Support Cabal flags in the generated Nix expression about cabal2nix

[kosmikus]

Main reason I haven't just done this is because I'm not sure how to best structure the inputs. There are several options here. Since Nix isn't a very typed language, doing the most accurate thing may easily lead to unreadable code.

[peti]

Unless I'm totally mistaken, flags in Cabal are always boolean values, right? There's only enable/disable; you cannot pass string values into the build or anything.

In that case, I'd say that a flag "foo" should be a parameter "enableFoo" to the expression that has whatever default value the Cabal file specifies. Now, the choice for "enableFoo" affects extraBuildInputs and propagatedBuildInputs, nothing else, right? So those lists would have to be extended at evaluation time using some kind of conditional guard.

Am I missing something?

[kosmikus]

Flags are only booleans, right. The choice is whether to require the maximum set of build inputs as function arguments and have them be null if they turn out not to be needed (this is what I currently favour), or whether to require build inputs as some nested record where conditional inputs are grouped with their flags.

[peti]

My intuitive choice would require all possible build inputs as flat arguments to the expression. I would not add them to the propagatedBuildInputs or extraBuildInput fields, though, unless the corresponding "enableFoo" tells me to.

The advantage I see in requiring all of them unconditionally is that missing inputs are detected quickly (because the function call will fail). We wouldn't know whether that expression actually builds in all cases, but we could be reasonably sure that all dependencies can be found in scope somewhere.

[kosmikus]

Yes, I think I'm coming to the same conclusion.

[kosmikus]

I'm starting to work on this in branch flag-support.

[peti]

As an intermediate solution, I've added the ability to hard-code certain flag assignments on a per-package basis.

[peti]

It's probably safe to close this issue for the time being. There is no immediate effort underway that touches this subject in any way.