@@ -1,12 +1,35 @@
1−# Serving an API
1+---
2+title: Serving an API
3+toc: true
4+---
25
36Enough chit-chat about type-level combinators and representing an API as a
47type. Can we have a webservice already?
58
6−## A first example
9+If you want to follow along with the code and run the examples while you read this guide:
710
8−Equipped with some basic knowledge about the way we represent APIs, let's now
9−write our first webservice.
11+``` bash
12+cabal get servant-examples
13+cd servant-examples-<VERSION>
14+cabal sandbox init
15+cabal install --dependencies-only
16+cabal configure && cabal build
17+```
18+
19+This will produce a `tutorial` executable in the
20+`dist/build/tutorial` directory that just runs the example corresponding
21+to the number specified as a command line argument:
22+
23+``` bash
24+$ dist/build/tutorial/tutorial
25+Usage: tutorial N
26+ where N is the number of the example you want to run.
27+```
28+
29+A first example
30+===============
31+
32+Equipped with some basic knowledge about the way we represent API, let's now write our first webservice.
1033
1134The source for this tutorial section is a literate haskell file, so first we
1235need to have some language extensions and imports:
@@ -18,23 +41,20 @@ need to have some language extensions and imports:
1841{-# LANGUAGE GeneralizedNewtypeDeriving #-}
1942{-# LANGUAGE MultiParamTypeClasses #-}
2043{-# LANGUAGE OverloadedStrings #-}
21−{-# LANGUAGE RankNTypes #-}
2244{-# LANGUAGE ScopedTypeVariables #-}
2345{-# LANGUAGE TypeOperators #-}
2446
2547module Server where
2648
27−import Prelude ()
28−import Prelude.Compat
29−
30−import Control.Monad.Except
49+import Control.Monad.IO.Class
3150import Control.Monad.Reader
51+import Control.Monad.Trans.Either
3252import Data.Aeson
3353import Data.Aeson.Types
3454import Data.Attoparsec.ByteString
3555import Data.ByteString (ByteString)
56+import Data.Int
3657import Data.List
37−import Data.Maybe
3858import Data.String.Conversions
3959import Data.Time.Calendar
4060import GHC.Generics
@@ -46,18 +66,15 @@ import Servant
4666import System.Directory
4767import Text.Blaze
4868import Text.Blaze.Html.Renderer.Utf8
49−import Servant.Types.SourceT (source)
5069import qualified Data.Aeson.Parser
5170import qualified Text.Blaze.Html
5271```
5372
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.
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.
6178
6279We will write a server that will serve the following API.
6380
@@ -96,6 +113,12 @@ users1 =
96113 ]
97114```
98115
116+Let's also write our API type.
117+
118+``` haskell ignore
119+type UserAPI1 = "users" :> Get '[JSON] [User]
120+```
121+
99122We can now take care of writing the actual webservice that will handle requests
100123to such an API. This one will be very simple, being reduced to just a single
101124endpoint. The type of the web application is determined by the API type,
@@ -107,11 +130,11 @@ corresponding API type.
107130The first thing to know about the `Server` type family is that behind the
108131scenes it will drive the routing, letting you focus only on the business
109132logic. The second thing to know is that for each endpoint, your handlers will
110−by default run in the `Handler` monad. This is overridable very
133+by default run in the `EitherT ServantErr IO` monad. This is overridable very
111134easily, as explained near the end of this guide. Third thing, the type of the
112135value returned in that monad must be the same as the second argument of the
113136HTTP method combinator used for the corresponding endpoint. In our case, it
114−means we must provide a handler of type `Handler [User]`. Well,
137+means we must provide a handler of type `EitherT ServantErr IO [User]`. Well,
115138we have a monad, let's just `return` our list:
116139
117140``` haskell
@@ -119,9 +142,7 @@ server1 :: Server UserAPI1
119142server1 = return users1
120143```
121144
122−That's it. Now we can turn `server` into an actual webserver using
123−[wai](http://hackage.haskell.org/package/wai) and
124−[warp](http://hackage.haskell.org/package/warp):
145+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):
125146
126147``` haskell
127148userAPI :: Proxy UserAPI1
@@ -146,8 +167,9 @@ main = run 8081 app1
146167
147168You can put this all into a file or just grab [servant's
148169repo](http://github.com/haskell-servant/servant) and look at the
149−*doc/tutorial* directory. This code (the source of this web page) is in
150−*doc/tutorial/Server.lhs*.
170+*servant-examples* directory. The code we have just explored is in
171+*tutorial/T1.hs*, runnable with
172+`dist/build/tutorial/tutorial 1`.
151173
152174If you run it, you can go to `http://localhost:8081/users` in your browser or
153175query it with curl and you see:
@@ -157,10 +179,10 @@ $ curl http://localhost:8081/users
157179[{"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"}]
158180```
159181
160−## More endpoints
182+More endpoints
183+==============
161184
162−What if we want more than one endpoint? Let's add `/albert` and `/isaac` to
163−view the corresponding users encoded in JSON.
185+What if we want more than one endpoint? Let's add `/albert` and `/isaac` to view the corresponding users encoded in JSON.
164186
165187``` haskell
166188type UserAPI2 = "users" :> Get '[JSON] [User]
@@ -183,7 +205,7 @@ users2 = [isaac, albert]
183205
184206Now, just like we separate the various endpoints in `UserAPI` with `:<|>`, we
185207are going to separate the handlers with `:<|>` too! They must be provided in
186−the same order as in the API type.
208+the same order as the one they appear in in the API type.
187209
188210``` haskell
189211server2 :: Server UserAPI2
@@ -192,21 +214,22 @@ server2 = return users2
192214 :<|> return isaac
193215```
194216
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`.
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`.
