The code below can be found in its final form in this repository on github:

[email protected]:rubenmoor/sf-servant-tutorial.git

Initialize new project with stack

$ cd sf-servant-tutorial
$ stack new sf-servant-tutorial --bare new-template --resolver lts-4.1
$ stack build # just for fun
$ stack exec sf-servant-tutorial-exe

Edit files to run a web server

-- app/Main.hs

module Main where

import Lib

main :: IO ()
main = startServer

-- src/Lib.hs

{-# LANGUAGE OverloadedStrings #-}

module Lib
    ( startServer
    ) where

import           Network.HTTP.Types       (status200)
import           Network.Wai              (Request, Response, responseBuilder)
import qualified Network.Wai.Handler.Warp as Warp

startServer :: IO ()
startServer = Warp.run 3000 app

app :: Request -> (Response -> t) -> t
app _ respond = respond $ responseBuilder
                          status200
                          [("Content-Type", "text/plain")]
                          "hello world"


-- sf-servant-tutorial.cabal

-- ...
library
  hs-source-dirs:      src
  exposed-modules:     Lib
  build-depends:       base >= 4.7 && < 5
                     , http-types
                     , wai
                     , warp
  default-language:    Haskell2010
-- ...

Check if everythings works as expected

$ stack exec sf-servant-tutorial-exe

Inside a browser, go to

http://localhost:3000/

Now add servant to serve an API. Starting with required packages:

-- sf-servant-tutorial.cabal

-- ...
library
  hs-source-dirs:      src
  exposed-modules:     Lib
  build-depends:       base >= 4.7 && < 5
                     , aeson
                     , servant-server
                     , transformers
                     , wai
                     , warp
  default-language:    Haskell2010
-- ...

-- src/Lib.hs

Compiler extensions. Servant uses DataKinds for compile-time check of the content-types. DeriveGeneric for generic JSON instance. Servant uses TypeOperators to allow operator syntax in type definitions.

{-# LANGUAGE DataKinds         #-}
{-# LANGUAGE DeriveGeneric     #-}
{-# LANGUAGE OverloadedStrings #-}
{-# LANGUAGE TypeOperators     #-}

module Lib
    ( startServer
    ) where

import           Control.Monad.IO.Class   (liftIO)
import           Data.Aeson               (ToJSON)
import           GHC.Generics             (Generic)

import           Network.Wai              (Application)
import qualified Network.Wai.Handler.Warp as Warp
import           Servant                  ((:>), Get, JSON, Proxy (Proxy),
                                           Server, serve)

Now serving an app :: Application, taken care of by servant.

startServer :: IO ()
startServer = Warp.run 3000 app

Mocking some data entries.

-- schema

data User = User
  { userName  :: String
  , userEmail :: String
  } deriving (Generic)

-- a ghc bug generates a warning here
instance ToJSON User

-- instad of a database

alice :: User
alice = User "Alice" "[email protected]"

bob :: User
bob = User "Robert" "[email protected]"

-- imagine a database action
getUsers :: IO [User]
getUsers = pure [alice, bob]

Now build the actual "servant application"

-- the app

app :: Application
app = serve api server

-- the API, or the routes

type Api = "users" :> Get '[JSON] [User]

-- Server Api == EitherT ServantErr IO [User]
server :: Server Api
server = liftIO getUsers

api :: Proxy Api
api = Proxy

Why is Proxy needed here? Further information can be found in this stackoverflow question.