{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE TypeOperators #-}
{-# LANGUAGE ScopedTypeVariables #-}
module Servant.API.Alternative where

import Data.Proxy
import Servant.Client
import Servant.Docs
import Servant.Server

-- | Union of two APIs, first takes precedence in case of overlap.
--
-- Example:
--
-- > type MyApi = "books" :> Get [Book] -- GET /books
-- >         :<|> "books" :> ReqBody Book :> Post Book -- POST /books
data a :<|> b = a :<|> b
infixr 8 :<|>

-- | 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 = ...
instance (HasServer a, HasServer b) => HasServer (a :<|> b) where
  type Server (a :<|> b) = Server a :<|> Server b
  route Proxy (a :<|> b) request respond =
    route pa a request $ \ mResponse ->
      if isMismatch mResponse
        then route pb b request $ \mResponse' -> respond (mResponse <> mResponse')
        else respond mResponse

    where pa = Proxy :: Proxy a
          pb = Proxy :: Proxy b

-- | A client querying function for @a ':<|>' b@ will actually hand you
--   one function for querying @a@ and another one for querying @b@,
--   stitching them together with ':<|>', which really is just like a pair.
--
-- > type MyApi = "books" :> Get [Book] -- GET /books
-- >         :<|> "books" :> ReqBody Book :> Post Book -- POST /books
-- >
-- > myApi :: Proxy MyApi
-- > myApi = Proxy
-- >
-- > getAllBooks :: BaseUrl -> EitherT String IO [Book]
-- > postNewBook :: Book -> BaseUrl -> EitherT String IO Book
-- > (getAllBooks :<|> postNewBook) = client myApi
instance (HasClient a, HasClient b) => HasClient (a :<|> b) where
  type Client (a :<|> b) = Client a :<|> Client b
  clientWithRoute Proxy req =
    clientWithRoute (Proxy :: Proxy a) req :<|>
    clientWithRoute (Proxy :: Proxy b) req

-- | The generated docs for @a ':<|>' b@ just appends the docs
--   for @a@ with the docs for @b@.
instance (HasDocs layout1, HasDocs layout2)
      => HasDocs (layout1 :<|> layout2) where

  docsFor Proxy (ep, action) = docsFor p1 (ep, action) <> docsFor p2 (ep, action)

    where p1 :: Proxy layout1
          p1 = Proxy

          p2 :: Proxy layout2
          p2 = Proxy