@@ -3,30 +3,10 @@
33Enough chit-chat about type-level combinators and representing an API as a
44type. Can we have a webservice already?
55
6−If you want to follow along with the code and run the examples while you read this guide:
6+## A first example
77
8−``` bash
9−cabal get servant-examples
10−cd servant-examples-<VERSION>
11−cabal sandbox init
12−cabal install --dependencies-only
13−cabal configure && cabal build
14−```
15−
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:
19−
20−``` bash
21−$ dist/build/tutorial/tutorial
22−Usage: tutorial N
23− where N is the number of the example you want to run.
24−```
25−
26−A first example
27−===============
28−
29−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.
3010
3111The source for this tutorial section is a literate haskell file, so first we
3212need to have some language extensions and imports:
@@ -38,6 +18,7 @@ need to have some language extensions and imports:
3818{-# LANGUAGE GeneralizedNewtypeDeriving #-}
3919{-# LANGUAGE MultiParamTypeClasses #-}
4020{-# LANGUAGE OverloadedStrings #-}
21+{-# LANGUAGE RankNTypes #-}
4122{-# LANGUAGE ScopedTypeVariables #-}
4223{-# LANGUAGE TypeOperators #-}
4324
@@ -46,15 +27,14 @@ module Server where
4627import Prelude ()
4728import Prelude.Compat
4829
49−import Control.Monad.IO.Class
30+import Control.Monad.Except
5031import Control.Monad.Reader
51−import Control.Monad.Trans.Except
5232import Data.Aeson
5333import Data.Aeson.Types
5434import Data.Attoparsec.ByteString
5535import Data.ByteString (ByteString)
56−import Data.Int
5736import Data.List
37+import Data.Maybe
5838import Data.String.Conversions
5939import Data.Time.Calendar
6040import GHC.Generics
@@ -66,15 +46,18 @@ import Servant
6646import System.Directory
6747import Text.Blaze
6848import Text.Blaze.Html.Renderer.Utf8
49+import Servant.Types.SourceT (source)
6950import qualified Data.Aeson.Parser
7051import qualified Text.Blaze.Html
7152```
7253
73−``` haskell ignore
74−{-# LANGUAGE TypeFamilies #-}
75−```
76−
77−**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.
54+**Important**: the `Servant` module comes from the **servant-server** package,
55+the one that lets us run webservers that implement a particular API type. It
56+reexports all the types from the **servant** package that let you declare API
57+types as well as everything you need to turn your request handlers into a
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
60+else.
7861
7962We will write a server that will serve the following API.
8063
@@ -113,12 +96,6 @@ users1 =
11396 ]
11497```
11598
116−Let's also write our API type.
117−
118−``` haskell ignore
119−type UserAPI1 = "users" :> Get '[JSON] [User]
120−```
121−
12299We can now take care of writing the actual webservice that will handle requests
123100to such an API. This one will be very simple, being reduced to just a single
124101endpoint. The type of the web application is determined by the API type,
@@ -130,11 +107,11 @@ corresponding API type.
130107The first thing to know about the `Server` type family is that behind the
131108scenes it will drive the routing, letting you focus only on the business
132109logic. The second thing to know is that for each endpoint, your handlers will
133−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
134111easily, as explained near the end of this guide. Third thing, the type of the
135112value returned in that monad must be the same as the second argument of the
136113HTTP method combinator used for the corresponding endpoint. In our case, it
137−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,
138115we have a monad, let's just `return` our list:
139116
140117``` haskell
@@ -142,7 +119,9 @@ server1 :: Server UserAPI1
142119server1 = return users1
143120```
144121
145−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):
146125
147126``` haskell
148127userAPI :: Proxy UserAPI1
@@ -152,7 +131,7 @@ userAPI = Proxy
152131-- which you can think of as an "abstract" web application,
153132-- not yet a webserver.
154133app1 :: Application
155−app1 = serve userAPI EmptyConfig server1
134+app1 = serve userAPI server1
156135```
157136
158137The `userAPI` bit is, alas, boilerplate (we need it to guide type inference).
@@ -167,9 +146,8 @@ main = run 8081 app1
167146
168147You can put this all into a file or just grab [servant's
169148repo](http://github.com/haskell-servant/servant) and look at the
170−*servant-examples* directory. The code we have just explored is in
171−*tutorial/T1.hs*, runnable with
172−`dist/build/tutorial/tutorial 1`.
149+*doc/tutorial* directory. This code (the source of this web page) is in
150+*doc/tutorial/Server.lhs*.
173151
174152If you run it, you can go to `http://localhost:8081/users` in your browser or
175153query it with curl and you see:
@@ -179,10 +157,10 @@ $ curl http://localhost:8081/users
179157[{"email":"isaac@newton.co.uk","registration_date":"1683-03-01","age":372,"name":"Isaac Newton"},{"email":"ae@mc2.org","registration_date":"1905-12-01","age":136,"name":"Albert Einstein"}]
180158```
181159
182−More endpoints
183−==============
160+## More endpoints
184161
185−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.
186164
187165``` haskell
188166type UserAPI2 = "users" :> Get '[JSON] [User]
@@ -205,7 +183,7 @@ users2 = [isaac, albert]
205183
206184Now, just like we separate the various endpoints in `UserAPI` with `:<|>`, we
207185are going to separate the handlers with `:<|>` too! They must be provided in
208−the same order as the one they appear in in the API type.
186+the same order as in the API type.
209187
210188``` haskell
211189server2 :: Server UserAPI2
@@ -214,22 +192,21 @@ server2 = return users2
214192 :<|> return isaac
215193```
216194
217−And that's it! You can run this example with
218−`dist/build/tutorial/tutorial 2` and check out the data available
219−at `/users`, `/albert` and `/isaac`.
195+And that's it! You can run this example in the same way that we showed for
196+`server1` and check out the data available at `/users`, `/albert` and `/isaac`.
220197
221−From combinators to handler arguments
222−=====================================
198+## From combinators to handler arguments
223199
224200Fine, we can write trivial webservices easily, but none of the two above use
225201any "fancy" combinator from servant. Let's address this and use `QueryParam`,
226−`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
227203combinators in an endpoint makes the corresponding handler receive an
228204argument of the appropriate type automatically. You don't have to worry about
229205manually looking up URL captures or query string parameters, or
230206decoding/encoding data from/to JSON. Never.
231207
232−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
209+server for `API`.