197220
198−## From combinators to handler arguments
221+From combinators to handler arguments
222+=====================================
199223
200224Fine, we can write trivial webservices easily, but none of the two above use
201225any "fancy" combinator from servant. Let's address this and use `QueryParam`,
202−`Capture` and `ReqBody` right away. You'll see how each occurrence of these
226+`Capture` and `ReqBody` right away. You'll see how each occurence of these
203227combinators in an endpoint makes the corresponding handler receive an
204228argument of the appropriate type automatically. You don't have to worry about
205229manually looking up URL captures or query string parameters, or
206230decoding/encoding data from/to JSON. Never.
207231
208−We are going to use the following data types and functions to implement a
209−server for `API`.
232+We are going to use the following data types and functions to implement a server for `API`.
210233
211234``` haskell
212235type API = "position" :> Capture "x" Int :> Capture "y" Int :> Get '[JSON] Position
@@ -214,8 +237,8 @@ type API = "position" :> Capture "x" Int :> Capture "y" Int :> Get '[JSON] Posit
214237 :<|> "marketing" :> ReqBody '[JSON] ClientInfo :> Post '[JSON] Email
215238
216239data Position = Position
217− { xCoord :: Int
218− , yCoord :: Int
240+ { x :: Int
241+ , y :: Int
219242 } deriving Generic
220243
221244instance ToJSON Position
@@ -265,15 +288,15 @@ server3 = position
265288 :<|> hello
266289 :<|> marketing
267290
268− where position :: Int -> Int -> Handler Position
291+ where position :: Int -> Int -> EitherT ServantErr IO Position
269292 position x y = return (Position x y)
270293
271− hello :: Maybe String -> Handler HelloMessage
294+ hello :: Maybe String -> EitherT ServantErr IO HelloMessage
272295 hello mname = return . HelloMessage $ case mname of
273296 Nothing -> "Hello, anonymous coward"
274297 Just n -> "Hello, " ++ n
275298
276− marketing :: ClientInfo -> Handler Email
299+ marketing :: ClientInfo -> EitherT ServantErr IO Email
277300 marketing clientinfo = return (emailForClient clientinfo)
278301```
279302
@@ -288,86 +311,114 @@ parameter might not always be there);
288311
289312 - a `ReqBody contentTypeList a` becomes an argument of type `a`;
290313
291−And that's it. Here's the example in action:
314+And that's it. You can see this example in action by running `dist/build/tutorial/tutorial 3`.
292315
293316``` bash
294317$ curl http://localhost:8081/position/1/2
295−{"xCoord":1,"yCoord":2}
318+{"x":1,"y":2}
296319$ curl http://localhost:8081/hello
297320{"msg":"Hello, anonymous coward"}
298321$ curl http://localhost:8081/hello?name=Alp
299322{"msg":"Hello, Alp"}
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
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
301324{"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"}
302325```
303326
304−For reference, here's a list of some combinators from **servant**:
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.
305329
306− > - `Delete`, `Get`, `Patch`, `Post`, `Put`: these do not become arguments. They provide the return type of handlers, which usually is `Handler <something>`.
330+ > - `Delete`, `Get`, `Patch`, `Post`, `Put`: these do not become arguments. They provide the return type of handlers, which usually is `EitherT ServantErr IO <something>`.
307331 > - `Capture "something" a` becomes an argument 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]`.
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]`.
311335 > - `ReqBody contentTypes a` gets turned into an argument of type `a`.
312336
313−## The `FromHttpApiData`/`ToHttpApiData` classes
337+The `FromText`/`ToText` classes
338+===============================
314339
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.
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.
318343
319344`Capture`s and `QueryParam`s are represented by some textual value in URLs.
320345`Header`s are similarly represented by a pair of a header name and a
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)):
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:
325350
326351``` haskell ignore
327−class FromHttpApiData a where
328− {-# MINIMAL parseUrlPiece | parseQueryParam #-}
329− -- | Parse URL path piece.
330− parseUrlPiece :: Text -> Either Text a
331− parseUrlPiece = parseQueryParam
352+class FromText a where
353+ fromText :: Text -> Maybe a
354+
355+class ToText a where
356+ toText :: a -> Text
357+```
358+
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.
363+
364+``` haskell
365+-- A typical enumeration
366+data Direction
367+ = Up
368+ | Down
369+ | Left
370+ | Right
371+
372+instance FromText Direction where
373+ -- requires {-# LANGUAGE OverloadedStrings #-}
374+ fromText "up" = Just Up
375+ fromText "down" = Just Down
376+ fromText "left" = Just Server.Left
377+ fromText "right" = Just Server.Right
378+ fromText _ = Nothing
332379
333− -- | Parse HTTP header value.
334− parseHeader :: ByteString -> Either Text a
335− parseHeader = parseUrlPiece . decodeUtf8
380+instance ToText Direction where
381+ toText Up = "up"
382+ toText Down = "down"
383+ toText Server.Left = "left"
384+ toText Server.Right = "right"
336385
337− -- | Parse query param value.
338− parseQueryParam :: Text -> Either Text a
339− parseQueryParam = parseUrlPiece
386+newtype UserId = UserId Int64
387+ deriving (FromText, ToText)
340388```
341389
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.
390+or writing the instances by hand:
345391
346−**http-api-data** provides a decent number of instances, helpers for defining new
347−ones, and wonderful documentation.
392+``` haskell ignore
393+instance FromText UserId where
394+ fromText = fmap UserId fromText
395+
396+instance ToText UserId where
397+ toText (UserId i) = toText i
398+```
348399
349400There's not much else to say about these classes. You will need instances for
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).
401+them when using `Capture`, `QueryParam`, `QueryParams`, `MatrixParam`,
402+`MatrixParams` and `Header` with your types. You will need `FromText` instances
403+for server-side request handlers and `ToText` instances only when using
404+*servant-client*, as described in the [section about deriving haskell
405+functions to query an API](/tutorial/client.html).
