Monthly Downloads: 13
Programming language: Haskell
License: MIT License
Tags: Utils     Utility    
Latest version: v0.4.0.0

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This library uses Data.Typeable to generate documentation for Servant API types.

It relies on the typeRep of Servant's combinators and other datatypes used in the API to generate the documentation.

Why do we need this?

1) We need the API Format types inside the documentation. They can be used as keys to look up examples, fields, and other miscellaneous details.

2) We want to generate an overview of our endpoints from the information our API type provides, without having to write instances.

In-depth explanation here


Example usage

Using this script

-- stack runghc --package servant-docs-simple
{-# LANGUAGE DataKinds        #-}
{-# LANGUAGE TypeApplications #-}
{-# LANGUAGE TypeOperators    #-}

module Main where

import Servant.API ((:>), Post, ReqBody)
import Servant.Docs.Simple (writeDocsJson, rriteDocsPlainText)

-- Our API type
type API = "hello" :> "world" :> Request :> Response
type Request = ReqBody '[()] ()
type Response = Post '[()] ()

main :: IO ()
main = do
  -- Writes to the file $PWD/docs.json
  writeDocsJson @API "docs.json"

  -- Writes to the file $PWD/docs.txt
  writeDocsPlainText @API "docs.txt"

Expected Output

Files should be generated relative to $PWD

$ ls | grep docs


    "/hello/world": {
        "Response": {
            "Format": "[()]",
            "ContentType": "()"
        "RequestType": "'POST",
        "RequestBody": {
            "Format": "[()]",
            "ContentType": "()"


    Format: [()]
    ContentType: ()
RequestType: 'POST
    Format: [()]
    ContentType: ()


Click on these links for tutorials

Generating plaintext/JSON documentation from api types

Generating the intermediate documentation structure

Writing our own rendering format

Parsing custom API combinators

To run tutorial scripts

# clone this repository
git clone [email protected]:Holmusk/servant-docs-simple.git

# run the examples
stack examples/<source file>


What's the usecase for this?

1) Lightweight documentation solution by providing an overview of endpoints.

2) Keeping documentation for types in one place

Suppose we have a library of format types. These are shared between different
languages and formats. If we centralize all the information in this
library, we have one source of truth.

``` haskell
--                                           --- Format Types ---
--                                          |                    |
--                                          v                    v
type SingleAPI = "hello" :> ReqBody '[JSON] User :> POST '[JSON] Message
data User = User Name Password deriving Typeable 
data Messsage = Message Id Content deriving Typeable


We use these format types in our `API` as illustrated above.
Developers can use these format types to index the library, to find information
(examples, instances, ...) they need about the type.

What is required to achieve this?

1) We need documentation to be generated from the API type, without writing extra instances for each format type we use.

We can do this with `typeRep` from `Data.Typeable`.

``` sh
$ stack ghci
Prelude> import Data.Typeable
Prelude Data.Typeable> data Alignment = Good | Bad deriving Typeable
Prelude Data.Typeable> typeRep (Proxy :: Proxy Alignment)

2) We need access to the API format types (Email, Users, ...) in documentation.

This is so we can index our library with these types to get the relevant fields, examples for these.

How does our usecase differ from Servant.Docs?

1) Servant.Docs generates documentation only if Format Types have implemented the necessary instances.

``` haskell
-- Instances for format types; they provide examples for format types

instance ToSample User where
    toSamples _ = <some example>

instance ToSample Message where
    toSamples _ = <some example>

Hence, we cannot generate documentation solely from the `Format Type`.

2) In documentation generated by Servant.Docs, the format types mentioned above (User, Message) are not included. This means we can't use them to index our library to look for the relevant information.

What are the trade-offs?

1) Currently Servant.Docs.Simple does not support as many formats as Servant.Docs (via Servant.Docs.Pandoc).

2) Examples are not mandatory as you do not have to write instances for your format types.

This means that **if developers don't provide examples**, the generated
documentation would be **without them**.

We can still provide these examples through:

- The shared library of types

- Using `Servant combinators` such as `Summary` and `Description`.
  These are generated as part of Documentation through `Servant.Docs.Simple`.

*Note that all licence references and agreements mentioned in the servant-docs-simple README section above are relevant to that project's source code only.