233210
234211``` haskell
235212type API = "position" :> Capture "x" Int :> Capture "y" Int :> Get '[JSON] Position
@@ -237,8 +214,8 @@ type API = "position" :> Capture "x" Int :> Capture "y" Int :> Get '[JSON] Posit
237214 :<|> "marketing" :> ReqBody '[JSON] ClientInfo :> Post '[JSON] Email
238215
239216data Position = Position
240− { x :: Int
241− , y :: Int
217+ { xCoord :: Int
218+ , yCoord :: Int
242219 } deriving Generic
243220
244221instance ToJSON Position
@@ -288,15 +265,15 @@ server3 = position
288265 :<|> hello
289266 :<|> marketing
290267
291− where position :: Int -> Int -> ExceptT ServantErr IO Position
268+ where position :: Int -> Int -> Handler Position
292269 position x y = return (Position x y)
293270
294− hello :: Maybe String -> ExceptT ServantErr IO HelloMessage
271+ hello :: Maybe String -> Handler HelloMessage
295272 hello mname = return . HelloMessage $ case mname of
296273 Nothing -> "Hello, anonymous coward"
297274 Just n -> "Hello, " ++ n
298275
299− marketing :: ClientInfo -> ExceptT ServantErr IO Email
276+ marketing :: ClientInfo -> Handler Email
300277 marketing clientinfo = return (emailForClient clientinfo)
301278```
302279
@@ -311,99 +288,86 @@ parameter might not always be there);
311288
312289 - a `ReqBody contentTypeList a` becomes an argument of type `a`;
313290
314−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:
315292
316293``` bash
317294$ curl http://localhost:8081/position/1/2
318−{"x":1,"y":2}
295+{"xCoord":1,"yCoord":2}
319296$ curl http://localhost:8081/hello
320297{"msg":"Hello, anonymous coward"}
321298$ curl http://localhost:8081/hello?name=Alp
322299{"msg":"Hello, Alp"}
323−$ 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
324301{"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"}
325302```
326303
327−For reference, here's a list of some combinators from *servant* and for those
328−that get turned into arguments to the handlers, the type of the argument.
304+For reference, here's a list of some combinators from **servant**:
329305
330− > - `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>`.
331307 > - `Capture "something" a` becomes an argument of type `a`.
332− > - `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.
333− > - `QueryFlag "something"` and `MatrixFlag "something"` get turned into arguments of type `Bool`.
334− > - `QueryParams "something" a` and `MatrixParams "something" a` get turned into arguments of type `[a]`.
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.
309+ > - `QueryFlag "something"` gets turned into an argument of type `Bool`.
310+ > - `QueryParams "something" a` gets turned into an argument of type `[a]`.
335311 > - `ReqBody contentTypes a` gets turned into an argument of type `a`.
336312
337−The `FromText`/`ToText` classes
338−===============================
313+## The `FromHttpApiData`/`ToHttpApiData` classes
339314
340−Wait... How does *servant* know how to decode the `Int`s from the URL? Or how
341−to decode a `ClientInfo` value from the request body? This is what this and the
342−following two sections address.
315+Wait... How does **servant** know how to decode the `Int`s from the URL? Or how
316+to decode a `ClientInfo` value from the request body? The following three sections will
317+help us answer these questions.
343318
344319`Capture`s and `QueryParam`s are represented by some textual value in URLs.
345320`Header`s are similarly represented by a pair of a header name and a
346−corresponding (textual) value in the request's "metadata". This is why we
347−decided to provide a pair of typeclasses, `FromText` and `ToText` which just
348−let you say that you can respectively *extract* or *encode* values of some type
349−*from*/*to* text. Here are the definitions:
321+corresponding (textual) value in the request's "metadata". How types are
322+decoded from headers, captures, and query params is expressed in a class
323+`FromHttpApiData` (from the package
324+[**http-api-data**](http://hackage.haskell.org/package/http-api-data)):
350325
351326``` haskell ignore
352−class FromText a where
353− fromText :: Text -> Maybe a
354−
355−class ToText a where
356− toText :: a -> Text
357−```
327+class FromHttpApiData a where
328+ {-# MINIMAL parseUrlPiece | parseQueryParam #-}
329+ -- | Parse URL path piece.
330+ parseUrlPiece :: Text -> Either Text a
331+ parseUrlPiece = parseQueryParam
358332
359−And as long as the type that a `Capture`/`QueryParam`/`Header`/etc will be
360−decoded to provides a `FromText` instance, it will Just Work. *servant*
361−provides a decent number of instances, but here are some examples of defining
362−your own.
333+ -- | Parse HTTP header value.
334+ parseHeader :: ByteString -> Either Text a
335+ parseHeader = parseUrlPiece . decodeUtf8
363336
364−``` haskell
365−-- A typical enumeration
366−data Direction
367− = Up
368− | Down
369− | Left
370− | Right
371−
372−newtype UserId = UserId Int64
337+ -- | Parse query param value.
338+ parseQueryParam :: Text -> Either Text a
339+ parseQueryParam = parseUrlPiece
373340```
374341
375−or writing the instances by hand:
342+As you can see, as long as you provide either `parseUrlPiece` (for `Capture`s)
343+or `parseQueryParam` (for `QueryParam`s), the other methods will be defined in
344+terms of this.
376345
377−``` haskell ignore
378−instance FromText UserId where
379− fromText = fmap UserId fromText
380−
381−instance ToText UserId where
382− toText (UserId i) = toText i
383−```
346+**http-api-data** provides a decent number of instances, helpers for defining new
347+ones, and wonderful documentation.
384348
385349There's not much else to say about these classes. You will need instances for
386−them when using `Capture`, `QueryParam`, `QueryParams`, `MatrixParam`,
387−`MatrixParams` and `Header` with your types. You will need `FromText` instances
388−for server-side request handlers and `ToText` instances only when using
389−*servant-client*, as described in the [section about deriving haskell
390−functions to query an API](/tutorial/client.html).
350+them when using `Capture`, `QueryParam`, `QueryParams`, and `Header` with your
351+types. You will need `FromHttpApiData` instances for server-side request
352+handlers and `ToHttpApiData` instances only when using
353+**servant-client**, as described in the [section about deriving haskell
354+functions to query an API](Client.html).
391355
392−Using content-types with your data types
393−========================================
356+## Using content-types with your data types
394357
395358The same principle was operating when decoding request bodies from JSON, and
396359responses *into* JSON. (JSON is just the running example - you can do this with
397360any content-type.)
398361
399−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
400363all of this work.
401364
402−The truth behind `JSON`
403−-----------------------
365+### The truth behind `JSON`
366+
404367
405−What exactly is `JSON`? Like the 3 other content types provided out of the box
406−by *servant*, it's a really dumb data type.
