`, with no arguments. In other words:
956``` haskell
861``` haskell
957server8 :: Server UserAPI3
862server8 :: Server UserAPI3
958server8 = getUser :<|> deleteUser
863server8 = getUser :<|> deleteUser
960− where getUser :: Int -> ExceptT ServantErr IO User
865+ where getUser :: Int -> Handler User
961 getUser _userid = error "..."
866 getUser _userid = error "..."
963− deleteUser :: Int -> ExceptT ServantErr IO ()
868+ deleteUser :: Int -> Handler NoContent
964 deleteUser _userid = error "..."
869 deleteUser _userid = error "..."
966-- notice how getUser and deleteUser
871-- notice how getUser and deleteUser
@@ -969,10 +874,10 @@ server8 = getUser :<|> deleteUser
969server9 :: Server UserAPI4
874server9 :: Server UserAPI4
970server9 userid = getUser userid :<|> deleteUser userid
875server9 userid = getUser userid :<|> deleteUser userid
972− where getUser :: Int -> ExceptT ServantErr IO User
877+ where getUser :: Int -> Handler User
973 getUser = error "..."
878 getUser = error "..."
975− deleteUser :: Int -> ExceptT ServantErr IO ()
880+ deleteUser :: Int -> Handler NoContent
976 deleteUser = error "..."
881 deleteUser = error "..."
@@ -991,13 +896,13 @@ type API1 = "users" :>
991-- we factor out the Request Body
896-- we factor out the Request Body
992type API2 = ReqBody '[JSON] User :>
897type API2 = ReqBody '[JSON] User :>
993 ( 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
994− :<|> Post '[JSON] () -- register the user. empty response
899+ :<|> PostNoContent -- register the user. empty response
997-- we factor out a Header
902-- we factor out a Header
998type API3 = Header "Authorization" Token :>
903type API3 = Header "Authorization" Token :>
999 ( Get '[JSON] SecretData -- get some secret data, if authorized
904 ( Get '[JSON] SecretData -- get some secret data, if authorized
1000− :<|> ReqBody '[JSON] SecretData :> Post '[] () -- add some secret data, if authorized
905+ :<|> ReqBody '[JSON] SecretData :> PostNoContent -- add some secret data, if authorized
1003newtype Token = Token ByteString
908newtype Token = Token ByteString
@@ -1010,44 +915,44 @@ API type only at the end.
1010``` haskell
915``` haskell
1011type UsersAPI =
916type UsersAPI =
1012 Get '[JSON] [User] -- list users
917 Get '[JSON] [User] -- list users
1013− :<|> ReqBody '[JSON] User :> Post '[] () -- add a user
918+ :<|> ReqBody '[JSON] User :> PostNoContent -- add a user
1014 :<|> Capture "userid" Int :>
919 :<|> Capture "userid" Int :>
1015 ( Get '[JSON] User -- view a user
920 ( Get '[JSON] User -- view a user
1016− :<|> ReqBody '[JSON] User :> Put '[] () -- update a user
921+ :<|> ReqBody '[JSON] User :> PutNoContent -- update a user
1017− :<|> Delete '[] () -- delete a user
922+ :<|> DeleteNoContent -- delete a user
1020usersServer :: Server UsersAPI
925usersServer :: Server UsersAPI
1021usersServer = getUsers :<|> newUser :<|> userOperations
926usersServer = getUsers :<|> newUser :<|> userOperations
1023− where getUsers :: ExceptT ServantErr IO [User]
928+ where getUsers :: Handler [User]
1024 getUsers = error "..."
929 getUsers = error "..."
1026− newUser :: User -> ExceptT ServantErr IO ()
931+ newUser :: User -> Handler NoContent
1027 newUser = error "..."
932 newUser = error "..."
