> mempty = NotFound
>
> NotFound    mappend           x = x
> WrongMethod mappend InvalidBody = InvalidBody
> WrongMethod mappend           _ = WrongMethod
> InvalidBody mappend           _ = InvalidBody

If we get a Right, it has precedence over everything else.

This in particular means that if we could get several Rights, only the first we encounter would be taken into account.

If you have a Delete endpoint in your API, the handler for this endpoint is meant to delete a resource.

The code of the handler will, just like for Get, Post and Put, run in EitherT (Int, String) IO (). The Int represents the status code and the String a message to be returned. You can use left to painlessly error out if the conditions for a successful deletion are not met.

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"

When implementing the handler for a Get endpoint, just like for Delete, Post and Put, the handler code runs in the EitherT (Int, String) IO monad, where the Int represents the status code and the String a message, returned in case of failure. You can quite handily use left to quickly fail if some conditions are not met.

If successfully returning a value, we just require that its type has a ToJSON instance and servant takes care of encoding it for you, yielding status code 200 along the way.

When implementing the handler for a Post endpoint, just like for Delete, Get and Put, the handler code runs in the EitherT (Int, String) IO monad, where the Int represents the status code and the String a message, returned in case of failure. You can quite handily use left to quickly fail if some conditions are not met.

If successfully returning a value, we just require that its type has a ToJSON instance and servant takes care of encoding it for you, yielding status code 201 along the way.

When implementing the handler for a Put endpoint, just like for Delete, Get and Post, the handler code runs in the EitherT (Int, String) IO monad, where the Int represents the status code and the String a message, returned in case of failure. You can quite handily use left to quickly fail if some conditions are not met.

If successfully returning a value, we just require that its type has a ToJSON instance and servant takes care of encoding it for you, yielding status code 200 along the way.

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

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

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

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 FromText for your type.

Example:

type MyApi = "books" :> Capture "isbn" Text :> Get Book

server :: Server MyApi
server = getBook
  where getBook :: Text -> EitherT (Int, String) IO Book
        getBook isbn = ...

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 [Book]

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

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 FromText for your type.

Example:

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

server :: Server MyApi
server = getBooksBy
  where getBooksBy :: [Text] -> EitherT (Int, String) IO [Book]
        getBooksBy authors = ...return all books by these authors...

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 FromText for your type.

Example:

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

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

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. 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 Book :> Post Book

server :: Server MyApi
server = postBook
  where postBook :: Book -> EitherT (Int, String) IO Book
        postBook book = ...insert into your db...

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