368+What exactly is `JSON` (the type as used in `Get '[JSON] User`)? Like the 3
369+other content-types provided out of the box by **servant**, it's a really dumb
370+data type.
407371
408372``` haskell ignore
409373data JSON
@@ -413,14 +377,15 @@ data OctetStream
413377```
414378
415379Obviously, this is not all there is to `JSON`, otherwise it would be quite
416−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
417381a special *symbol* that's associated with encoding (resp. decoding) to (resp.
418382from) the *JSON* format. The way this association is performed can be
419383decomposed into two steps.
420384
421385The first step is to provide a proper
422−[`MediaType`](https://hackage.haskell.org/package/http-media-0.6.2/docs/Network-HTTP-Media.html)
423−representation for `JSON`, or for your own content types. If you look at the
386+`MediaType` (from
387+[**http-media**](https://hackage.haskell.org/package/http-media-0.6.2/docs/Network-HTTP-Media.html))
388+representation for `JSON`, or for your own content-types. If you look at the
424389haddocks from this link, you can see that we just have to specify
425390`application/json` using the appropriate functions. In our case, we can just
426391use `(//) :: ByteString -> ByteString -> MediaType`. The precise way to specify
@@ -436,14 +401,14 @@ instance Accept JSON where
436401```
437402
438403The second step is centered around the `MimeRender` and `MimeUnrender` classes.
439−These classes just let you specify a way to respectively encode and decode
440−values respectively into or from your content-type's representation.
404+These classes just let you specify a way to encode and decode
405+values into or from your content-type's representation.
441406
442407``` haskell ignore
443408class Accept ctype => MimeRender ctype a where
444− mimeRender :: Proxy ctype -> a -> ByteString
409+ mimeRender :: Proxy ctype -> a -> ByteString
445410 -- alternatively readable as:
446− mimeRender :: Proxy ctype -> (a -> ByteString)
411+ mimeRender :: Proxy ctype -> (a -> ByteString)
447412```
448413
449414Given a content-type and some user type, `MimeRender` provides a function that
@@ -464,14 +429,12 @@ And now the `MimeUnrender` class, which lets us extract values from lazy
464429``` haskell ignore
465430class Accept ctype => MimeUnrender ctype a where
466431 mimeUnrender :: Proxy ctype -> ByteString -> Either String a
467− -- alternatively:
468− mimeUnrender :: Proxy ctype -> (ByteString -> Either String a)
469432```
470433
471434We don't have much work to do there either, `Data.Aeson.eitherDecode` is
472435precisely what we need. However, it only allows arrays and objects as toplevel
473436JSON values and this has proven to get in our way more than help us so we wrote
474−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
475438JSON value at the toplevel of a "JSON document". Here's the definition in case
476439you are curious.
477440
@@ -489,21 +452,20 @@ instance FromJSON a => MimeUnrender JSON a where
489452 mimeUnrender _ = eitherDecodeLenient
490453```
491454
492−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`,
493456`Post` and friends. We can check our understanding by implementing support
494−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
495458HTML representation of the data they want, ready to be included in any HTML
496−document, e.g. using [jQuery's `load` function](https://api.jquery.com/load/), simply by adding `Accept:
497−text/html` to their request headers.
459+document, e.g. using [jQuery's `load` function](https://api.jquery.com/load/),
460+simply by adding `Accept: text/html` to their request headers.
498461
499−Case-studies: *servant-blaze* and *servant-lucid*
500−-------------------------------------------------
462+### Case-studies: **servant-blaze** and **servant-lucid**
501463
502464These days, most of the haskellers who write their HTML UIs directly from
503−Haskell use either [blaze-html](http://hackage.haskell.org/package/blaze-html)
504−or [lucid](http://hackage.haskell.org/package/lucid). The best option for
505−*servant* is obviously to support both (and hopefully other templating
506−solutions!).
465+Haskell use either [**blaze-html**](http://hackage.haskell.org/package/blaze-html)
466+or [**lucid**](http://hackage.haskell.org/package/lucid). The best option for
467+**servant** is obviously to support both (and hopefully other templating
468+solutions!). We're first going to look at **lucid**:
507469
508470``` haskell
509471data HTMLLucid
@@ -511,24 +473,20 @@ data HTMLLucid
511473
512474Once again, the data type is just there as a symbol for the encoding/decoding
513475functions, except that this time we will only worry about encoding since
514−*blaze-html* and *lucid* don't provide a way to extract data from HTML.
515−
516−Both packages also have the same `Accept` instance for their `HTMLLucid` type.
476+**lucid** doesn't provide a way to extract data from HTML.
517477
518478``` haskell
519479instance Accept HTMLLucid where
520480 contentType _ = "text" // "html" /: ("charset", "utf-8")
521481```
522482
523−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
524484us specify additional information about a content-type, like the charset here.
525485
526−The rendering instances for both packages both call similar functions that take
486+The rendering instances call similar functions that take
527487types with an appropriate instance to an "abstract" HTML representation and
528488then write that to a `ByteString`.
529489
530−For *lucid*:
531−
532490``` haskell
533491instance ToHtml a => MimeRender HTMLLucid a where
534492 mimeRender _ = renderBS . toHtml
@@ -539,7 +497,7 @@ instance MimeRender HTMLLucid (Html a) where
539497 mimeRender _ = renderBS
540498```
541499
542−For *blaze-html*:
500+For **blaze-html** everything works very similarly:
543501
544502``` haskell
545503-- For this tutorial to compile 'HTMLLucid' and 'HTMLBlaze' have to be
@@ -559,15 +517,13 @@ instance MimeRender HTMLBlaze Text.Blaze.Html.Html where
559517 mimeRender _ = renderHtml
560518```
561519
562−Both [servant-blaze](http://hackage.haskell.org/package/servant-blaze) and
563−[servant-lucid](http://hackage.haskell.org/package/servant-lucid) let you use
564−`HTMLLucid` in any content type list as long as you provide an instance of the
565−appropriate class (`ToMarkup` for *blaze-html*, `ToHtml` for *lucid*).
566−
567−We can now write webservice that uses *servant-lucid* to show the `HTMLLucid`
568−content type in action. First off, imports and pragmas as usual.
520+Both [**servant-blaze**](http://hackage.haskell.org/package/servant-blaze) and
521+[**servant-lucid**](http://hackage.haskell.org/package/servant-lucid) let you use
522+`HTMLLucid` and `HTMLBlaze` in any content-type list as long as you provide an instance of the
523+appropriate class (`ToMarkup` for **blaze-html**, `ToHtml` for **lucid**).