355406
356−## Using content-types with your data types
407+Using content-types with your data types
408+========================================
357409
358410The same principle was operating when decoding request bodies from JSON, and
359411responses *into* JSON. (JSON is just the running example - you can do this with
360412any content-type.)
361413
362−This section introduces a couple of typeclasses provided by **servant** that make
414+This section introduces a couple of typeclasses provided by *servant* that make
363415all of this work.
364416
365−### The truth behind `JSON`
366−
417+The truth behind `JSON`
418+-----------------------
367419
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.
420+What exactly is `JSON`? Like the 3 other content types provided out of the box
421+by *servant*, it's a really dumb data type.
371422
372423``` haskell ignore
373424data JSON
@@ -377,15 +428,14 @@ data OctetStream
377428```
378429
379430Obviously, this is not all there is to `JSON`, otherwise it would be quite
380−pointless. Like most of the data types in **servant**, `JSON` is mostly there as
431+pointless. Like most of the data types in *servant*, `JSON` is mostly there as
381432a special *symbol* that's associated with encoding (resp. decoding) to (resp.
382433from) the *JSON* format. The way this association is performed can be
383434decomposed into two steps.
384435
385436The first step is to provide a proper
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
437+[`MediaType`](https://hackage.haskell.org/package/http-media-0.6.2/docs/Network-HTTP-Media.html)
438+representation for `JSON`, or for your own content types. If you look at the
389439haddocks from this link, you can see that we just have to specify
390440`application/json` using the appropriate functions. In our case, we can just
391441use `(//) :: ByteString -> ByteString -> MediaType`. The precise way to specify
@@ -401,14 +451,14 @@ instance Accept JSON where
401451```
402452
403453The second step is centered around the `MimeRender` and `MimeUnrender` classes.
404−These classes just let you specify a way to encode and decode
405−values into or from your content-type's representation.
454+These classes just let you specify a way to respectively encode and decode
455+values respectively into or from your content-type's representation.
406456
407457``` haskell ignore
408458class Accept ctype => MimeRender ctype a where
409− mimeRender :: Proxy ctype -> a -> ByteString
459+ mimeRender :: Proxy ctype -> a -> ByteString
410460 -- alternatively readable as:
411− mimeRender :: Proxy ctype -> (a -> ByteString)
461+ mimeRender :: Proxy ctype -> (a -> ByteString)
412462```
413463
414464Given a content-type and some user type, `MimeRender` provides a function that
@@ -429,12 +479,14 @@ And now the `MimeUnrender` class, which lets us extract values from lazy
429479``` haskell ignore
430480class Accept ctype => MimeUnrender ctype a where
431481 mimeUnrender :: Proxy ctype -> ByteString -> Either String a
482+ -- alternatively:
483+ mimeUnrender :: Proxy ctype -> (ByteString -> Either String a)
432484```
433485
434486We don't have much work to do there either, `Data.Aeson.eitherDecode` is
435487precisely what we need. However, it only allows arrays and objects as toplevel
436488JSON values and this has proven to get in our way more than help us so we wrote
437−our own little function around **aeson** and **attoparsec** that allows any type of
489+our own little function around *aeson* and *attoparsec* that allows any type of
438490JSON value at the toplevel of a "JSON document". Here's the definition in case
439491you are curious.
440492
@@ -452,20 +504,21 @@ instance FromJSON a => MimeUnrender JSON a where
452504 mimeUnrender _ = eitherDecodeLenient
453505```
454506
455−And this is all the code that lets you use `JSON` with `ReqBody`, `Get`,
507+And this is all the code that lets you use `JSON` for with `ReqBody`, `Get`,
456508`Post` and friends. We can check our understanding by implementing support
457−for an `HTML` content-type, so that users of your webservice can access an
509+for an `HTML` content type, so that users of your webservice can access an
458510HTML representation of the data they want, ready to be included in any HTML
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.
511+document, e.g. using [jQuery's `load` function](https://api.jquery.com/load/), simply by adding `Accept:
512+text/html` to their request headers.
461513
462−### Case-studies: **servant-blaze** and **servant-lucid**
514+Case-studies: *servant-blaze* and *servant-lucid*
515+-------------------------------------------------
463516
464517These days, most of the haskellers who write their HTML UIs directly from
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**:
518+Haskell use either [blaze-html](http://hackage.haskell.org/package/blaze-html)
519+or [lucid](http://hackage.haskell.org/package/lucid). The best option for
520+*servant* is obviously to support both (and hopefully other templating
521+solutions!).
469522
470523``` haskell
471524data HTMLLucid
@@ -473,20 +526,24 @@ data HTMLLucid
473526
474527Once again, the data type is just there as a symbol for the encoding/decoding
475528functions, except that this time we will only worry about encoding since
476−**lucid** doesn't provide a way to extract data from HTML.
529+*blaze-html* and *lucid* don't provide a way to extract data from HTML.
530+
531+Both packages also have the same `Accept` instance for their `HTMLLucid` type.
477532
478533``` haskell
479534instance Accept HTMLLucid where
480535 contentType _ = "text" // "html" /: ("charset", "utf-8")
481536```
482537
483−Note that this instance uses the `(/:)` operator from **http-media** which lets
538+Note that this instance uses the `(/:)` operator from *http-media* which lets
484539us specify additional information about a content-type, like the charset here.
485540
486−The rendering instances call similar functions that take
541+The rendering instances for both packages both call similar functions that take
487542types with an appropriate instance to an "abstract" HTML representation and
488543then write that to a `ByteString`.
