Access to "default component"

[shlevy]

If I do cabal repl or cabal build in the root of a project, cabal will select a default component. It would be nice if this component were available from Nix on the project value.

I believe the heuristic is to choose the cabal package in the same dir as cabal.project (if any), then choose its lib component or its first exe if none. But I haven't verified this.

[michaelpj]

Can you give an example of a use case? This is just saving people from having to be explicit, is that really a problem?

[shlevy]

I'd like to use this to automatically have packages.<system>.default and apps.<system>.default in a flake match what cabal does.

[michaelpj]

Okay, so the use case is "flake containing a single haskell package has default outputs that match cabal"? That does sound nice, although of course it stops being useful as soon as you have more than one package in the project.

[shlevy]

Why the "single package" constraint? I also want a default output for multi-package projects.

[michaelpj]

cabal build or cabal repl in a muliti-package project will give you an error and not do anything. What would you expect it to do?

[shlevy]

https://github.com/shlevy/eventuo11y has multiple packages but bare cabal build and cabal repl work. I think it's because there's a package in ..

[michaelpj]

I see. I hadn't seen that sort of thing before.

Anyway, I think you could write such a function and contribute it to haskell.nix easily enough if you know the heuristic. selectDefaultComponent or whatever.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[andreabedini]

@shlevy are you still interested in this?

IIRC cabal is not always able to pick a default component. I would have to look into this.

[shlevy]

Yes, still interested.

I think #1777 (comment) captures the logic.