569524
570−We will be serving the following API:
525+We can now write a webservice that uses **servant-lucid** to show the `HTMLLucid`
526+content-type in action. We will be serving the following API:
571527
572528``` haskell
573529type PersonAPI = "persons" :> Get '[JSON, HTMLLucid] [Person]
@@ -584,7 +540,7 @@ data Person = Person
584540instance ToJSON Person
585541```
586542
587−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
588544a list of `Person`s as a table with a row per person.
589545
590546``` haskell
@@ -615,8 +571,8 @@ instance ToHtml [Person] where
615571We create some `Person` values and serve them as a list:
616572
617573``` haskell
618−persons :: [Person]
619−persons =
574+people :: [Person]
575+people =
620576 [ Person "Isaac" "Newton"
621577 , Person "Albert" "Einstein"
622578 ]
@@ -625,75 +581,69 @@ personAPI :: Proxy PersonAPI
625581personAPI = Proxy
626582
627583server4 :: Server PersonAPI
628−server4 = return persons
584+server4 = return people
629585
630586app2 :: Application
631−app2 = serve personAPI EmptyConfig server4
587+app2 = serve personAPI server4
632588```
633589
634−And we're good to go. You can run this example with `dist/build/tutorial/tutorial 4`.
590+And we're good to go:
635591
636592``` bash
637− $ curl http://localhost:8081/persons
638− [{"lastName":"Newton","firstName":"Isaac"},{"lastName":"Einstein","firstName":"Albert"}]
639− $ curl -H 'Accept: text/html' http://localhost:8081/persons
640− <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>
641− # or just point your browser to http://localhost:8081/persons
593+$ curl http://localhost:8081/persons
594+[{"lastName":"Newton","firstName":"Isaac"},{"lastName":"Einstein","firstName":"Albert"}]
595+$ curl -H 'Accept: text/html' http://localhost:8081/persons
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>
597+# or just point your browser to http://localhost:8081/persons
642598```
643599
644−The `ExceptT ServantErr IO` monad
645−=================================
600+## The `Handler` monad
646601
647−At the heart of the handlers is the monad they run in, namely `ExceptT
648−ServantErr IO`. One might wonder: why this monad? The answer is that it is the
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)).
604+One might wonder: why this monad? The answer is that it is the
649605simplest monad with the following properties:
650606
651−- it lets us both return a successful result (with the `Right` branch of
652−`Either`) or "fail" with a descriptive error (with the `Left` branch of
653−`Either`);
607+- it lets us both return a successful result (using `return`)
608+or "fail" with a descriptive error (using `throwError`);
654609- it lets us perform IO, which is absolutely vital since most webservices exist
655−as interfaces to databases that we interact with in `IO`;
610+as interfaces to databases that we interact with in `IO`.
656611
657612Let's recall some definitions.
658613
659614``` haskell ignore
660−-- from the Prelude
661−data Either e a = Left e | Right a
662−
663−-- from the 'either' package at
664−-- http://hackage.haskell.org/package/either-4.3.3.2/docs/Control-Monad-Trans-Either.html
665−newtype ExceptT e m a
666− = ExceptT { runEitherT :: m (Either e a) }
615+-- from the 'mtl' package at
616+newtype ExceptT e m a = ExceptT (m (Either e a))
667617```
668618
669−In short, this means that a handler of type `ExceptT ServantErr IO a` is simply
670−equivalent to a computation of type `IO (Either ServantErr a)`, that is, an IO
619+In short, this means that a handler of type `Handler a` is simply
620+equivalent to a computation of type `IO (Either ServerError a)`, that is, an IO
671621action that either returns an error or a result.
672622
673−The aforementioned `either` package is worth taking a look at. Perhaps most
674−importantly:
675−
676−``` haskell ignore
677−left :: Monad m => e -> ExceptT e m a
678−```
679−Allows you to return an error from your handler (whereas `return` is enough to
680−return a success).
623+The module [`Control.Monad.Except`](https://hackage.haskell.org/package/mtl/docs/Control-Monad-Except.html#t:ExceptT)
624+from which `ExceptT` comes is worth looking at.
625+Perhaps most importantly, `ExceptT` and `Handler` are instances of `MonadError`, so
626+`throwError` can be used to return an error from your handler (whereas `return`
627+ is enough to return a success).
681628
682629Most of what you'll be doing in your handlers is running some IO and,
683630depending on the result, you might sometimes want to throw an error of some
684631kind and abort early. The next two sections cover how to do just that.
685632
686−Performing IO
687−-------------
633+### Performing IO
688634
689−Another important instance from the list above is `MonadIO m => MonadIO (ExceptT e m)`. [`MonadIO`](http://hackage.haskell.org/package/transformers-0.4.3.0/docs/Control-Monad-IO-Class.html) is a class from the *transformers* package defined as:
635+Other important instances from the list above are `MonadIO m => MonadIO
636+(ExceptT e m)`, and therefore also `MonadIO Handler` as there is a `MonadIO IO` instance.
637+[`MonadIO`](http://hackage.haskell.org/package/base/docs/Control-Monad-IO-Class.html#t:MonadIO)
638+is a class from the **transformers** package defined as:
690639
691640``` haskell ignore
692641class Monad m => MonadIO m where
693642 liftIO :: IO a -> m a
694643```
695644
696−Obviously, the `IO` monad provides a `MonadIO` instance. Hence for any type `e`, `ExceptT e IO` has a `MonadIO` instance. So if you want to run any kind of IO computation in your handlers, just use `liftIO`:
645+So if you want to run any kind of
646+IO computation in your handlers, just use `liftIO`:
697647
698648``` haskell
699649type IOAPI1 = "myfile.txt" :> Get '[JSON] FileContent
@@ -710,17 +660,16 @@ server5 = do
710660 return (FileContent filecontent)
711661```
712662
713−Failing, through `ServantErr`
714−-----------------------------
663+### Failing, through `ServerError`
715664
716665If you want to explicitly fail at providing the result promised by an endpoint
717666using the appropriate HTTP status code (not found, unauthorized, etc) and some
718−error message, all you have to do is use the `left` function mentioned above
719−and provide it with the appropriate value of type `ServantErr`, which is
667+error message, all you have to do is use the `throwError` function mentioned above
668+and provide it with the appropriate value of type `ServerError`, which is
720669defined as:
721670
722671``` haskell ignore
723−data ServantErr = ServantErr
672+data ServerError = ServerError
724673 { errHTTPCode :: Int
725674 , errReasonPhrase :: String
726675 , errBody :: ByteString -- lazy bytestring
@@ -733,10 +682,10 @@ module. If you want to use these values but add a body or some headers, just
733682use record update syntax:
734683
735684``` haskell
736−failingHandler :: ExceptT ServantErr IO ()
737−failingHandler = throwE myerr
685+failingHandler :: Handler ()
686+failingHandler = throwError myerr
738687
739− where myerr :: ServantErr
688+ where myerr :: ServerError
740689 myerr = err503 { errBody = "Sorry dear user." }
741690```
742691
@@ -749,48 +698,47 @@ server6 = do
749698 exists <- liftIO (doesFileExist "myfile.txt")
750699 if exists
751700 then liftIO (readFile "myfile.txt") >>= return . FileContent
752− else throwE custom404Err
701+ else throwError custom404Err
753702
754703 where custom404Err = err404 { errBody = "myfile.txt just isn't there, please leave this server alone." }
755704```
756705
757−Let's run this server (`dist/build/tutorial/tutorial 5`) and
758−query it, first without the file and then with the file.