1029 userOperations userid =
934 userOperations userid =
1030 viewUser userid :<|> updateUser userid :<|> deleteUser userid
935 viewUser userid :<|> updateUser userid :<|> deleteUser userid
1033− viewUser :: Int -> ExceptT ServantErr IO User
938+ viewUser :: Int -> Handler User
1034 viewUser = error "..."
939 viewUser = error "..."
1036− updateUser :: Int -> User -> ExceptT ServantErr IO ()
941+ updateUser :: Int -> User -> Handler NoContent
1037 updateUser = error "..."
942 updateUser = error "..."
1039− deleteUser :: Int -> ExceptT ServantErr IO ()
944+ deleteUser :: Int -> Handler NoContent
1040 deleteUser = error "..."
945 deleteUser = error "..."
1043``` haskell
948``` haskell
1044type ProductsAPI =
949type ProductsAPI =
1045 Get '[JSON] [Product] -- list products
950 Get '[JSON] [Product] -- list products
1046− :<|> ReqBody '[JSON] Product :> Post '[] () -- add a product
951+ :<|> ReqBody '[JSON] Product :> PostNoContent -- add a product
1047 :<|> Capture "productid" Int :>
952 :<|> Capture "productid" Int :>
1048 ( Get '[JSON] Product -- view a product
953 ( Get '[JSON] Product -- view a product
1049− :<|> ReqBody '[JSON] Product :> Put '[] () -- update a product
954+ :<|> ReqBody '[JSON] Product :> PutNoContent -- update a product
1050− :<|> Delete '[] () -- delete a product
955+ :<|> DeleteNoContent -- delete a product
1053data Product = Product { productId :: Int }
958data Product = Product { productId :: Int }
@@ -1055,23 +960,23 @@ data Product = Product { productId :: Int }
1055productsServer :: Server ProductsAPI
960productsServer :: Server ProductsAPI
1056productsServer = getProducts :<|> newProduct :<|> productOperations
961productsServer = getProducts :<|> newProduct :<|> productOperations
1058− where getProducts :: ExceptT ServantErr IO [Product]
963+ where getProducts :: Handler [Product]
1059 getProducts = error "..."
964 getProducts = error "..."
1061− newProduct :: Product -> ExceptT ServantErr IO ()
966+ newProduct :: Product -> Handler NoContent
1062 newProduct = error "..."
967 newProduct = error "..."
1064 productOperations productid =
969 productOperations productid =
1065 viewProduct productid :<|> updateProduct productid :<|> deleteProduct productid
970 viewProduct productid :<|> updateProduct productid :<|> deleteProduct productid
1068− viewProduct :: Int -> ExceptT ServantErr IO Product
973+ viewProduct :: Int -> Handler Product
1069 viewProduct = error "..."
974 viewProduct = error "..."
1071− updateProduct :: Int -> Product -> ExceptT ServantErr IO ()
976+ updateProduct :: Int -> Product -> Handler NoContent
1072 updateProduct = error "..."
977 updateProduct = error "..."
1074− deleteProduct :: Int -> ExceptT ServantErr IO ()
979+ deleteProduct :: Int -> Handler NoContent
1075 deleteProduct = error "..."
980 deleteProduct = error "..."
@@ -1091,39 +996,49 @@ abstract that away:
1091-- indexed by values of type 'i'
996-- indexed by values of type 'i'
1092type APIFor a i =
997type APIFor a i =
1093 Get '[JSON] [a] -- list 'a's
998 Get '[JSON] [a] -- list 'a's
1094− :<|> ReqBody '[JSON] a :> Post '[] () -- add an 'a'
999+ :<|> ReqBody '[JSON] a :> PostNoContent -- add an 'a'
1095 :<|> Capture "id" i :>
1000 :<|> Capture "id" i :>
1096 ( 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'
1097− :<|> ReqBody '[JSON] a :> Put '[] () -- update an 'a'
1002+ :<|> ReqBody '[JSON] a :> PutNoContent -- update an 'a'
1098− :<|> Delete '[] () -- delete an 'a'
1003+ :<|> DeleteNoContent -- delete an 'a'
1101-- Build the appropriate 'Server'
1006-- Build the appropriate 'Server'
1102-- given the handlers of the right type.
