A simple servant sample of error

simple_servant_samplel_error.hs

{-# LANGUAGE DataKinds                  #-}
{-# LANGUAGE DeriveGeneric              #-}
{-# LANGUAGE FlexibleInstances          #-}
{-# LANGUAGE GeneralizedNewtypeDeriving #-}
{-# LANGUAGE MultiParamTypeClasses      #-}
{-# LANGUAGE OverloadedStrings          #-}
{-# LANGUAGE ScopedTypeVariables        #-}
{-# LANGUAGE TypeOperators              #-}

import Control.Monad.IO.Class             -- liftIO
import Control.Monad.Trans.Except         -- Handler
import Data.Aeson.Types                   -- JSON
import Data.Aeson.Parser                  -- JSON
import GHC.Generics
import Network.Wai.Handler.Warp
import Servant

data RetDiv = RetDiv { fst :: Double, snd :: Double, ret :: Double} deriving (Eq, Show, Generic)

instance ToJSON RetDiv

type MyAPI =  "div" :> Capture "fst" Double :> Capture "snd" Double:> Get '[JSON] RetDiv

div' :: Double -> Double -> Handler RetDiv
div' f s =  do
   liftIO $ (putStrLn $ logDiv f s )
   if s == 0 
   then throwError err400 {errBody="zero division"} 
   else return $ RetDiv f s (f/s)

logDiv :: Double -> Double -> String
logDiv f s = (show f) ++ "/" ++ (show s) ++ "=" ++ (show (f / s))

myServer :: Server MyAPI
myServer = div'

myProxy :: Proxy MyAPI
myProxy = Proxy

myApp :: Application
myApp = serve myProxy myServer

main :: IO ()
main = run 8081 myApp

$ curl -i http://127.0.0.1:8081/div/5/4
HTTP/1.1 200 OK
Transfer-Encoding: chunked
Date: Sat, 18 Feb 2017 11:30:30 GMT
Server: Warp/3.2.9
Content-Type: application/json

$ curl -i http://127.0.0.1:8081/div/5/0
HTTP/1.1 303 See Other
Transfer-Encoding: chunked
Date: Sat, 18 Feb 2017 11:31:12 GMT
Server: Warp/3.2.9