489544
545+For *lucid*:
546+
490547``` haskell
491548instance ToHtml a => MimeRender HTMLLucid a where
492549 mimeRender _ = renderBS . toHtml
@@ -497,7 +554,7 @@ instance MimeRender HTMLLucid (Html a) where
497554 mimeRender _ = renderBS
498555```
499556
500−For **blaze-html** everything works very similarly:
557+For *blaze-html*:
501558
502559``` haskell
503560-- For this tutorial to compile 'HTMLLucid' and 'HTMLBlaze' have to be
@@ -517,13 +574,15 @@ instance MimeRender HTMLBlaze Text.Blaze.Html.Html where
517574 mimeRender _ = renderHtml
518575```
519576
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**).
577+Both [servant-blaze](http://hackage.haskell.org/package/servant-blaze) and
578+[servant-lucid](http://hackage.haskell.org/package/servant-lucid) let you use
579+`HTMLLucid` in any content type list as long as you provide an instance of the
580+appropriate class (`ToMarkup` for *blaze-html*, `ToHtml` for *lucid*).
581+
582+We can now write webservice that uses *servant-lucid* to show the `HTMLLucid`
583+content type in action. First off, imports and pragmas as usual.
524584
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:
585+We will be serving the following API:
527586
528587``` haskell
529588type PersonAPI = "persons" :> Get '[JSON, HTMLLucid] [Person]
@@ -540,7 +599,7 @@ data Person = Person
540599instance ToJSON Person
541600```
542601
543−Now, let's teach **lucid** how to render a `Person` as a row in a table, and then
602+Now, let's teach *lucid* how to render a `Person` as a row in a table, and then
544603a list of `Person`s as a table with a row per person.
545604
546605``` haskell
@@ -571,8 +630,8 @@ instance ToHtml [Person] where
571630We create some `Person` values and serve them as a list:
572631
573632``` haskell
574−people :: [Person]
575−people =
633+persons :: [Person]
634+persons =
576635 [ Person "Isaac" "Newton"
577636 , Person "Albert" "Einstein"
578637 ]
@@ -581,69 +640,75 @@ personAPI :: Proxy PersonAPI
581640personAPI = Proxy
582641
583642server4 :: Server PersonAPI
584−server4 = return people
643+server4 = return persons
585644
586645app2 :: Application
587646app2 = serve personAPI server4
588647```
589648
590−And we're good to go:
649+And we're good to go. You can run this example with `dist/build/tutorial/tutorial 4`.
591650
592651``` bash
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
652+ $ curl http://localhost:8081/persons
653+ [{"lastName":"Newton","firstName":"Isaac"},{"lastName":"Einstein","firstName":"Albert"}]
654+ $ curl -H 'Accept: text/html' http://localhost:8081/persons
655+ <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>
656+ # or just point your browser to http://localhost:8081/persons
598657```
599658
600−## The `Handler` monad
659+The `EitherT ServantErr IO` monad
660+=================================
601661
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
662+At the heart of the handlers is the monad they run in, namely `EitherT
663+ServantErr IO`. One might wonder: why this monad? The answer is that it is the
605664simplest monad with the following properties:
606665
607−- it lets us both return a successful result (using `return`)
608−or "fail" with a descriptive error (using `throwError`);
666+- it lets us both return a successful result (with the `Right` branch of
667+`Either`) or "fail" with a descriptive error (with the `Left` branch of
668+`Either`);
609669- it lets us perform IO, which is absolutely vital since most webservices exist
610−as interfaces to databases that we interact with in `IO`.
670+as interfaces to databases that we interact with in `IO`;
611671
612672Let's recall some definitions.
613673
614674``` haskell ignore
615−-- from the 'mtl' package at
616−newtype ExceptT e m a = ExceptT (m (Either e a))
675+-- from the Prelude
676+data Either e a = Left e | Right a
677+
678+-- from the 'either' package at
679+-- http://hackage.haskell.org/package/either-4.3.3.2/docs/Control-Monad-Trans-Either.html
680+newtype EitherT e m a
681+ = EitherT { runEitherT :: m (Either e a) }
617682```
618683
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
684+In short, this means that a handler of type `EitherT ServantErr IO a` is simply
685+equivalent to a computation of type `IO (Either ServantErr a)`, that is, an IO
621686action that either returns an error or a result.
622687
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).
688+The aforementioned `either` package is worth taking a look at. Perhaps most
689+importantly:
690+
691+``` haskell ignore
692+left :: Monad m => e -> EitherT e m a
693+```
694+Allows you to return an error from your handler (whereas `return` is enough to
695+return a success).
628696
629697Most of what you'll be doing in your handlers is running some IO and,
630698depending on the result, you might sometimes want to throw an error of some
631699kind and abort early. The next two sections cover how to do just that.
632700
633−### Performing IO
701+Performing IO
702+-------------
634703
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:
704+Another important instance from the list above is `MonadIO m => MonadIO (EitherT 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:
639705
640706``` haskell ignore
641707class Monad m => MonadIO m where
642708 liftIO :: IO a -> m a
643709```
644710
645−So if you want to run any kind of
646−IO computation in your handlers, just use `liftIO`:
711+Obviously, the `IO` monad provides a `MonadIO` instance. Hence for any type `e`, `EitherT e IO` has a `MonadIO` instance. So if you want to run any kind of IO computation in your handlers, just use `liftIO`:
647712
648713``` haskell
649714type IOAPI1 = "myfile.txt" :> Get '[JSON] FileContent
@@ -660,16 +725,17 @@ server5 = do
660725 return (FileContent filecontent)
661726```
662727
663−### Failing, through `ServerError`
728+Failing, through `ServantErr`
729+-----------------------------
664730
665731If you want to explicitly fail at providing the result promised by an endpoint
666732using the appropriate HTTP status code (not found, unauthorized, etc) and some
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
733+error message, all you have to do is use the `left` function mentioned above
734+and provide it with the appropriate value of type `ServantErr`, which is
669735defined as:
670736
671737``` haskell ignore
672−data ServerError = ServerError
738+data ServantErr = ServantErr
673739 { errHTTPCode :: Int
674740 , errReasonPhrase :: String
675741 , errBody :: ByteString -- lazy bytestring
@@ -682,10 +748,10 @@ module. If you want to use these values but add a body or some headers, just
682748use record update syntax:
683749
684750``` haskell
685−failingHandler :: Handler ()
686−failingHandler = throwError myerr
751+failingHandler :: EitherT ServantErr IO ()
752+failingHandler = left myerr
687753
688− where myerr :: ServerError
754+ where myerr :: ServantErr
689755 myerr = err503 { errBody = "Sorry dear user." }
690756```
691757
@@ -698,47 +764,48 @@ server6 = do
698764 exists <- liftIO (doesFileExist "myfile.txt")
699765 if exists
700766 then liftIO (readFile "myfile.txt") >>= return . FileContent
701− else throwError custom404Err
767+ else left custom404Err
702768
703769 where custom404Err = err404 { errBody = "myfile.txt just isn't there, please leave this server alone." }
704770```
705771
706−Here's how that server looks in action:
772+Let's run this server (`dist/build/tutorial/tutorial 5`) and
773+query it, first without the file and then with the file.