706+Here's how that server looks in action:
759707
760708``` bash
761− $ curl --verbose http://localhost:8081/myfile.txt
762− [snip]
763− * Connected to localhost (127.0.0.1) port 8081 (#0)
764− > GET /myfile.txt HTTP/1.1
765− > User-Agent: curl/7.30.0
766− > Host: localhost:8081
767− > Accept: */*
768− >
769− < HTTP/1.1 404 Not Found
770− [snip]
771− myfile.txt just isnt there, please leave this server alone.
772−
773− $ echo Hello > myfile.txt
774−
775− $ curl --verbose http://localhost:8081/myfile.txt
776− [snip]
777− * Connected to localhost (127.0.0.1) port 8081 (#0)
778− > GET /myfile.txt HTTP/1.1
779− > User-Agent: curl/7.30.0
780− > Host: localhost:8081
781− > Accept: */*
782− >
783− < HTTP/1.1 200 OK
784− [snip]
785− < Content-Type: application/json
786− [snip]
787− {"content":"Hello\n"}
788−```
789−
790−Response headers
791−================
792−
793−To add headers to your response, use [addHeader](http://hackage.haskell.org/package/servant-0.4.4/docs/Servant-API-ResponseHeaders.html).
709+$ curl --verbose http://localhost:8081/myfile.txt
710+[snip]
711+* Connected to localhost (127.0.0.1) port 8081 (#0)
712+> GET /myfile.txt HTTP/1.1
713+> User-Agent: curl/7.30.0
714+> Host: localhost:8081
715+> Accept: */*
716+>
717+< HTTP/1.1 404 Not Found
718+[snip]
719+myfile.txt just isn't there, please leave this server alone.
720+
721+$ echo Hello > myfile.txt
722+
723+$ curl --verbose http://localhost:8081/myfile.txt
724+[snip]
725+* Connected to localhost (127.0.0.1) port 8081 (#0)
726+> GET /myfile.txt HTTP/1.1
727+> User-Agent: curl/7.30.0
728+> Host: localhost:8081
729+> Accept: */*
730+>
731+< HTTP/1.1 200 OK
732+[snip]
733+< Content-Type: application/json
734+[snip]
735+{"content":"Hello\n"}
736+```
737+
738+## Response headers
739+
740+To add headers to your response, use
741+[addHeader](http://hackage.haskell.org/package/servant/docs/Servant-API-ResponseHeaders.html).
794742Note that this changes the type of your API, as we can see in the following example:
795743
796744``` haskell
@@ -800,158 +748,89 @@ myHandler :: Server MyHandler
800748myHandler = return $ addHeader 1797 albert
801749```
802750
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:
753+
754+``` haskell
755+type MyHeadfulHandler = Get '[JSON] (Headers '[Header "X-A-Bool" Bool, Header "X-An-Int" Int] User)
756+
757+myHeadfulHandler :: Server MyHeadfulHandler
758+myHeadfulHandler = return $ addHeader True $ addHeader 1797 albert
759+```
760+
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`:
803765
804−Serving static files
805−====================
766+``` haskell
767+type MyMaybeHeaderHandler
768+ = Capture "withHeader" Bool :> Get '[JSON] (Headers '[Header "X-An-Int" Int] User)
806769
807−*servant-server* also provides a way to just serve the content of a directory
770+myMaybeHeaderHandler :: Server MyMaybeHeaderHandler
771+myMaybeHeaderHandler x = return $ if x then addHeader 1797 albert
772+ else noHeader albert
773+```
774+
775+## Serving static files
776+
777+**servant-server** also provides a way to just serve the content of a directory
808778under some path in your web API. As mentioned earlier in this document, the
809779`Raw` combinator can be used in your APIs to mean "plug here any WAI
810−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
811781directory serving WAI application, namely:
812782
813783``` haskell ignore
814784-- exported by Servant and Servant.Server
815−serveDirectory :: FilePath -> Server Raw
785+serveDirectoryWebApp :: FilePath -> Server Raw
816786```
817787
818−`serveDirectory`'s argument must be a path to a valid directory. You can see an
819−example below, runnable with `dist/build/tutorial/tutorial 6`
820−(you **must** run it from within the *servant-examples/* directory!), which is
821−a webserver that serves the various bits of code covered in this
822−getting-started.
788+`serveDirectoryWebApp`'s argument must be a path to a valid directory.
823789
824−The API type will be the following.
790+Here's an example API that will serve some static files:
825791
826792``` haskell
827−type CodeAPI = "code" :> Raw
793+type StaticAPI = "static" :> Raw
828794```
829795
830796And the server:
831797
832798``` haskell
833−codeAPI :: Proxy CodeAPI
834−codeAPI = Proxy
799+staticAPI :: Proxy StaticAPI
800+staticAPI = Proxy
835801```
836802
837803``` haskell
838−server7 :: Server CodeAPI
839−server7 = serveDirectory "tutorial"
804+server7 :: Server StaticAPI
805+server7 = serveDirectoryWebApp "static-files"
840806
841807app3 :: Application
842−app3 = serve codeAPI EmptyConfig server7
808+app3 = serve staticAPI server7
843809```
844810
845−This server will match any request whose path starts with `/code` and will look for a file at the path described by the rest of the request path, inside the *tutorial/* directory of the path you run the program from.
811+This server will match any request whose path starts with `/static` and will look
812+for a file at the path described by the rest of the request path, inside the
813+ *static-files/* directory of the path you run the program from.
846814
847−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.
