loop.hs

{-# LANGUAGE TypeOperators #-}
{-# LANGUAGE DataKinds #-}

{-
This piece of code makes GHC never terminate.

$ ghc --version
The Glorious Glasgow Haskell Compilation System, version 8.0.2
$ ghc-pkg list | grep servant
    servant-0.11
    servant-server-0.11.0.1
-}

module Test where

import Servant

type MyAPI = Post '[JSON] () :<|> Get '[JSON] ()

server :: Server IO
server = enter (undefined :: IO :~> Handler) (undefined :: ServerT MyAPI IO)

Line 10 should be server :: Server MyAPI for the program to be type-correct.