707774
708775``` bash
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).
776+ $ curl --verbose http://localhost:8081/myfile.txt
777+ [snip]
778+ * Connected to localhost (127.0.0.1) port 8081 (#0)
779+ > GET /myfile.txt HTTP/1.1
780+ > User-Agent: curl/7.30.0
781+ > Host: localhost:8081
782+ > Accept: */*
783+ >
784+ < HTTP/1.1 404 Not Found
785+ [snip]
786+ myfile.txt just isnt there, please leave this server alone.
787+
788+ $ echo Hello > myfile.txt
789+
790+ $ curl --verbose http://localhost:8081/myfile.txt
791+ [snip]
792+ * Connected to localhost (127.0.0.1) port 8081 (#0)
793+ > GET /myfile.txt HTTP/1.1
794+ > User-Agent: curl/7.30.0
795+ > Host: localhost:8081
796+ > Accept: */*
797+ >
798+ < HTTP/1.1 200 OK
799+ [snip]
800+ < Content-Type: application/json
801+ [snip]
802+ {"content":"Hello\n"}
803+```
804+
805+Response headers
806+================
807+
808+To add headers to your response, use [addHeader](http://hackage.haskell.org/package/servant-0.4.4/docs/Servant-API-ResponseHeaders.html).
742809Note that this changes the type of your API, as we can see in the following example:
743810
744811``` haskell
@@ -748,89 +815,158 @@ myHandler :: Server MyHandler
748815myHandler = return $ addHeader 1797 albert
749816```
750817
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`:
765818
766−``` haskell
767−type MyMaybeHeaderHandler
768− = Capture "withHeader" Bool :> Get '[JSON] (Headers '[Header "X-An-Int" Int] User)
769−
770−myMaybeHeaderHandler :: Server MyMaybeHeaderHandler
771−myMaybeHeaderHandler x = return $ if x then addHeader 1797 albert
772− else noHeader albert
773−```
819+Serving static files
820+====================
774821
775−## Serving static files
776−
777−**servant-server** also provides a way to just serve the content of a directory
822+*servant-server* also provides a way to just serve the content of a directory
778823under some path in your web API. As mentioned earlier in this document, the
779824`Raw` combinator can be used in your APIs to mean "plug here any WAI
780−application". Well, **servant-server** provides a function to get a file and
825+application". Well, servant-server provides a function to get a file and
781826directory serving WAI application, namely:
782827
783828``` haskell ignore
784829-- exported by Servant and Servant.Server
785−serveDirectoryWebApp :: FilePath -> Server Raw
830+serveDirectory :: FilePath -> Server Raw
786831```
787832
788−`serveDirectoryWebApp`'s argument must be a path to a valid directory.
833+`serveDirectory`'s argument must be a path to a valid directory. You can see an
834+example below, runnable with `dist/build/tutorial/tutorial 6`
835+(you **must** run it from within the *servant-examples/* directory!), which is
836+a webserver that serves the various bits of code covered in this
837+getting-started.
789838
790−Here's an example API that will serve some static files:
839+The API type will be the following.
791840
792841``` haskell
793−type StaticAPI = "static" :> Raw
842+type CodeAPI = "code" :> Raw
794843```
795844
796845And the server:
797846
798847``` haskell
799−staticAPI :: Proxy StaticAPI
800−staticAPI = Proxy
848+codeAPI :: Proxy CodeAPI
849+codeAPI = Proxy
801850```
802851
803852``` haskell
804−server7 :: Server StaticAPI
805−server7 = serveDirectoryWebApp "static-files"
853+server7 :: Server CodeAPI
854+server7 = serveDirectory "tutorial"
806855
807856app3 :: Application
808−app3 = serve staticAPI server7
857+app3 = serve codeAPI server7
809858```
810859
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.
860+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.
861+
862+In other words:
863+
864+- If a client requests `/code/foo.txt`, the server will look for a file at `./tutorial/foo.txt` (and fail)
865+- If a client requests `/code/T1.hs`, the server will look for a file at `./tutorial/T1.hs` (and succeed)
866+- 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)
867+
868+Here is our little server in action.
