handler.ex

defmodule Handler do
	def init(_type, req, []) do
    	{:ok, req, :no_state}
  	end

	def handle(request, state) do
    	{ :ok, reply } = :cowboy_req.reply(
    		200, 
        [ {"content-type", "text/plain"} ],
        "Hello World",
        request
    	)
	
	    {:ok, reply, state}
  	end

  	def terminate(_reason, _request, _state) do
	    :ok
  	end
end

hello_world.ex

defmodule HelloWorld do
	def start(_type, _args) do	
		dispatch = :cowboy_router.compile([
			{ :_, 
				[
    	      		{"/", Handler, []}
    	  		]
    	  	}
    	])
	
    	{ :ok, _ } = :cowboy.start_http(:http, 100, [{:port, 8080}], [{ :env, [{:dispatch, dispatch}]}]) 
	end
end