case modRewriteAppUrl "tutorial/registered" of
Left e -> ...
Right page -> ...
Given a page path, try to return the home page of a happs server, basically, the host plus an optional path.
At the same time, try to deal sanely with HAppS serving behind apache mod rewrite,
a common situation for me at least
because it lets you have multiple happs servers listening on different ports, all
looking to the casual user like they are being served on port 80, and also unblocked by firewalls.
Can fail monadically
host with port number stripped out, if any.
Useful, for example, for getting the right address to ssh to.