869+
870+``` haskell ignore
871+$ curl http://localhost:8081/code/T1.hs
872+{-# LANGUAGE DataKinds #-}
873+{-# LANGUAGE TypeFamilies #-}
874+{-# LANGUAGE DeriveGeneric #-}
875+{-# LANGUAGE TypeOperators #-}
876+module T1 where
877+
878+import Data.Aeson
879+import Data.Time.Calendar
880+import GHC.Generics
881+import Network.Wai
882+import Servant
883+
884+data User = User
885+ { name :: String
886+ , age :: Int
887+ , email :: String
888+ , registration_date :: Day
889+ } deriving (Eq, Show, Generic)
890+
891+-- orphan ToJSON instance for Day. necessary to derive one for User
892+instance ToJSON Day where
893+ -- display a day in YYYY-mm-dd format
894+ toJSON d = toJSON (showGregorian d)
895+
896+instance ToJSON User
897+
898+type UserAPI = "users" :> Get '[JSON] [User]
899+
900+users :: [User]
901+users =
902+ [ User "Isaac Newton" 372 "isaac@newton.co.uk" (fromGregorian 1683 3 1)
903+ , User "Albert Einstein" 136 "ae@mc2.org" (fromGregorian 1905 12 1)
904+ ]
905+
906+userAPI :: Proxy UserAPI
907+userAPI = Proxy
908+
909+server :: Server UserAPI
910+server = return users
911+
912+app :: Application
913+app = serve userAPI server
914+$ curl http://localhost:8081/code/tutorial.hs
915+import Network.Wai
916+import Network.Wai.Handler.Warp
917+import System.Environment
918+
919+import qualified T1
920+import qualified T2
921+import qualified T3
922+import qualified T4
923+import qualified T5
924+import qualified T6
925+import qualified T7
926+import qualified T9
927+import qualified T10
928+
929+app :: String -> (Application -> IO ()) -> IO ()
930+app n f = case n of
931+ "1" -> f T1.app
932+ "2" -> f T2.app
933+ "3" -> f T3.app
934+ "4" -> f T4.app
935+ "5" -> f T5.app
936+ "6" -> f T6.app
937+ "7" -> f T7.app
938+ "8" -> f T3.app
939+ "9" -> T9.writeJSFiles >> f T9.app
940+ "10" -> f T10.app
941+ _ -> usage
814942
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.
943+main :: IO ()
944+main = do
945+ args <- getArgs
946+ case args of
947+ [n] -> app n (run 8081)
948+ _ -> usage
818949
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.
950+usage :: IO ()
951+usage = do
952+ putStrLn "Usage:\t tutorial N"
953+ putStrLn "\t\twhere N is the number of the example you want to run."
822954
823−## Nested APIs
955+$ curl http://localhost:8081/foo
956+not found
957+```
824958
825−Let's see how you can define APIs in a modular way, while avoiding repetition.
826−Consider this simple example:
959+Nested APIs
960+===========
961+
962+Let's see how you can define APIs in a modular way, while avoiding repetition. Consider this simple example:
827963
828964``` haskell
829965type UserAPI3 = -- view the user with given userid, in JSON
830966 Capture "userid" Int :> Get '[JSON] User
831967
832968 :<|> -- delete the user with given userid. empty response
833− Capture "userid" Int :> DeleteNoContent
969+ Capture "userid" Int :> Delete '[] ()
834970```
835971
836972We can instead factor out the `userid`:
@@ -838,34 +974,32 @@ We can instead factor out the `userid`:
838974``` haskell
839975type UserAPI4 = Capture "userid" Int :>
840976 ( Get '[JSON] User
841− :<|> DeleteNoContent
977+ :<|> Delete '[] ()
842978 )
843979```
844980
845−However, you have to be aware that this has an effect on the type of the
846−corresponding `Server`:
981+However, you have to be aware that this has an effect on the type of the corresponding `Server`:
847982
848983``` haskell ignore
849−Server UserAPI3 = (Int -> Handler User)
850− :<|> (Int -> Handler NoContent)
984+Server UserAPI3 = (Int -> EitherT ServantErr IO User)
985+ :<|> (Int -> EitherT ServantErr IO ())
851986
852−Server UserAPI4 = Int -> ( Handler User
853− :<|> Handler NoContent
987+Server UserAPI4 = Int -> ( EitherT ServantErr IO User
988+ :<|> EitherT ServantErr IO ()
854989 )
855990```
856991
857992In the first case, each handler receives the *userid* argument. In the latter,
858−the whole `Server` takes the *userid* and has handlers that are just
859−computations in `Handler`, with no arguments. In other words:
993+the whole `Server` takes the *userid* and has handlers that are just computations in `EitherT`, with no arguments. In other words:
860994
861995``` haskell
862996server8 :: Server UserAPI3
863997server8 = getUser :<|> deleteUser
864998
865− where getUser :: Int -> Handler User
999+ where getUser :: Int -> EitherT ServantErr IO User
8661000 getUser _userid = error "..."
8671001
868− deleteUser :: Int -> Handler NoContent
1002+ deleteUser :: Int -> EitherT ServantErr IO ()
8691003 deleteUser _userid = error "..."
8701004
8711005-- notice how getUser and deleteUser
@@ -874,17 +1008,14 @@ server8 = getUser :<|> deleteUser
8741008server9 :: Server UserAPI4
8751009server9 userid = getUser userid :<|> deleteUser userid
8761010
877− where getUser :: Int -> Handler User
1011+ where getUser :: Int -> EitherT ServantErr IO User
8781012 getUser = error "..."
8791013
880− deleteUser :: Int -> Handler NoContent
1014+ deleteUser :: Int -> EitherT ServantErr IO ()
8811015 deleteUser = error "..."
8821016```
8831017
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`.
1018+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`.
8881019
8891020``` haskell
8901021-- we just factor out the "users" path fragment
@@ -896,63 +1027,62 @@ type API1 = "users" :>
8961027-- we factor out the Request Body
8971028type API2 = ReqBody '[JSON] User :>
8981029 ( Get '[JSON] User -- just display the same user back, don't register it
899− :<|> PostNoContent -- register the user. empty response
1030+ :<|> Post '[JSON] () -- register the user. empty response
9001031 )
9011032
9021033-- we factor out a Header
9031034type API3 = Header "Authorization" Token :>
9041035 ( Get '[JSON] SecretData -- get some secret data, if authorized
905− :<|> ReqBody '[JSON] SecretData :> PostNoContent -- add some secret data, if authorized
1036+ :<|> ReqBody '[JSON] SecretData :> Post '[] () -- add some secret data, if authorized
9061037 )
9071038
9081039newtype Token = Token ByteString
9091040newtype SecretData = SecretData ByteString
9101041```
9111042
912−This approach lets you define APIs modularly and assemble them all into one big
913−API type only at the end.
