Haskell Language Web Development Servant
Example
Servant is a library for declaring APIs at the type-level and then:
- write servers (this part of servant can be considered a web framework),
- obtain client functions (in haskell),
- generate client functions for other programming languages,
- generate documentation for your web applications
- and more...
Servant has a concise yet powerful API. A simple API can be written in very few lines of code:
{-# LANGUAGE DataKinds #-}
{-# LANGUAGE TypeOperators #-}
import Data.Text
import Data.Aeson.Types
import GHC.Generics
import Servant.API
data SortBy = Age | Name
data User = User {
name :: String,
age :: Int
} deriving (Eq, Show, Generic)
instance ToJSON User -- automatically convert User to JSON
Now we can declare our API:
type UserAPI = "users" :> QueryParam "sortby" SortBy :> Get '[JSON] [User]
which states that we wish to expose /users to GET requests with a query param sortby of type SortBy and return JSON of type User in the response.
Now we can define our handler:
-- This is where we'd return our user data, or e.g. do a database lookup
server :: Server UserAPI
server = return [User "Alex" 31]
userAPI :: Proxy UserAPI
userAPI = Proxy
app1 :: Application
app1 = serve userAPI server
And the main method which listens on port 8081 and serves our user API:
main :: IO ()
main = run 8081 app1
Note, Stack has a template for generating basic APIs in Servant, which is useful for getting up and running very quick.
