Support for common stanzas

[ramirez7]

I saw this #513

I'm opening a new issue about the same thing.

YAML includes are inadequate for DRYing up even moderately-complex cabal config. Mostly due to the lack of recursive merge.

For instance, if I define a shared YAML file with default-extensions for my entire monorepo but a single package wants to add another default-extension just for itself, it clobbers the include. There's no good way around this using just YAML includes.

Now, if I could use cabal common stanzas, then YAML includes could help me:

• I could define the common stanzas in shared YAML across my monorepo
• My individual packages' package.yaml could include the common stanzas
• Each package.yaml could then use cabal import accordingly.

I guess an alternative would be for hpack to add support for proper YAML recursive merge. But even if it had that, I don't see why hpack shouldn't support the cabal feature.

[ramirez7]

(This seems like a pretty trivial feature to add, so if I find time, I'll just add it myself ✌️)

[MrQubo]

That's so much needed. I've tried using hpack a few times, but always had to go back to using Cabal.

YAML's anchors are rarely enough. E.g. you can't use it with ghc-options, because this is a list and YAML doesn't support list concatenation. You can hack it like this (but that's very ugly):

when:
  - condition: true
    ghc-options: *ghc-options-warnings
  - condition: true
    ghc-options: *ghc-options-opt