Documenting an API

The source for this tutorial section is a literate haskell file, so first we need to have some language extensions and imports:

{-# LANGUAGE DataKinds #-}
{-# LANGUAGE DeriveGeneric #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE OverloadedStrings #-}
{-# LANGUAGE TypeOperators #-}
{-# OPTIONS_GHC -fno-warn-orphans #-}

module Docs where

import Data.ByteString.Lazy (ByteString)
import Data.Proxy
import Data.Text.Lazy.Encoding (encodeUtf8)
import Data.Text.Lazy (pack)
import Network.HTTP.Types
import Network.Wai
import Servant.API
import Servant.Docs
import Servant.Server
import Web.FormUrlEncoded(FromForm(..), ToForm(..))

And we’ll import some things from one of our earlier modules (Serving an API):

import Server (Email(..), ClientInfo(..), Position(..), HelloMessage(..),
  server3, emailForClient)

Like client function generation, documentation generation amounts to inspecting the API type and extracting all the data we need to then present it in some format to users of your API.

This time however, we have to assist servant. While it is able to deduce a lot of things about our API, it can’t magically come up with descriptions of the various pieces of our APIs that are human-friendly and explain what’s going on “at the business-logic level”. A good example to study for documentation generation is our webservice with the /position, /hello and /marketing endpoints from earlier:

type ExampleAPI = "position" :> Capture "x" Int :> Capture "y" Int :> Get '[JSON] Position
      :<|> "hello" :> QueryParam "name" String :> Get '[JSON] HelloMessage
      :<|> "marketing" :> ReqBody '[JSON] ClientInfo :> Post '[JSON] Email

exampleAPI :: Proxy ExampleAPI
exampleAPI = Proxy

While servant can see e.g. that there are 3 endpoints and that the response bodies will be in JSON, it doesn’t know what influence the captures, parameters, request bodies and other combinators have on the webservice. This is where some manual work is required.

For every capture, request body, response body, query param, we have to give some explanations about how it influences the response, what values are possible and the likes. Here’s how it looks like for the parameters we have above.

instance ToCapture (Capture "x" Int) where
  toCapture _ =
    DocCapture "x"                                -- name
               "(integer) position on the x axis" -- description

instance ToCapture (Capture "y" Int) where
  toCapture _ =
    DocCapture "y"                                -- name
               "(integer) position on the y axis" -- description

instance ToSample Position where
  toSamples _ = singleSample (Position 3 14) -- example of output

instance ToParam (QueryParam "name" String) where
  toParam _ =
    DocQueryParam "name"                     -- name
                  ["Alp", "John Doe", "..."] -- example of values (not necessarily exhaustive)
                  "Name of the person to say hello to." -- description
                  Normal -- Normal, List or Flag

instance ToSample HelloMessage where
  toSamples _ =
    [ ("When a value is provided for 'name'", HelloMessage "Hello, Alp")
    , ("When 'name' is not specified", HelloMessage "Hello, anonymous coward")
    -- multiple examples to display this time

ci :: ClientInfo
ci = ClientInfo "Alp" "" 26 ["haskell", "mathematics"]

instance ToSample ClientInfo where
  toSamples _ = singleSample ci

instance ToSample Email where
  toSamples _ = singleSample (emailForClient ci)

Types that are used as request or response bodies have to instantiate the ToSample typeclass which lets you specify one or more examples of values. Captures and QueryParams have to instantiate their respective ToCapture and ToParam classes and provide a name and some information about the concrete meaning of that argument, as illustrated in the code above. The EmptyAPI combinator needs no special treatment as it generates no documentation: an empty API has no endpoints to document.

With all of this, we can derive docs for our API.

apiDocs :: API
apiDocs = docs exampleAPI

API is a type provided by servant-docs that stores all the information one needs about a web API in order to generate documentation in some format. Out of the box, servant-docs only provides a pretty documentation printer that outputs Markdown, but the servant-pandoc package can be used to target many useful formats.

servant’s markdown pretty printer is a function named markdown.

markdown :: API -> String

That lets us see what our API docs look like in markdown, by looking at markdown apiDocs.

