Servant.Server

This module lets you implement Servers for defined APIs. You'll most likely just need serve.

Run a wai application from an API

serve :: HasServer layout `[]` => Proxy layout -> Server layout -> Application

serve allows you to implement an API and produce a wai Application.

Example:

type MyApi = "books" :> Get '[JSON] [Book] -- GET /books
        :<|> "books" :> ReqBody Book :> Post '[JSON] Book -- POST /books

server :: Server MyApi
server = listAllBooks :<|> postBook
  where listAllBooks = ...
        postBook book = ...

myApi :: Proxy MyApi
myApi = Proxy

app :: Application
app = serve myApi server

main :: IO ()
main = Network.Wai.Handler.Warp.run 8080 app

serveWithContext :: HasServer layout context => Proxy layout -> Context context -> Server layout -> Application

Construct a wai Application from an API

toApplication :: RoutingApplication -> Application

Handlers for all standard combinators

class HasServer layout context where

Associated Types

type ServerT layout m :: *

Methods

route :: Proxy layout -> Context context -> Delayed env (Server layout) -> Router env

Instances

HasServer * Raw context

Just pass the request to the underlying application and serve its response.

Example:

type MyApi = "images" :> Raw

server :: Server MyApi
server = serveDirectory "/var/www/images"

(HasServer * a context, HasServer * b context) => HasServer * ((:<|>) a b) context

A server for a :<|> b first tries to match the request against the route represented by a and if it fails tries b. You must provide a request handler for each route.

type MyApi = "books" :> Get '[JSON] [Book] -- GET /books
        :<|> "books" :> ReqBody Book :> Post '[JSON] Book -- POST /books

server :: Server MyApi
server = listAllBooks :<|> postBook
  where listAllBooks = ...
        postBook book = ...

(HasContextEntry context (NamedContext name subContext), HasServer * subApi subContext) => HasServer * (WithNamedContext name subContext subApi) context

(KnownSymbol realm, HasServer k api context, HasContextEntry context (BasicAuthCheck usr)) => HasServer * ((:>) * k (BasicAuth realm usr) api) context

Basic Authentication

HasServer k api context => HasServer * ((:>) * k HttpVersion api) context

HasServer k api context => HasServer * ((:>) * k Vault api) context

HasServer k api context => HasServer * ((:>) * k IsSecure api) context

HasServer k api context => HasServer * ((:>) * k RemoteHost api) context

(AllCTUnrender list a, HasServer k sublayout context) => HasServer * ((:>) * k (ReqBody * list a) sublayout) context

If you use ReqBody in one of the endpoints for your API, this automatically requires your server-side handler to be a function that takes an argument of the type specified by ReqBody. The Content-Type header is inspected, and the list provided is used to attempt deserialization. If the request does not have a Content-Type header, it is treated as application/octet-stream (as specified in RFC7231. This lets servant worry about extracting it from the request and turning it into a value of the type you specify.

All it asks is for a FromJSON instance.

Example:

type MyApi = "books" :> ReqBody '[JSON] Book :> Post '[JSON] Book

server :: Server MyApi
server = postBook
  where postBook :: Book -> Handler Book
        postBook book = ...insert into your db...

(KnownSymbol sym, HasServer k sublayout context) => HasServer * ((:>) * k (QueryFlag sym) sublayout) context

If you use QueryFlag "published" in one of the endpoints for your API, this automatically requires your server-side handler to be a function that takes an argument of type Bool.

Example:

type MyApi = "books" :> QueryFlag "published" :> Get '[JSON] [Book]

server :: Server MyApi
server = getBooks
  where getBooks :: Bool -> Handler [Book]
        getBooks onlyPublished = ...return all books, or only the ones that are already published, depending on the argument...

(KnownSymbol sym, FromHttpApiData a, HasServer k sublayout context) => HasServer * ((:>) * k (QueryParams * sym a) sublayout) context

If you use QueryParams "authors" Text in one of the endpoints for your API, this automatically requires your server-side handler to be a function that takes an argument of type [Text].

