|
| 1 | +# Using generics |
| 2 | + |
| 3 | +```haskell |
| 4 | +{-# LANGUAGE DataKinds #-} |
| 5 | +{-# LANGUAGE DeriveGeneric #-} |
| 6 | +{-# LANGUAGE RankNTypes #-} |
| 7 | +{-# LANGUAGE TypeOperators #-} |
| 8 | +module Main (main, api, getLink, routesLinks, cliGet) where |
| 9 | +
|
| 10 | +import Control.Exception (throwIO) |
| 11 | +import Control.Monad.Trans.Reader (ReaderT, runReaderT) |
| 12 | +import Data.Proxy (Proxy (..)) |
| 13 | +import Network.Wai.Handler.Warp (run) |
| 14 | +import System.Environment (getArgs) |
| 15 | +
|
| 16 | +import Servant |
| 17 | +import Servant.Client |
| 18 | +
|
| 19 | +import Servant.API.Generic |
| 20 | +import Servant.Client.Generic |
| 21 | +import Servant.Server.Generic |
| 22 | +``` |
| 23 | +
|
| 24 | +The usage is simple, if you only need a collection of routes. |
| 25 | +First you define a record with field types prefixed by a parameter `route`: |
| 26 | +
|
| 27 | +```haskell |
| 28 | +data Routes route = Routes |
| 29 | + { _get :: route :- Capture "id" Int :> Get '[JSON] String |
| 30 | + , _put :: route :- ReqBody '[JSON] Int :> Put '[JSON] Bool |
| 31 | + } |
| 32 | + deriving (Generic) |
| 33 | +``` |
| 34 | +
|
| 35 | +Then we'll use this data type to define API, links, server and client. |
| 36 | +
|
| 37 | +## API |
| 38 | +
|
| 39 | +You can get a `Proxy` of the API using `genericApi`: |
| 40 | +
|
| 41 | +```haskell |
| 42 | +api :: Proxy (ToServantApi Routes) |
| 43 | +api = genericApi (Proxy :: Proxy Routes) |
| 44 | +``` |
| 45 | +
|
| 46 | +It's recommended to use `genericApi` function, as then you'll get |
| 47 | +better error message, for example if you forget to `derive Generic`. |
| 48 | +
|
| 49 | +## Links |
| 50 | +
|
| 51 | +The clear advantage of record-based generics approach, is that |
| 52 | +we can get safe links very conveniently. We don't need to define endpoint types, |
| 53 | +as field accessors work as proxies: |
| 54 | +
|
| 55 | +```haskell |
| 56 | +getLink :: Int -> Link |
| 57 | +getLink = fieldLink _get |
| 58 | +``` |
| 59 | +
|
| 60 | +We can also get all links at once, as a record: |
| 61 | +
|
| 62 | +```haskell |
| 63 | +routesLinks :: Routes (AsLink Link) |
| 64 | +routesLinks = allFieldLinks |
| 65 | +``` |
| 66 | +
|
| 67 | +## Client |
| 68 | +
|
| 69 | +Even more power starts to show when we generate a record of client functions. |
| 70 | +Here we use `genericClientHoist` function, which lets us simultaneously |
| 71 | +hoist the monad, in this case from `ClientM` to `IO`. |
| 72 | +
|
| 73 | +```haskell |
| 74 | +cliRoutes :: Routes (AsClientT IO) |
| 75 | +cliRoutes = genericClientHoist |
| 76 | + (\x -> runClientM x env >>= either throwIO return) |
| 77 | + where |
| 78 | + env = error "undefined environment" |
| 79 | +
|
| 80 | +cliGet :: Int -> IO String |
| 81 | +cliGet = _get cliRoutes |
| 82 | +``` |
| 83 | +
|
| 84 | +## Server |
| 85 | +
|
| 86 | +Finally, probably the most handy usage: we can convert record of handlers into |
| 87 | +the server implementation: |
| 88 | +
|
| 89 | +```haskell |
| 90 | +record :: Routes AsServer |
| 91 | +record = Routes |
| 92 | + { _get = return . show |
| 93 | + , _put = return . odd |
| 94 | + } |
| 95 | +
|
| 96 | +app :: Application |
| 97 | +app = genericServe record |
| 98 | +
|
| 99 | +main :: IO () |
| 100 | +main = do |
| 101 | + args <- getArgs |
| 102 | + case args of |
| 103 | + ("run":_) -> do |
| 104 | + putStrLn "Starting cookbook-generic at http://localhost:8000" |
| 105 | + run 8000 app |
| 106 | + -- see this cookbook below for custom-monad explanation |
| 107 | + ("run-custom-monad":_) -> do |
| 108 | + putStrLn "Starting cookbook-generic with a custom monad at http://localhost:8000" |
| 109 | + run 8000 (appMyMonad AppCustomState) |
| 110 | + _ -> putStrLn "To run, pass 'run' argument: cabal new-run cookbook-generic run" |
| 111 | +``` |
| 112 | +
|
| 113 | +## Using generics together with a custom monad |
| 114 | +
|
| 115 | +If your app uses a custom monad, here's how you can combine it with |
| 116 | +generics. |
| 117 | +
|
| 118 | +```haskell |
| 119 | +data AppCustomState = |
| 120 | + AppCustomState |
| 121 | +
|
| 122 | +type AppM = ReaderT AppCustomState Handler |
| 123 | +
|
| 124 | +apiMyMonad :: Proxy (ToServantApi Routes) |
| 125 | +apiMyMonad = genericApi (Proxy :: Proxy Routes) |
| 126 | +
|
| 127 | +getRouteMyMonad :: Int -> AppM String |
| 128 | +getRouteMyMonad = return . show |
| 129 | +
|
| 130 | +putRouteMyMonad :: Int -> AppM Bool |
| 131 | +putRouteMyMonad = return . odd |
| 132 | +
|
| 133 | +recordMyMonad :: Routes (AsServerT AppM) |
| 134 | +recordMyMonad = Routes {_get = getRouteMyMonad, _put = putRouteMyMonad} |
| 135 | +
|
| 136 | +-- natural transformation |
| 137 | +nt :: AppCustomState -> AppM a -> Handler a |
| 138 | +nt s x = runReaderT x s |
| 139 | +
|
| 140 | +appMyMonad :: AppCustomState -> Application |
| 141 | +appMyMonad state = genericServeT (nt state) recordMyMonad |
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