1043+This approach lets you define APIs modularly and assemble them all into one big API type only at the end.
9141044
9151045``` haskell
9161046type UsersAPI =
9171047 Get '[JSON] [User] -- list users
918− :<|> ReqBody '[JSON] User :> PostNoContent -- add a user
1048+ :<|> ReqBody '[JSON] User :> Post '[] () -- add a user
9191049 :<|> Capture "userid" Int :>
9201050 ( Get '[JSON] User -- view a user
921− :<|> ReqBody '[JSON] User :> PutNoContent -- update a user
922− :<|> DeleteNoContent -- delete a user
1051+ :<|> ReqBody '[JSON] User :> Put '[] () -- update a user
1052+ :<|> Delete '[] () -- delete a user
9231053 )
9241054
9251055usersServer :: Server UsersAPI
9261056usersServer = getUsers :<|> newUser :<|> userOperations
9271057
928− where getUsers :: Handler [User]
1058+ where getUsers :: EitherT ServantErr IO [User]
9291059 getUsers = error "..."
9301060
931− newUser :: User -> Handler NoContent
1061+ newUser :: User -> EitherT ServantErr IO ()
9321062 newUser = error "..."
9331063
9341064 userOperations userid =
9351065 viewUser userid :<|> updateUser userid :<|> deleteUser userid
9361066
9371067 where
938− viewUser :: Int -> Handler User
1068+ viewUser :: Int -> EitherT ServantErr IO User
9391069 viewUser = error "..."
9401070
941− updateUser :: Int -> User -> Handler NoContent
1071+ updateUser :: Int -> User -> EitherT ServantErr IO ()
9421072 updateUser = error "..."
9431073
944− deleteUser :: Int -> Handler NoContent
1074+ deleteUser :: Int -> EitherT ServantErr IO ()
9451075 deleteUser = error "..."
9461076```
9471077
9481078``` haskell
9491079type ProductsAPI =
9501080 Get '[JSON] [Product] -- list products
951− :<|> ReqBody '[JSON] Product :> PostNoContent -- add a product
1081+ :<|> ReqBody '[JSON] Product :> Post '[] () -- add a product
9521082 :<|> Capture "productid" Int :>
9531083 ( Get '[JSON] Product -- view a product
954− :<|> ReqBody '[JSON] Product :> PutNoContent -- update a product
955− :<|> DeleteNoContent -- delete a product
1084+ :<|> ReqBody '[JSON] Product :> Put '[] () -- update a product
1085+ :<|> Delete '[] () -- delete a product
9561086 )
9571087
9581088data Product = Product { productId :: Int }
@@ -960,23 +1090,23 @@ data Product = Product { productId :: Int }
9601090productsServer :: Server ProductsAPI
9611091productsServer = getProducts :<|> newProduct :<|> productOperations
9621092
963− where getProducts :: Handler [Product]
1093+ where getProducts :: EitherT ServantErr IO [Product]
9641094 getProducts = error "..."
9651095
966− newProduct :: Product -> Handler NoContent
1096+ newProduct :: Product -> EitherT ServantErr IO ()
9671097 newProduct = error "..."
9681098
9691099 productOperations productid =
9701100 viewProduct productid :<|> updateProduct productid :<|> deleteProduct productid
9711101
9721102 where
973− viewProduct :: Int -> Handler Product
1103+ viewProduct :: Int -> EitherT ServantErr IO Product
9741104 viewProduct = error "..."
9751105
976− updateProduct :: Int -> Product -> Handler NoContent
1106+ updateProduct :: Int -> Product -> EitherT ServantErr IO ()
9771107 updateProduct = error "..."
9781108
979− deleteProduct :: Int -> Handler NoContent
1109+ deleteProduct :: Int -> EitherT ServantErr IO ()
9801110 deleteProduct = error "..."
9811111```
9821112
@@ -988,213 +1118,139 @@ server10 :: Server CombinedAPI
9881118server10 = usersServer :<|> productsServer
9891119```
9901120
991−Finally, we can realize the user and product APIs are quite similar and
992−abstract that away:
1121+Finally, we can realize the user and product APIs are quite similar and abstract that away:
9931122
9941123``` haskell
9951124-- API for values of type 'a'
9961125-- indexed by values of type 'i'
9971126type APIFor a i =
9981127 Get '[JSON] [a] -- list 'a's
999− :<|> ReqBody '[JSON] a :> PostNoContent -- add an 'a'
1128+ :<|> ReqBody '[JSON] a :> Post '[] () -- add an 'a'
10001129 :<|> Capture "id" i :>
10011130 ( Get '[JSON] a -- view an 'a' given its "identifier" of type 'i'
1002− :<|> ReqBody '[JSON] a :> PutNoContent -- update an 'a'
1003− :<|> DeleteNoContent -- delete an 'a'
1131+ :<|> ReqBody '[JSON] a :> Put '[] () -- update an 'a'
1132+ :<|> Delete '[] () -- delete an 'a'
10041133 )
10051134
10061135-- Build the appropriate 'Server'
10071136-- given the handlers of the right type.
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
1137+serverFor :: EitherT ServantErr IO [a] -- handler for listing of 'a's
1138+ -> (a -> EitherT ServantErr IO ()) -- handler for adding an 'a'
1139+ -> (i -> EitherT ServantErr IO a) -- handler for viewing an 'a' given its identifier of type 'i'
1140+ -> (i -> a -> EitherT ServantErr IO ()) -- updating an 'a' with given id
1141+ -> (i -> EitherT ServantErr IO ()) -- deleting an 'a' given its id
10131142 -> Server (APIFor a i)
10141143serverFor = error "..."
