Hello Cafe, I have this project here: https://sprinkles.tobiasdammers.nl/ (source on github: https://github.com/tdammers/sprinkles), and it has a bunch of runtime dependencies on C libraries. Many of them are only required for certain use cases, e.g., the libmysqlclient dependency is only relevant if you want to access MySQL database backends, libfcgi is only required for hosting as FastCGI (which isn't even the recommended first choice), etc. Still, in order to build and run Sprinkles, all the dependencies must be installed, which is of course not ideal. The pipe dream would be for those dependencies to be optional *at run time*, such that the application would test for their presence on startup, or load them dynamically when and if they're needed. However, I'm also fine with making them compile-time feature toggles, such that users can enable only the ones they need. Doing some sort of autotools-style dependency discovery would of course be cool as well (i.e., only enable mysql support if libmysqlclient is present). So the broad question is, which of these are realistic, and how would I go about it? And, regarding feature toggles, this bit in the Cabal FAQ baffles me a little:
Question: I like to let the user enable extended functionality using a Cabal flag. Is this the right way?
Answer: Certainly not. Since other packages can distinguish packages only according to their name and their version, it is not a good idea to allow different APIs for the same package version. Cumbersome as it is you have to move extra features to a separate package.
Moving the features to a separate package wouldn't really solve anything AFAICT, because I'd still have to bring them together somehow. -- Tobias Dammers - tdammers@gmail.com