Notes as I build a site with Opium

[avsm]

I hope you don't mind if I maintain a little issue list that I'm running across as I make my Opium site. I'll submit pull requests for these right after the site is done and if you think they are good ideas. Will edit this as I go along.

• [ ] not clear from docs how to do more modern auth middleware which redirects to a login page and uses session cookies. This requires overriding the handler with a login-specific one.
• [x] helper function to do a HTTP redirect directly as a handler shortcut would be useful.
• [ ] I've got a little REST module that glues together get/put/delete handlers (via a higher order rest db get put del base = base |> get db |> del db |> put db interface that also passes a database handle into each of the sub handlers.
• [x] What is the difference between Opium and Rock? App can be converted from one to the other, but I'm still not entirely sure (aside from that Rock is middleware and lower level?)
• [x] cant get cookies working. I added App.middleware Cookie.m but Cookie.get in a subsequent middleware fails. Can middlewares depend on previous ones setting the environment like this? Cookies work fine via manually adding them to the Rock.Request.headers using Cohttp's header generation directly.
• [x] since the move to Lwt, the README is out of date and still refers to Command instead of Cmdliner.
• [x] Lwt_log debugging is annoying to get working -- need to set LWT_DEBUG env in addition to the -d flag to the Opium app (I think). Also would be nice to know how to activate the middleware tracing. (#27)
• [x] HTTP headers get overwritten for non-String responses (#29)
• [x] On a more positive note, Opium is really fun to work with :)

[rgrinberg]

The first 3 are good points and number 3 is something that I've looked at especially before.

I'll address the last point a little more thoroughly

• Rock is a specification for what an app is. E.g. a rock app is nothing more than type Handler.t = Request.t -> Response.t Lwt.t. The types in Opium_rock don't make this very clear because of all the synonyms.
• Opium is simply a set of modules to eventually create such a rock app. Opium has its own app type as well but it's really just a big record of input that get transformed to a rock app.
• Middleware works on the rock level, since it's simply Handler.t -> Handler.t(with a few small details).

[avsm]

Thanks, makes sense. So Opium can be thought of as a 'Rock executor' since it takes a set of Rock middleware and links it through to a command-line interface and other such niceties. So a well-designed modular use of Opium is to have as much logic as possible be pure Rock, and then just glue it together into an Opium app. (It's working out very well so far btw, will open the source just as soon as it's all tied together).

[avsm]

New entry: cant get cookies working. I added App.middleware Cookie.m but Cookie.get in a subsequent middleware fails. Can middlewares depend on previous ones setting the environment like this? Cookies work fine via manually adding them to the Rock.Request.headers using Cohttp's header generation directly.

[rgrinberg]

Hmm I just tested it and setting cookies as in examples/sample.ml works fine. Can I see how you're doing it?

You may set the cookies directly into the cohttp header if you want. The only reason why I have the wrapper middleware is because I want the values to be base64'd by default to avoid having to remember to escape them myself.

[rgrinberg]

Addressing your second point https://github.com/rgrinberg/opium/blob/master/opium/app.mli#L88

Isn't that good enough?

[rgrinberg]

I'll make a ticket for the Lwt_log thing. Setting debug should definitely affect it but it should also be its own command line flag.

[avsm]

@rgrinberg I'll have to extricate the cookie problem into a smaller test case (my current code is annoyingly part of a big blob and hard to pull out as-is). Will do so as soon as current hacking settles down.

Re: the redirect, there seem to be a bunch of convenience functions defined in Opium that aren't in Rock. I do the redirect in a handler, so I really need a Rock.redirect rather than an Opium.redirect so that middleware can do the redirection (in this case, to go to a login page)

[rgrinberg]

Yes, some of these helpers are actually general enough to just be in rock. I will move them over and re-export in opium.

[anuragsoni]

@avsm Not sure if you are still using Opium. From the points that are left unanswered is there anything else that I can improve in Opium? :)