1007-- given the handlers of the right type.
1103−serverFor :: ExceptT ServantErr IO [a] -- handler for listing of 'a's
1008+serverFor :: Handler [a] -- handler for listing of 'a's
1104− -> (a -> ExceptT ServantErr IO ()) -- handler for adding an 'a'
1009+ -> (a -> Handler NoContent) -- handler for adding an 'a'
1105− -> (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'
1106− -> (i -> a -> ExceptT ServantErr IO ()) -- updating an 'a' with given id
1011+ -> (i -> a -> Handler NoContent) -- updating an 'a' with given id
1107− -> (i -> ExceptT ServantErr IO ()) -- deleting an 'a' given its id
1012+ -> (i -> Handler NoContent) -- deleting an 'a' given its id
1108 -> Server (APIFor a i)
1013 -> Server (APIFor a i)
1109serverFor = error "..."
1014serverFor = error "..."
1110-- implementation left as an exercise. contact us on IRC
1015-- implementation left as an exercise. contact us on IRC
1111-- 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
1114## Using another monad for your handlers
1030## Using another monad for your handlers
1116−Remember how `Server` turns combinators for HTTP methods into `ExceptT
1032+Remember how `Server` turns combinators for HTTP methods into `Handler`? Well, actually, there's more to that. `Server` is actually a
1117−ServantErr IO`? Well, actually, there's more to that. `Server` is actually a
1118simple type synonym.
1033simple type synonym.
1120``` haskell ignore
1035``` haskell ignore
1121−type Server api = ServerT api (ExceptT ServantErr IO)
1036+type Server api = ServerT api Handler
1124`ServerT` is the actual type family that computes the required types for the
1039`ServerT` is the actual type family that computes the required types for the
1125handlers that's part of the `HasServer` class. It's like `Server` except that
1040handlers that's part of the `HasServer` class. It's like `Server` except that
1126−it takes a third parameter which is the monad you want your handlers to run in,
1041+it takes another parameter which is the monad you want your handlers to run in,
1127or more generally the return types of your handlers. This third parameter is
1042or more generally the return types of your handlers. This third parameter is
1128used for specifying the return type of the handler for an endpoint, e.g when
1043used for specifying the return type of the handler for an endpoint, e.g when
1129computing `ServerT (Get '[JSON] Person) SomeMonad`. The result would be
1044computing `ServerT (Get '[JSON] Person) SomeMonad`. The result would be
@@ -1131,98 +1046,155 @@ computing `ServerT (Get '[JSON] Person) SomeMonad`. The result would be
1132The first and main question one might have then is: how do we write handlers
1047The first and main question one might have then is: how do we write handlers
1133that run in another monad? How can we "bring back" the value from a given monad
1048that run in another monad? How can we "bring back" the value from a given monad
1134−into something *servant* can understand?
1049+into something **servant** can understand?
1136### Natural transformations
1051### Natural transformations
1138If 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
1139do we have?
1054do we have?
1141−``` haskell ignore
1056+``` haskell
1142−newtype m :~> n = Nat { unNat :: forall a. m a -> n a}
1057+type (~>) m n = forall a. m a -> n a
1144−-- For example
1060+For example:
1145−-- listToMaybeNat ::`[] :~> Maybe`
1061+
1146−-- listToMaybeNat = Nat listToMaybe -- from Data.Maybe
1062+``` haskell
1063+listToMaybe' :: [] ~> Maybe
1064+listToMaybe' = listToMaybe -- from Data.Maybe
1148−(`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.
