A more complete example (with rewriting cookie domains/paths) can be found at http://blog.haproxy.com/2014/04/28/howto-write-apache-proxypass-rules-in-haproxy/
We will try something roughly equivalent to the following ProxyPass
directives in Apache2:
ServerName www.example.com
...
ProxyPass /foo/ http://foo.local
ProxyPassReverse /foo/ http://foo.local
In haproxy.cfg
we define a backend, say foo, to reverse-proxy to foo.local
backend server.