10151144-- implementation left as an exercise. contact us on IRC
10161145-- or the mailing list if you get stuck!
10171146```
10181147
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:
1148+Using another monad for your handlers
1149+=====================================
10221150
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.
1151+Remember how `Server` turns combinators for HTTP methods into `EitherT ServantErr IO`? Well, actually, there's more to that. `Server` is actually a simple type synonym.
10341152
10351153``` haskell ignore
1036−type Server api = ServerT api Handler
1154+type Server api = ServerT api (EitherT ServantErr IO)
10371155```
10381156
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`.
1157+`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`.
10461158
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?
1159+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?
10501160
1051−### Natural transformations
1161+Natural transformations
1162+-----------------------
10521163
10531164If we have a function that gets us from an `m a` to an `n a`, for any `a`, what
10541165do we have?
10551166
1056−``` haskell
1057−type (~>) m n = forall a. m a -> n a
1058−```
1059−
1060−For example:
1167+``` haskell ignore
1168+newtype m :~> n = Nat { unNat :: forall a. m a -> n a}
10611169
1062−``` haskell
1063−listToMaybe' :: [] ~> Maybe
1064−listToMaybe' = listToMaybe -- from Data.Maybe
1170+-- For example
1171+-- listToMaybeNat ::`[] :~> Maybe`
1172+-- listToMaybeNat = Nat listToMaybe -- from Data.Maybe
10651173```
1174+(`Nat` comes from "natural transformation", in case you're wondering.)
10661175
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
1176+So if you want to write handlers using another monad/type than `EitherT
1177+ServantErr IO`, say the `Reader String` monad, the first thing you have to
10711178prepare is a function:
10721179
10731180``` haskell ignore
1074−readerToHandler :: Reader String a -> Handler a
1181+readerToEither :: Reader String :~> EitherT ServantErr IO
10751182```
10761183
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`.
1184+Let's start with `readerToEither'`. We obviously have to run the `Reader`
1185+computation by supplying it with a `String`, like `"hi"`. We get an `a` out
1186+from that and can then just `return` it into `EitherT`. We can then just wrap
1187+that function with the `Nat` constructor to make it have the fancier type.
10801188
10811189``` haskell
1082−readerToHandler :: Reader String a -> Handler a
1083−readerToHandler r = return (runReader r "hi")
1190+readerToEither' :: forall a. Reader String a -> EitherT ServantErr IO a
1191+readerToEither' r = return (runReader r "hi")
1192+
1193+readerToEither :: Reader String :~> EitherT ServantErr IO
1194+readerToEither = Nat readerToEither'
10841195```
10851196
10861197We can write some simple webservice with the handlers running in `Reader String`.
10871198
10881199``` haskell
10891200type ReaderAPI = "a" :> Get '[JSON] Int
1090− :<|> "b" :> ReqBody '[JSON] Double :> Get '[JSON] Bool
1201+ :<|> "b" :> Get '[JSON] String
10911202
10921203readerAPI :: Proxy ReaderAPI
10931204readerAPI = Proxy
10941205
10951206readerServerT :: ServerT ReaderAPI (Reader String)
1096−readerServerT = a :<|> b where
1097− a :: Reader String Int
1098− a = return 1797
1207+readerServerT = a :<|> b
1208+
1209+ where a :: Reader String Int
1210+ a = return 1797
10991211
1100− b :: Double -> Reader String Bool
1101− b _ = asks (== "hi")
1212+ b :: Reader String String
1213+ b = ask
11021214```
11031215
11041216We unfortunately can't use `readerServerT` as an argument of `serve`, because
1105−`serve` wants a `Server ReaderAPI`, i.e., with handlers running in `Handler`. But there's a simple solution to this.
1217+`serve` wants a `Server ReaderAPI`, i.e., with handlers running in `EitherT
1218+ServantErr IO`. But there's a simple solution to this.
11061219
1107−### Welcome `hoistServer`
1220+Enter `enter`
1221+-------------
11081222
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
1223+That's right. We have just written `readerToEither`, which is exactly what we
1224+would need to apply to the results of all handlers to make the handlers have the
11111225right type for `serve`. Being cumbersome to do by hand, we provide a function
1112−`hoistServer` which takes a natural transformation between two parameterized types `m`
1226+`enter` which takes a natural transformation between two parametrized types `m`
11131227and `n` and a `ServerT someapi m`, and returns a `ServerT someapi n`.
11141228
1115−In our case, we can wrap up our little webservice by using
1116−`hoistServer readerAPI readerToHandler` on our handlers.
1229+In our case, we can wrap up our little webservice by using `enter readerToEither` on our handlers.
11171230
11181231``` haskell
11191232readerServer :: Server ReaderAPI
1120−readerServer = hoistServer readerAPI readerToHandler readerServerT
1233+readerServer = enter readerToEither readerServerT
11211234
11221235app4 :: Application
11231236app4 = serve readerAPI readerServer
11241237```
11251238
1126−This is the webservice in action:
1239+And we can indeed see this webservice in action by running `dist/build/tutorial/tutorial 7`.
11271240
11281241``` bash
11291242$ curl http://localhost:8081/a
113012431797
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)
1244+$ curl http://localhost:8081/b
1245+"hi"
11851246```
11861247
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.
1248+Conclusion
1249+==========
11951250
1196−## Conclusion
1251+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*.
11971252
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**.
1253+<div style="text-align: center;">
1254+ <p><a href="/tutorial/api-type.html">Previous page: A web API as a type</a></p>
1255+ <p><a href="/tutorial/client.html">Next page: Deriving Haskell functions to query an API</a></p>
1256+</div>