This lets servant worry about looking up 0 or more values in the query string associated to authors and turning each of them into a value of the type you specify.

You can control how the individual values are converted from Text to your type by simply providing an instance of FromHttpApiData for your type.

Example:

type MyApi = "books" :> QueryParams "authors" Text :> Get '[JSON] [Book]

server :: Server MyApi
server = getBooksBy
  where getBooksBy :: [Text] -> Handler [Book]
        getBooksBy authors = ...return all books by these authors...

(KnownSymbol sym, FromHttpApiData a, HasServer k sublayout context) => HasServer * ((:>) * k (QueryParam * sym a) sublayout) context

If you use QueryParam "author" Text in one of the endpoints for your API, this automatically requires your server-side handler to be a function that takes an argument of type Maybe Text.

This lets servant worry about looking it up in the query string and turning it into a value of the type you specify, enclosed in Maybe, because it may not be there and servant would then hand you Nothing.

You can control how it'll be converted from Text to your type by simply providing an instance of FromHttpApiData for your type.

Example:

type MyApi = "books" :> QueryParam "author" Text :> Get '[JSON] [Book]

server :: Server MyApi
server = getBooksBy
  where getBooksBy :: Maybe Text -> Handler [Book]
        getBooksBy Nothing       = ...return all books...
        getBooksBy (Just author) = ...return books by the given author...

(KnownSymbol sym, FromHttpApiData a, HasServer k sublayout context) => HasServer * ((:>) * k (Header sym a) sublayout) context

If you use Header in one of the endpoints for your API, this automatically requires your server-side handler to be a function that takes an argument of the type specified by Header. This lets servant worry about extracting it from the request and turning it into a value of the type you specify.

All it asks is for a FromHttpApiData instance.

Example:

newtype Referer = Referer Text
  deriving (Eq, Show, FromHttpApiData)

           -- GET /view-my-referer
type MyApi = "view-my-referer" :> Header "Referer" Referer :> Get '[JSON] Referer

server :: Server MyApi
server = viewReferer
  where viewReferer :: Referer -> Handler referer
        viewReferer referer = return referer

(KnownSymbol capture, FromHttpApiData a, HasServer k sublayout context) => HasServer * ((:>) * k (Capture * capture a) sublayout) context

If you use Capture in one of the endpoints for your API, this automatically requires your server-side handler to be a function that takes an argument of the type specified by the Capture. This lets servant worry about getting it from the URL and turning it into a value of the type you specify.

You can control how it'll be converted from Text to your type by simply providing an instance of FromHttpApiData for your type.

Example:

type MyApi = "books" :> Capture "isbn" Text :> Get '[JSON] Book

server :: Server MyApi
server = getBook
  where getBook :: Text -> Handler Book
        getBook isbn = ...

(KnownSymbol path, HasServer k sublayout context) => HasServer * ((:>) Symbol k path sublayout) context

Make sure the incoming request starts with "/path", strip it and pass the rest of the request path to sublayout.

(AllCTRender ctypes a, ReflectMethod k method, KnownNat status, GetHeaders (Headers h a)) => HasServer * (Verb k * method status ctypes (Headers h a)) context

(AllCTRender ctypes a, ReflectMethod k method, KnownNat status) => HasServer * (Verb k * method status ctypes a) context

type Server layout = ServerT layout Handler

type Handler = ExceptT ServantErr IO

Debugging the server layout

layout :: HasServer layout `[]` => Proxy layout -> Text

The function layout produces a textual description of the internal router layout for debugging purposes. Note that the router layout is determined just by the API, not by the handlers.

Example:

For the following API

type API =
       "a" :> "d" :> Get '[JSON] ()
  :<|> "b" :> Capture "x" Int :> Get '[JSON] Bool
  :<|> "c" :> Put '[JSON] Bool
  :<|> "a" :> "e" :> Get '[JSON] Int
  :<|> "b" :> Capture "x" Int :> Put '[JSON] Bool
  :<|> Raw

we get the following output:

/
├─ a/
│  ├─ d/
│  │  └─•
│  └─ e/
│     └─•
├─ b/
│  └─ <capture>/
│     ├─•
│     ┆
│     └─•
├─ c/
│  └─•
┆
└─ <raw>

Explanation of symbols:

├

Normal lines reflect static branching via a table.

a/

Nodes reflect static path components.