848818
849−- If a client requests `/code/foo.txt`, the server will look for a file at `./tutorial/foo.txt` (and fail)
850−- If a client requests `/code/T1.hs`, the server will look for a file at `./tutorial/T1.hs` (and succeed)
851−- 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)
819+`serveDirectoryWebApp` uses some standard settings that fit the use case of
820+serving static files for most web apps. You can find out about the other
821+options in the documentation of the `Servant.Server.StaticFiles` module.
852822
853−Here is our little server in action.
823+## Nested APIs
854824
855−``` haskell ignore
856−$ curl http://localhost:8081/code/T1.hs
857−{-# LANGUAGE DataKinds #-}
858−{-# LANGUAGE TypeFamilies #-}
859−{-# LANGUAGE DeriveGeneric #-}
860−{-# LANGUAGE TypeOperators #-}
861−module T1 where
862−
863−import Data.Aeson
864−import Data.Time.Calendar
865−import GHC.Generics
866−import Network.Wai
867−import Servant
868−
869−data User = User
870− { name :: String
871− , age :: Int
872− , email :: String
873− , registration_date :: Day
874− } deriving (Eq, Show, Generic)
875−
876−-- orphan ToJSON instance for Day. necessary to derive one for User
877−instance ToJSON Day where
878− -- display a day in YYYY-mm-dd format
879− toJSON d = toJSON (showGregorian d)
880−
881−instance ToJSON User
882−
883−type UserAPI = "users" :> Get '[JSON] [User]
884−
885−users :: [User]
886−users =
887− [ User "Isaac Newton" 372 "isaac@newton.co.uk" (fromGregorian 1683 3 1)
888− , User "Albert Einstein" 136 "ae@mc2.org" (fromGregorian 1905 12 1)
889− ]
890−
891−userAPI :: Proxy UserAPI
892−userAPI = Proxy
893−
894−server :: Server UserAPI
895−server = return users
896−
897−app :: Application
898−app = serve userAPI server
899−$ curl http://localhost:8081/code/tutorial.hs
900−import Network.Wai
901−import Network.Wai.Handler.Warp
902−import System.Environment
903−
904−import qualified T1
905−import qualified T2
906−import qualified T3
907−import qualified T4
908−import qualified T5
909−import qualified T6
910−import qualified T7
911−import qualified T9
912−import qualified T10
913−
914−app :: String -> (Application -> IO ()) -> IO ()
915−app n f = case n of
916− "1" -> f T1.app
917− "2" -> f T2.app
918− "3" -> f T3.app
919− "4" -> f T4.app
920− "5" -> f T5.app
921− "6" -> f T6.app
922− "7" -> f T7.app
923− "8" -> f T3.app
924− "9" -> T9.writeJSFiles >> f T9.app
925− "10" -> f T10.app
926− _ -> usage
927−
928−main :: IO ()
929−main = do
930− args <- getArgs
931− case args of
932− [n] -> app n (run 8081)
933− _ -> usage
934−
935−usage :: IO ()
936−usage = do
937− putStrLn "Usage:\t tutorial N"
938− putStrLn "\t\twhere N is the number of the example you want to run."
939−
940−$ curl http://localhost:8081/foo
941−not found
942−```
943−
944−Nested APIs
945−===========
946−
947−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:
948827
949828``` haskell
950829type UserAPI3 = -- view the user with given userid, in JSON
951830 Capture "userid" Int :> Get '[JSON] User
952831
953832 :<|> -- delete the user with given userid. empty response
954− Capture "userid" Int :> Delete '[] ()
833+ Capture "userid" Int :> DeleteNoContent
955834```
956835
957836We can instead factor out the `userid`:
@@ -959,32 +838,34 @@ We can instead factor out the `userid`:
959838``` haskell
960839type UserAPI4 = Capture "userid" Int :>
961840 ( Get '[JSON] User
962− :<|> Delete '[] ()
841+ :<|> DeleteNoContent
963842 )
964843```
965844
966−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`:
967847
968848``` haskell ignore
969−Server UserAPI3 = (Int -> ExceptT ServantErr IO User)
970− :<|> (Int -> ExceptT ServantErr IO ())
849+Server UserAPI3 = (Int -> Handler User)
850+ :<|> (Int -> Handler NoContent)
971851
972−Server UserAPI4 = Int -> ( ExceptT ServantErr IO User
973− :<|> ExceptT ServantErr IO ()
852+Server UserAPI4 = Int -> ( Handler User
853+ :<|> Handler NoContent
974854 )
975855```
976856
977857In the first case, each handler receives the *userid* argument. In the latter,
978−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:
979860
980861``` haskell
981862server8 :: Server UserAPI3
982863server8 = getUser :<|> deleteUser
983864
984− where getUser :: Int -> ExceptT ServantErr IO User
865+ where getUser :: Int -> Handler User
985866 getUser _userid = error "..."
986867
987− deleteUser :: Int -> ExceptT ServantErr IO ()
868+ deleteUser :: Int -> Handler NoContent
988869 deleteUser _userid = error "..."
989870
990871-- notice how getUser and deleteUser
@@ -993,14 +874,17 @@ server8 = getUser :<|> deleteUser
993874server9 :: Server UserAPI4
994875server9 userid = getUser userid :<|> deleteUser userid
995876
996− where getUser :: Int -> ExceptT ServantErr IO User
877+ where getUser :: Int -> Handler User
997878 getUser = error "..."
998879
999− deleteUser :: Int -> ExceptT ServantErr IO ()
880+ deleteUser :: Int -> Handler NoContent
1000881 deleteUser = error "..."
1001882```
1002883
1003−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
887+`Server`.
1004888
1005889``` haskell
1006890-- we just factor out the "users" path fragment
@@ -1012,62 +896,63 @@ type API1 = "users" :>
1012896-- we factor out the Request Body
1013897type API2 = ReqBody '[JSON] User :>
1014898 ( Get '[JSON] User -- just display the same user back, don't register it
1015− :<|> Post '[JSON] () -- register the user. empty response
899+ :<|> PostNoContent -- register the user. empty response
1016900 )
1017901
1018902-- we factor out a Header
1019903type API3 = Header "Authorization" Token :>
1020904 ( Get '[JSON] SecretData -- get some secret data, if authorized
1021− :<|> ReqBody '[JSON] SecretData :> Post '[] () -- add some secret data, if authorized
905+ :<|> ReqBody '[JSON] SecretData :> PostNoContent -- add some secret data, if authorized
1022906 )
1023907
1024908newtype Token = Token ByteString
1025909newtype SecretData = SecretData ByteString
1026910```
1027911
1028−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.