## GET /hello

#### GET Parameters:

- name
     - **Values**: *Alp, John Doe, ...*
     - **Description**: Name of the person to say hello to.

#### Response:

- Status code 200
- Headers: []

- Supported content types are:

    - `application/json;charset=utf-8`
    - `application/json`

- When a value is provided for 'name' (`application/json;charset=utf-8`, `application/json`):

{"msg":"Hello, Alp"}

- When 'name' is not specified (`application/json;charset=utf-8`, `application/json`):

{"msg":"Hello, anonymous coward"}

## POST /marketing

#### Request:

- Supported content types are:

    - `application/json;charset=utf-8`
    - `application/json`

- Example (`application/json;charset=utf-8`, `application/json`):


#### Response:

- Status code 200
- Headers: []

- Supported content types are:

    - `application/json;charset=utf-8`
    - `application/json`

- Example (`application/json;charset=utf-8`, `application/json`):

{"subject":"Hey Alp, we miss you!","body":"Hi Alp,\n\nSince you've recently turned 26, have you checked out our latest haskell, mathematics products? Give us a visit!","to":"","from":""}

## GET /position/:x/:y

#### Captures:

- *x*: (integer) position on the x axis
- *y*: (integer) position on the y axis

#### Response:

- Status code 200
- Headers: []

- Supported content types are:

    - `application/json;charset=utf-8`
    - `application/json`

- Example (`application/json;charset=utf-8`, `application/json`):


However, we can also add one or more introduction sections to the document. We just need to tweak the way we generate apiDocs. We will also convert the content to a lazy ByteString since this is what wai expects for Raw endpoints.

docsBS :: ByteString
docsBS = encodeUtf8
       . pack
       . markdown
       $ docsWithIntros [intro] exampleAPI

  where intro = DocIntro "Welcome" ["This is our super webservice's API.", "Enjoy!"]

docsWithIntros just takes an additional parameter, a list of DocIntros that must be displayed before any endpoint docs.

More customisation can be done with the markdownWith function, which allows customising some of the parameters used when generating Markdown. The most obvious of these is how to handle when a request or response body has multiple content types. For example, if we make a slight change to the /marketing endpoint of our API so that the request body can also be encoded as a form:

type ExampleAPI2 = "position" :> Capture "x" Int :> Capture "y" Int :> Get '[JSON] Position
      :<|> "hello" :> QueryParam "name" String :> Get '[JSON] HelloMessage
      :<|> "marketing" :> ReqBody '[JSON, FormUrlEncoded] ClientInfo :> Post '[JSON] Email

instance ToForm ClientInfo
instance FromForm ClientInfo

exampleAPI2 :: Proxy ExampleAPI2
exampleAPI2 = Proxy

api2Docs :: API
api2Docs = docs exampleAPI2

The relevant output of markdown api2Docs is now:

#### Request:

- Supported content types are:

    - `application/json;charset=utf-8`
    - `application/json`
    - `application/x-www-form-urlencoded`

- Example (`application/json;charset=utf-8`, `application/json`):


- Example (`application/x-www-form-urlencoded`):


If, however, you don’t want the extra example encoding shown, then you can use markdownWith (defRenderingOptions & requestExamples .~ FirstContentType) to get behaviour identical to markdown apiDocs.

We can now serve the API and the API docs with a simple server.

type DocsAPI = ExampleAPI :<|> Raw

api :: Proxy DocsAPI
api = Proxy

server :: Server DocsAPI
server = Server.server3 :<|> Tagged serveDocs where
    serveDocs _ respond =
        respond $ responseLBS ok200 [plain] docsBS
    plain = ("Content-Type", "text/plain")

app :: Application
app = serve api server

And if you spin up this server and request anything else than /position, /hello and /marketing, you will see the API docs in markdown. This is because serveDocs is attempted if the 3 other endpoints don’t match and systematically succeeds since its definition is to just return some fixed bytestring with the text/plain content type.