─•

Leaves reflect endpoints.

<capture>/

This is a delayed capture of a path component.

<raw>

This is a part of the API we do not know anything about.

┆

Dashed lines suggest a dynamic choice between the part above and below. If there is a success for fatal failure in the first part, that one takes precedence. If both parts fail, the "better" error code will be returned.

layoutWithContext :: HasServer layout context => Proxy layout -> Context context -> Text

Variant of layout that takes an additional Context.

Enter

Sometimes our cherished ExceptT monad isn't quite the type you'd like for your handlers. Maybe you want to thread some configuration in a Reader monad. Or have your types ensure that your handlers don't do any IO. Enter enter.

With enter, you can provide a function, wrapped in the `(:~>)` / Nat newtype, to convert any number of endpoints from one type constructor to another. For example

>>> import Control.Monad.Reader
>>> import qualified Control.Category as C
>>> type ReaderAPI = "ep1" :> Get '[JSON] Int :<|> "ep2" :> Get '[JSON] String
>>> let readerServer = return 1797 :<|> ask :: ServerT ReaderAPI (Reader String)
>>> let mainServer = enter (generalizeNat C.. (runReaderTNat "hi")) readerServer :: Server ReaderAPI

Basic functions and datatypes

enter :: Enter typ arg ret => arg -> typ -> ret

newtype m :~> n

A natural transformation from m to n. Used to enter particular datatypes.

Constructors

Nat

Fields

unNat :: forall a. m a -> n a

Instances

Enter (m a) ((:~>) m n) (n a)

Category (* -> *) (:~>)

Nat utilities

liftNat :: (MonadTrans t, Monad m) => m :~> t m

Like lift.

runReaderTNat :: r -> ReaderT r m :~> m

evalStateTLNat :: Monad m => s -> StateT s m :~> m

evalStateTSNat :: Monad m => s -> StateT s m :~> m

logWriterTLNat :: MonadIO m => (w -> IO ()) -> WriterT w m :~> m

Like logWriterTSNat, but for strict WriterT.

logWriterTSNat :: MonadIO m => (w -> IO ()) -> WriterT w m :~> m

Log the contents of WriterT with the function provided as the first argument, and return the value of the WriterT computation

Functions based on mmorph

hoistNat :: (MFunctor t, Monad m) => (m :~> n) -> t m :~> t n

Like mmorph's hoist.

embedNat :: (MMonad t, Monad n) => (m :~> t n) -> t m :~> t n

Like mmorph's embed.

squashNat :: (Monad m, MMonad t) => t (t m) :~> t m

Like mmorph's squash.

generalizeNat :: Applicative m => Identity :~> m

Like mmorph's generalize.

tweakResponse :: (RouteResult Response -> RouteResult Response) -> Router env -> Router env

Apply a transformation to the response of a Router.

Context

data Context contextTypes where

Contexts are used to pass values to combinators. (They are not meant to be used to pass parameters to your handlers, i.e. they should not replace any custom ReaderT-monad-stack that you're using with enter.) If you don't use combinators that require any context entries, you can just use serve as always.

If you are using combinators that require a non-empty Context you have to use serveWithContext and pass it a Context that contains all the values your combinators need. A Context is essentially a heterogenous list and accessing the elements is being done by type (see getContextEntry). The parameter of the type Context is a type-level list reflecting the types of the contained context entries. To create a Context with entries, use the operator (:.):

>>> :type True :. () :. EmptyContext
True :. () :. EmptyContext :: Context '[Bool, ()]

Constructors

EmptyContext :: Context `[]`

(:.) :: x -> Context xs -> Context (x : xs) infixr 5

Instances

(Eq a, Eq (Context as)) => Eq (Context ((:) * a as))

Eq (Context ([] *))

(Show a, Show (Context as)) => Show (Context ((:) * a as))

Show (Context ([] *))

class HasContextEntry context val where

This class is used to access context entries in Contexts. getContextEntry returns the first value where the type matches:

>>> getContextEntry (True :. False :. EmptyContext) :: Bool
True

If the Context does not contain an entry of the requested type, you'll get an error:

>>> getContextEntry (True :. False :. EmptyContext) :: String
...
    No instance for (HasContextEntry '[] [Char])
...

