Use hpack for skeleton

[kmicklas]

This removes the need to keep exposed-modules up to date.

[ElvishJerricco]

It looks like cabal will support globbing in an upcoming version, and it already has common stanzas. I think hpack should be avoided. It's a 100% unnecessary point of failure that adds confusion to the file hierarchy of the project.

[kmicklas]

@ElvishJerricco Whoa, I didn't realize Cabal finally supported that. One thing that still appears to be missing is having a common stanza included from an external file. That would be super useful, for example, for maintaining a set of default-extensions shared across the packages in the project.

[ElvishJerricco]

@kmicklas I'd consider that minor enough that it's not worth the tremendous risks of complicating the tooling. Plus I'm not a fan of default-extensions to begin with ;)

[kmicklas]

@ElvishJerricco We'd like to use default-extensions to make it easier for beginners to use, not having to worry about understanding the risks of each and maintaining them per-file. In general I prefer to use it as well, as, for example, putting ScopedTypeVariables in every file is just needless verbosity. I leave the per-file declarations for "dangerous" extensions (e.g. UndecidableInstances, TemplateHaskell) which now stick out much more easily. I'm curious why you don't like this feature?

I'm also not convinced the tooling complexity is really an issue here since we have pretty deep control over the build infrastructure of an Obelisk project.

On the other hand, after testing I've discovered that sharing configuration across packages doesn't actually work since it will be outside the package root for the purposes of cabal2nix. (There may be a workaround for this but I'm closing this PR for the time being.)

[ElvishJerricco]

@kmicklas I don't like default-extensions because it means I can't visit a file and know what language I'm using. That said I'm not suggesting default-extensions should be avoided; it's just not my preference.

I would strongly suggest hpack be avoided though. It doesn't matter if it's the user or obelisk taking on the complexity; it's complexity nonetheless, so it adds points of failure that will be ever more opaque and surprising to the user. Using hpack is an easy way to buy build failures that are 100% avoidable.