1150−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
1151−ServantErr IO`, say the `Reader String` monad, the first thing you have to
1152prepare is a function:
1071prepare is a function:
1154``` haskell ignore
1073``` haskell ignore
1155−readerToEither :: Reader String :~> ExceptT ServantErr IO
1074+readerToHandler :: Reader String a -> Handler a
1158−Let's start with `readerToEither'`. We obviously have to run the `Reader`
1159−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
1160−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`
1161−that function with the `Nat` constructor to make it have the fancier type.
1079+it into `Handler`.
1163``` haskell
1081``` haskell
1164−readerToEither' :: forall a. Reader String a -> ExceptT ServantErr IO a
1082+readerToHandler :: Reader String a -> Handler a
1165−readerToEither' r = return (runReader r "hi")
1083+readerToHandler r = return (runReader r "hi")
1167−readerToEither :: Reader String :~> ExceptT ServantErr IO
1168−readerToEither = Nat readerToEither'
1171We 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`.
1173``` haskell
1088``` haskell
1174type ReaderAPI = "a" :> Get '[JSON] Int
1089type ReaderAPI = "a" :> Get '[JSON] Int
1175− :<|> "b" :> Get '[JSON] String
1090+ :<|> "b" :> ReqBody '[JSON] Double :> Get '[JSON] Bool
1177readerAPI :: Proxy ReaderAPI
1092readerAPI :: Proxy ReaderAPI
1178readerAPI = Proxy
1093readerAPI = Proxy
1180readerServerT :: ServerT ReaderAPI (Reader String)
1095readerServerT :: ServerT ReaderAPI (Reader String)
1181−readerServerT = a :<|> b
1096+readerServerT = a :<|> b where
1183− where a :: Reader String Int
1097+ a :: Reader String Int
1184− a = return 1797
1098+ a = return 1797
1186− b :: Reader String String
1100+ b :: Double -> Reader String Bool
1187− b = ask
1101+ b _ = asks (== "hi")
1190We unfortunately can't use `readerServerT` as an argument of `serve`, because
1104We unfortunately can't use `readerServerT` as an argument of `serve`, because
1191−`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.
1192−ServantErr IO`. But there's a simple solution to this.
1194−### Enter `enter`
1107+### Welcome `hoistServer`
1196−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
1197−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
1198right 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
1199−`enter` which takes a natural transformation between two parametrized types `m`
1112+`hoistServer` which takes a natural transformation between two parameterized types `m`
1200and `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`.
1202−In our case, we can wrap up our little webservice by using `enter
1115+In our case, we can wrap up our little webservice by using
1203−readerToEither` on our handlers.
1116+`hoistServer readerAPI readerToHandler` on our handlers.
1205``` haskell
1118``` haskell
1206readerServer :: Server ReaderAPI
1119readerServer :: Server ReaderAPI
1207−readerServer = enter readerToEither readerServerT
1120+readerServer = hoistServer readerAPI readerToHandler readerServerT
1209app4 :: Application
1122app4 :: Application
1210−app4 = serve readerAPI EmptyConfig readerServer
1123+app4 = serve readerAPI readerServer
1213−And we can indeed see this webservice in action by running
1126+This is the webservice in action:
1214−`dist/build/tutorial/tutorial 7`.
1217$ curl http://localhost:8081/a
1129$ curl http://localhost:8081/a
1219−$ curl http://localhost:8081/b
1131+$ curl http://localhost:8081/b -X GET -d '42.0' -H 'Content-Type: application/json'
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
1220−"hi"
1153+ b _ s = s == "hi"
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.
1223## Conclusion
1196## Conclusion
1225−You're now equipped to write any kind of webservice/web-application using
1198+You're now equipped to write webservices/web-applications using
1226−*servant*. One thing not covered here is how to incorporate your own
1227−combinators and will be the topic of a page on the website. The rest of this
1228−document focuses on *servant-client*, *servant-jquery* and *servant-docs*.
1199+**servant**. The rest of this document focuses on **servant-client**,
1200+**servant-js** and **servant-docs**.