Methods

getContextEntry :: Context context -> val

Instances

HasContextEntry ((:) * val xs) val

HasContextEntry xs val => HasContextEntry ((:) * notIt xs) val

NamedContext

data NamedContext name subContext

Normally context entries are accessed by their types. In case you need to have multiple values of the same type in your Context and need to access them, we provide NamedContext. You can think of it as sub-namespaces for Contexts.

Constructors

NamedContext (Context subContext)

descendIntoNamedContext :: forall context name subContext. HasContextEntry context (NamedContext name subContext) => Proxy (name :: Symbol) -> Context context -> Context subContext

descendIntoNamedContext allows you to access NamedContexts. Usually you won't have to use it yourself but instead use a combinator like WithNamedContext.

This is how descendIntoNamedContext works:

>>> :set -XFlexibleContexts
>>> let subContext = True :. EmptyContext
>>> :type subContext
subContext :: Context '[Bool]
>>> let parentContext = False :. (NamedContext subContext :: NamedContext "subContext" '[Bool]) :. EmptyContext
>>> :type parentContext
parentContext :: Context '[Bool, NamedContext "subContext" '[Bool]]
>>> descendIntoNamedContext (Proxy :: Proxy "subContext") parentContext :: Context '[Bool]
True :. EmptyContext

Basic Authentication

newtype BasicAuthCheck usr

Datatype wrapping a function used to check authentication.

Constructors

BasicAuthCheck

Fields

unBasicAuthCheck :: BasicAuthData -> IO (BasicAuthResult usr)

Instances

Functor BasicAuthCheck

Generic (BasicAuthCheck usr)

type Rep (BasicAuthCheck usr)

data BasicAuthResult usr

servant-server's current implementation of basic authentication is not immune to certian kinds of timing attacks. Decoding payloads does not take a fixed amount of time.

The result of authentication/authorization

Constructors

Unauthorized

BadPassword

NoSuchUser

Authorized usr

Instances

Functor BasicAuthResult

Eq usr => Eq (BasicAuthResult usr)

Read usr => Read (BasicAuthResult usr)

Show usr => Show (BasicAuthResult usr)

Generic (BasicAuthResult usr)

type Rep (BasicAuthResult usr)

General Authentication

Default error type

data ServantErr

Constructors

ServantErr

Fields

errHTTPCode :: Int

errReasonPhrase :: String

errBody :: ByteString

errHeaders :: [Header]

Instances

Eq ServantErr

Read ServantErr

Show ServantErr

Exception ServantErr

3XX

err300 :: ServantErr

err300 Multiple Choices

Example:

failingHandler :: Handler ()
failingHandler = throwErr $ err300 { errBody = "I can't choose." }

err301 :: ServantErr

err301 Moved Permanently

Example:

failingHandler :: Handler ()
failingHandler = throwErr err301

err302 :: ServantErr

err302 Found

Example:

failingHandler :: Handler ()
failingHandler = throwErr err302

err303 :: ServantErr

err303 See Other

Example:

failingHandler :: Handler ()
failingHandler = throwErr err303

err304 :: ServantErr

err304 Not Modified

Example:

failingHandler :: Handler ()
failingHandler = throwErr err304

err305 :: ServantErr

err305 Use Proxy

Example:

failingHandler :: Handler ()
failingHandler = throwErr err305

err307 :: ServantErr

err307 Temporary Redirect

Example:

failingHandler :: Handler ()
failingHandler = throwErr err307

4XX

err400 :: ServantErr

err400 Bad Request

Example:

failingHandler :: Handler ()
failingHandler = throwErr $ err400 { errBody = "Your request makes no sense to me." }

err401 :: ServantErr

err401 Unauthorized

Example:

failingHandler :: Handler ()
failingHandler = throwErr $ err401 { errBody = "Your credentials are invalid." }

err402 :: ServantErr

err402 Payment Required

Example:

failingHandler :: Handler ()
failingHandler = throwErr $ err402 { errBody = "You have 0 credits. Please give me $$$." }

err403 :: ServantErr

err403 Forbidden

Example:

failingHandler :: Handler ()
failingHandler = throwErr $ err403 { errBody = "Please login first." }

err404 :: ServantErr

err404 Not Found

Example:

failingHandler :: Handler ()
failingHandler = throwErr $ err404 { errBody = "(╯°□°）╯︵ ┻━┻)." }

err405 :: ServantErr

err405 Method Not Allowed

Example:

failingHandler :: Handler ()
failingHandler = throwErr $ err405 { errBody = "Your account privileges does not allow for this.  Please pay $$$." }

err406 :: ServantErr

err406 Not Acceptable

Example:

failingHandler :: Handler ()
failingHandler = throwErr err406

err407 :: ServantErr

err407 Proxy Authentication Required

Example:

failingHandler :: Handler ()
failingHandler = throwErr err407

err409 :: ServantErr

err409 Conflict

Example:

failingHandler :: Handler ()
failingHandler = throwErr $ err409 { errBody = "Transaction conflicts with 59879cb56c7c159231eeacdd503d755f7e835f74" }

err410 :: ServantErr

err410 Gone

Example:

failingHandler :: Handler ()
failingHandler = throwErr $ err410 { errBody = "I know it was here at some point, but.. I blame bad luck." }

err411 :: ServantErr

err411 Length Required

Example:

failingHandler :: Handler ()
failingHandler = throwErr err411

err412 :: ServantErr

err412 Precondition Failed

Example:

failingHandler :: Handler ()
failingHandler = throwErr $ err412 { errBody = "Precondition fail: x < 42 && y > 57" }

err413 :: ServantErr

err413 Request Entity Too Large

Example:

failingHandler :: Handler ()
failingHandler = throwErr $ err413 { errBody = "Request exceeded 64k." }

err414 :: ServantErr

err414 Request-URI Too Large

Example:

failingHandler :: Handler ()
failingHandler = throwErr $ err414 { errBody = "Maximum length is 64." }

err415 :: ServantErr

err415 Unsupported Media Type

Example:

failingHandler :: Handler ()
failingHandler = throwErr $ err415 { errBody = "Supported media types:  gif, png" }

err416 :: ServantErr

err416 Request range not satisfiable

Example:

failingHandler :: Handler ()
failingHandler = throwErr $ err416 { errBody = "Valid range is [0, 424242]." }

err417 :: ServantErr

err417 Expectation Failed

Example:

failingHandler :: Handler ()
failingHandler = throwErr $ err417 { errBody = "I found a quux in the request.  This isn't going to work." }

5XX

err500 :: ServantErr

err500 Internal Server Error

Example:

failingHandler :: Handler ()
failingHandler = throwErr $ err500 { errBody = "Exception in module A.B.C:55.  Have a great day!" }

err501 :: ServantErr

err501 Not Implemented

Example:

failingHandler :: Handler ()
failingHandler = throwErr $ err501 { errBody = "/v1/foo is not supported with quux in the request." }

err502 :: ServantErr

err502 Bad Gateway

Example:

failingHandler :: Handler ()
failingHandler = throwErr $ err502 { errBody = "Tried gateway foo, bar, and baz.  None responded." }

err503 :: ServantErr

err503 Service Unavailable

Example:

failingHandler :: Handler ()
failingHandler = throwErr $ err503 { errBody = "We're rewriting in PHP." }

err504 :: ServantErr

err504 Gateway Time-out

Example:

failingHandler :: Handler ()
failingHandler = throwErr $ err504 { errBody = "Backend foobar did not respond in 5 seconds." }

err505 :: ServantErr

err505 HTTP Version not supported

Example usage:

failingHandler :: Handler ()
failingHandler = throwErr $ err505 { errBody = "I support HTTP/4.0 only." }