Using HAProxy to loadbalance squid proxies

I want to use HAProxy as a forward proxy to load balance across a list of squid instances.
I’ve seen repositories using the http_proxy option, but this no longer exists.

How do i go about forwarding proxy requests to other proxy servers?
Here’s some example repos


After further testing, this turns out to work just fine out-of-the-box.
I had the ssl option set on my servers, which was making every request fail.