1029914
1030915``` haskell
1031916type UsersAPI =
1032917 Get '[JSON] [User] -- list users
1033− :<|> ReqBody '[JSON] User :> Post '[] () -- add a user
918+ :<|> ReqBody '[JSON] User :> PostNoContent -- add a user
1034919 :<|> Capture "userid" Int :>
1035920 ( Get '[JSON] User -- view a user
1036− :<|> ReqBody '[JSON] User :> Put '[] () -- update a user
1037− :<|> Delete '[] () -- delete a user
921+ :<|> ReqBody '[JSON] User :> PutNoContent -- update a user
922+ :<|> DeleteNoContent -- delete a user
1038923 )
1039924
1040925usersServer :: Server UsersAPI
1041926usersServer = getUsers :<|> newUser :<|> userOperations
1042927
1043− where getUsers :: ExceptT ServantErr IO [User]
928+ where getUsers :: Handler [User]
1044929 getUsers = error "..."
1045930
1046− newUser :: User -> ExceptT ServantErr IO ()
931+ newUser :: User -> Handler NoContent
1047932 newUser = error "..."
1048933
1049934 userOperations userid =
1050935 viewUser userid :<|> updateUser userid :<|> deleteUser userid
1051936
1052937 where
1053− viewUser :: Int -> ExceptT ServantErr IO User
938+ viewUser :: Int -> Handler User
1054939 viewUser = error "..."
1055940
1056− updateUser :: Int -> User -> ExceptT ServantErr IO ()
941+ updateUser :: Int -> User -> Handler NoContent
1057942 updateUser = error "..."
1058943
1059− deleteUser :: Int -> ExceptT ServantErr IO ()
944+ deleteUser :: Int -> Handler NoContent
1060945 deleteUser = error "..."
1061946```
1062947
1063948``` haskell
1064949type ProductsAPI =
1065950 Get '[JSON] [Product] -- list products
1066− :<|> ReqBody '[JSON] Product :> Post '[] () -- add a product
951+ :<|> ReqBody '[JSON] Product :> PostNoContent -- add a product
1067952 :<|> Capture "productid" Int :>
1068953 ( Get '[JSON] Product -- view a product
1069− :<|> ReqBody '[JSON] Product :> Put '[] () -- update a product
1070− :<|> Delete '[] () -- delete a product
954+ :<|> ReqBody '[JSON] Product :> PutNoContent -- update a product
955+ :<|> DeleteNoContent -- delete a product
1071956 )
1072957
1073958data Product = Product { productId :: Int }
@@ -1075,23 +960,23 @@ data Product = Product { productId :: Int }
1075960productsServer :: Server ProductsAPI
1076961productsServer = getProducts :<|> newProduct :<|> productOperations
1077962
1078− where getProducts :: ExceptT ServantErr IO [Product]
963+ where getProducts :: Handler [Product]
1079964 getProducts = error "..."
1080965
1081− newProduct :: Product -> ExceptT ServantErr IO ()
966+ newProduct :: Product -> Handler NoContent
1082967 newProduct = error "..."
1083968
1084969 productOperations productid =
1085970 viewProduct productid :<|> updateProduct productid :<|> deleteProduct productid
1086971
1087972 where
1088− viewProduct :: Int -> ExceptT ServantErr IO Product
973+ viewProduct :: Int -> Handler Product
1089974 viewProduct = error "..."
1090975
1091− updateProduct :: Int -> Product -> ExceptT ServantErr IO ()
976+ updateProduct :: Int -> Product -> Handler NoContent
1092977 updateProduct = error "..."
1093978
1094− deleteProduct :: Int -> ExceptT ServantErr IO ()
979+ deleteProduct :: Int -> Handler NoContent
1095980 deleteProduct = error "..."
1096981```
1097982
@@ -1103,139 +988,213 @@ server10 :: Server CombinedAPI
1103988server10 = usersServer :<|> productsServer
1104989```
1105990
1106−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
992+abstract that away:
1107993
1108994``` haskell
1109995-- API for values of type 'a'
1110996-- indexed by values of type 'i'
1111997type APIFor a i =
1112998 Get '[JSON] [a] -- list 'a's
1113− :<|> ReqBody '[JSON] a :> Post '[] () -- add an 'a'
999+ :<|> ReqBody '[JSON] a :> PostNoContent -- add an 'a'
11141000 :<|> Capture "id" i :>
11151001 ( Get '[JSON] a -- view an 'a' given its "identifier" of type 'i'
1116− :<|> ReqBody '[JSON] a :> Put '[] () -- update an 'a'
1117− :<|> Delete '[] () -- delete an 'a'
1002+ :<|> ReqBody '[JSON] a :> PutNoContent -- update an 'a'
1003+ :<|> DeleteNoContent -- delete an 'a'
11181004 )
11191005
11201006-- Build the appropriate 'Server'
11211007-- given the handlers of the right type.
1122−serverFor :: ExceptT ServantErr IO [a] -- handler for listing of 'a's
1123− -> (a -> ExceptT ServantErr IO ()) -- handler for adding an 'a'
1124− -> (i -> ExceptT ServantErr IO a) -- handler for viewing an 'a' given its identifier of type 'i'
1125− -> (i -> a -> ExceptT ServantErr IO ()) -- updating an 'a' with given id
1126− -> (i -> ExceptT ServantErr IO ()) -- deleting an 'a' given its id
1008+serverFor :: Handler [a] -- handler for listing of 'a's
1009+ -> (a -> Handler NoContent) -- handler for adding an 'a'
1010+ -> (i -> Handler a) -- handler for viewing an 'a' given its identifier of type 'i'
1011+ -> (i -> a -> Handler NoContent) -- updating an 'a' with given id
1012+ -> (i -> Handler NoContent) -- deleting an 'a' given its id
11271013 -> Server (APIFor a i)
11281014serverFor = error "..."
11291015-- implementation left as an exercise. contact us on IRC
11301016-- or the mailing list if you get stuck!
11311017```
11321018
1133−Using another monad for your handlers
1134−=====================================
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:
11351022
1136−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.
1023+``` haskell
1024+type CombinedAPI2 = API :<|> "empty" :> EmptyAPI
1025+
1026+server11 :: Server CombinedAPI2
1027+server11 = server3 :<|> emptyServer
1028+```
1029+
1030+## Using another monad for your handlers
1031+
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.
11371034
11381035``` haskell ignore
1139−type Server api = ServerT api (ExceptT ServantErr IO)
1036+type Server api = ServerT api Handler
11401037```
11411038
1142−`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
1045+`SomeMonad Person`.
11431046
1144−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?
11451050
1146−Natural transformations
1147−-----------------------
1051+### Natural transformations
11481052
11491053If we have a function that gets us from an `m a` to an `n a`, for any `a`, what
11501054do we have?
