Main.hs

{-# LANGUAGE DataKinds #-}
{-# LANGUAGE TypeOperators #-}
{-#LANGUAGE GeneralizedNewtypeDeriving #-}
{-# LANGUAGE DeriveGeneric     #-}
{-# LANGUAGE OverloadedStrings #-}
{-# LANGUAGE PolyKinds         #-}
{-# LANGUAGE TypeFamilies      #-}

module Main where

import Servant.API
import Servant.Server
import Control.Concurrent.STM
import Data.Aeson (FromJSON, ToJSON)
import Data.Proxy
import Control.Monad.IO.Class
import Network.Wai
import Network.Wai.Handler.Warp (run)

newtype CounterVal = CounterVal {getCounterVal :: Int}
                   deriving (Show, Num, FromJSON, ToJSON)

type GetCounter = Get '[JSON] CounterVal
type StepCounter = "step" :> Post '[] NoContent
type Counter = GetCounter :<|> StepCounter

counterAPI :: Proxy Counter
counterAPI = Proxy

handleGetCounter :: TVar CounterVal -> Server GetCounter
handleGetCounter ctr = liftIO $ readTVarIO ctr

handleStepCounter :: TVar CounterVal -> Server StepCounter
handleStepCounter ctr = do
  liftIO $ atomically $ modifyTVar ctr (+ 1)
  return NoContent

handleCounter :: TVar CounterVal -> Server Counter
handleCounter ctr = handleGetCounter ctr
               :<|> handleStepCounter ctr

start :: IO ()
start = do
  initCtr <- newTVarIO 0
  run 8000 (serve counterAPI (handleCounter initCtr))

main :: IO ()
main = start

output

$ cabal build
Building servant-counter-example-0.1.0.0...
Preprocessing executable 'servant-counter-example' for
servant-counter-example-0.1.0.0...
[1 of 1] Compiling Main             ( src/Main.hs, dist/build/servant-counter-example/servant-counter-example-tmp/Main.o )

src/Main.hs:45:13: error:
    • No instance for (Servant.API.ContentTypes.AllCTRender
                         '[] NoContent)
        arising from a use of ‘serve’
    • In the second argument of ‘run’, namely
        ‘(serve counterAPI (handleCounter initCtr))’
      In a stmt of a 'do' block:
        run 8000 (serve counterAPI (handleCounter initCtr))
      In the expression:
        do { initCtr <- newTVarIO 0;
             run 8000 (serve counterAPI (handleCounter initCtr)) }

servant-counter-example.cabal

-- Initial servant-greet-example.cabal generated by cabal init.  For 
-- further documentation, see http://haskell.org/cabal/users-guide/

-- The name of the package.
name:                servant-greet-example

-- The package version.  See the Haskell package versioning policy (PVP) 
-- for standards guiding when and how versions should be incremented.
-- https://wiki.haskell.org/Package_versioning_policy
-- PVP summary:      +-+------- breaking API changes
--                   | | +----- non-breaking API additions
--                   | | | +--- code changes with no API change
version:             0.1.0.0

-- A short (one-line) description of the package.
-- synopsis:            

-- A longer description of the package.
-- description:         

-- The license under which the package is released.
license:             BSD3

-- The file containing the license text.
license-file:        LICENSE

-- The package author(s).
-- author:              

-- An email address to which users can send suggestions, bug reports, and 
-- patches.
-- maintainer:          

-- A copyright notice.
-- copyright:           

category:            Web

build-type:          Simple

-- Extra files to be distributed with the package, such as examples or a 
-- README.
extra-source-files:  ChangeLog.md

-- Constraint on the version of Cabal needed to build this package.
cabal-version:       >=1.10


executable servant-greet-example
  -- .hs or .lhs file containing the Main module.
  main-is:             Main.hs
  
  -- Modules included in this executable, other than Main.
  -- other-modules:       
  
  -- LANGUAGE extensions used by modules in this package.
  -- other-extensions:    
  
  -- Other library packages from which modules are imported.
  build-depends:       base >=4.9 && <4.10
                     , servant ==0.13.*
                     , servant-server ==0.13.*
                     , aeson >=1.2.3.0 && <1.4
                     , stm ==2.4.5.*
                     , warp ==3.2.18.*
                     , text
                     , wai
  
  -- Directories containing source files.
  hs-source-dirs:      src
  
  -- Base language which the package is written in.
  default-language:    Haskell2010