cowboy_req:reply works by side-effect!

Let’s say you have the following cowboy handler:

reply_with_things(Things, Req, State) ->
    {ok, Req1} = cowboy_req:reply(200,
        [{<<"content-type">>, <<"application/json">>}],
        jiffy:encode({[{things, Things}]}), Req),
    _ = some_action(Things),
    {ok, Req1, State}.

and some_action/1 crashes. What gets returned to the client?

In turns out that the client will still get the encoded json response, as cowboy_req:reply works by side-effect.

Not that you’d want to have code like the above in your system. It’s here just for educational purposes.