11511055
1152−``` haskell ignore
1153−newtype m :~> n = Nat { unNat :: forall a. m a -> n a}
1056+``` haskell
1057+type (~>) m n = forall a. m a -> n a
1058+```
1059+
1060+For example:
11541061
1155−-- For example
1156−-- listToMaybeNat ::`[] :~> Maybe`
1157−-- listToMaybeNat = Nat listToMaybe -- from Data.Maybe
1062+``` haskell
1063+listToMaybe' :: [] ~> Maybe
1064+listToMaybe' = listToMaybe -- from Data.Maybe
11581065```
1159−(`Nat` comes from "natural transformation", in case you're wondering.)
11601066
1161−So if you want to write handlers using another monad/type than `ExceptT
1162−ServantErr IO`, say the `Reader String` monad, the first thing you have to
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.
1069+
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
11631071prepare is a function:
11641072
11651073``` haskell ignore
1166−readerToEither :: Reader String :~> ExceptT ServantErr IO
1074+readerToHandler :: Reader String a -> Handler a
11671075```
11681076
1169−Let's start with `readerToEither'`. We obviously have to run the `Reader`
1170−computation by supplying it with a `String`, like `"hi"`. We get an `a` out
1171−from that and can then just `return` it into `ExceptT`. We can then just wrap
1172−that function with the `Nat` constructor to make it have the fancier type.
1077+We obviously have to run the `Reader` computation by supplying it with a
1078+`String`, like `"hi"`. We get an `a` out from that and can then just `return`
1079+it into `Handler`.
11731080
11741081``` haskell
1175−readerToEither' :: forall a. Reader String a -> ExceptT ServantErr IO a
1176−readerToEither' r = return (runReader r "hi")
1177−
1178−readerToEither :: Reader String :~> ExceptT ServantErr IO
1179−readerToEither = Nat readerToEither'
1082+readerToHandler :: Reader String a -> Handler a
1083+readerToHandler r = return (runReader r "hi")
11801084```
11811085
11821086We can write some simple webservice with the handlers running in `Reader String`.
11831087
11841088``` haskell
11851089type ReaderAPI = "a" :> Get '[JSON] Int
1186− :<|> "b" :> Get '[JSON] String
1090+ :<|> "b" :> ReqBody '[JSON] Double :> Get '[JSON] Bool
11871091
11881092readerAPI :: Proxy ReaderAPI
11891093readerAPI = Proxy
11901094
11911095readerServerT :: ServerT ReaderAPI (Reader String)
1192−readerServerT = a :<|> b
1193−
1194− where a :: Reader String Int
1195− a = return 1797
1096+readerServerT = a :<|> b where
1097+ a :: Reader String Int
1098+ a = return 1797
11961099
1197− b :: Reader String String
1198− b = ask
1100+ b :: Double -> Reader String Bool
1101+ b _ = asks (== "hi")
11991102```
12001103
12011104We unfortunately can't use `readerServerT` as an argument of `serve`, because
1202−`serve` wants a `Server ReaderAPI`, i.e., with handlers running in `ExceptT
1203−ServantErr IO`. But there's a simple solution to this.
1105+`serve` wants a `Server ReaderAPI`, i.e., with handlers running in `Handler`. But there's a simple solution to this.
12041106
1205−Enter `enter`
1206−-------------
1107+### Welcome `hoistServer`
12071108
1208−That's right. We have just written `readerToEither`, which is exactly what we
1209−would need to apply to the results of all handlers to make the handlers have the
1109+That's right. We have just written `readerToHandler`, which is exactly what we
1110+would need to apply to all handlers to make the handlers have the
12101111right type for `serve`. Being cumbersome to do by hand, we provide a function
1211−`enter` which takes a natural transformation between two parametrized types `m`
1112+`hoistServer` which takes a natural transformation between two parameterized types `m`
12121113and `n` and a `ServerT someapi m`, and returns a `ServerT someapi n`.
12131114
1214−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.
12151117
12161118``` haskell
12171119readerServer :: Server ReaderAPI
1218−readerServer = enter readerToEither readerServerT
1120+readerServer = hoistServer readerAPI readerToHandler readerServerT
12191121
12201122app4 :: Application
1221−app4 = serve readerAPI EmptyConfig readerServer
1123+app4 = serve readerAPI readerServer
12221124```
12231125
1224−And we can indeed see this webservice in action by running `dist/build/tutorial/tutorial 7`.
1126+This is the webservice in action:
12251127
12261128``` bash
12271129$ curl http://localhost:8081/a
122811301797
1229−$ curl http://localhost:8081/b
1230−"hi"
1131+$ curl http://localhost:8081/b -X GET -d '42.0' -H 'Content-Type: application/json'
1132+true
1133+```
1134+
1135+### An arrow is a reader too.
1136+
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.
1139+
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`.
1142+
1143+We can rewrite the previous example without `Reader`:
1144+
1145+```haskell
1146+funServerT :: ServerT ReaderAPI ((->) String)
1147+funServerT = a :<|> b where
1148+ a :: String -> Int
1149+ a _ = 1797
1150+
1151+ -- unfortunately, we cannot make `String` the first argument.
1152+ b :: Double -> String -> Bool
1153+ b _ s = s == "hi"
1154+
1155+funToHandler :: (String -> a) -> Handler a
1156+funToHandler f = return (f "hi")
1157+
1158+app5 :: Application
1159+app5 = serve readerAPI (hoistServer readerAPI funToHandler funServerT)
1160+```
1161+
1162+## Streaming endpoints
1163+
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.
1174+
1175+``` haskell
1176+type StreamAPI = "userStream" :> StreamGet NewlineFraming JSON (SourceIO User)
1177+streamAPI :: Proxy StreamAPI
1178+streamAPI = Proxy
1179+
1180+streamUsers :: SourceIO User
1181+streamUsers = source [isaac, albert, albert]
1182+
1183+app6 :: Application
1184+app6 = serve streamAPI (return streamUsers)
12311185```
12321186
1233−Conclusion
1234−==========
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.
12351195
1236−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*.
1196+## Conclusion
12371197
1238−<div style="text-align: center;">
1239− <p><a href="/tutorial/api-type.html">Previous page: A web API as a type</a></p>
1240− <p><a href="/tutorial/client.html">Next page: Deriving Haskell functions to query an API</a></p>
1241−</div>
1198+You're now equipped to write webservices/web-applications using
1199+**servant**. The rest of this document focuses on **servant-client**,
1200+**servant-js** and **servant-docs**.