Global extra-dep, or, how to separate public and private code?

[mtolly]

This is both a specific question and a broader goal, so to avoid an XY problem I'll explain both.

I have an open source program built with Stack in a public repo. I also have a closed source library in a private repo. The open source program can optionally use the closed source library. I want to make it easy to build both with and without it.

My idea is to use a flag in the program's .cabal file:

flag UseClosedLib
  default:             False
  description:         Use optional extra library

executable myprog
  if flag(UseClosedLib)
    build-depends:       closed-lib
    cpp-options:         -DUSECLOSEDLIB

Then, since I have both repos on my local system, I'd like to just configure my global Stack settings to know where closed-lib is located:

packages:
- location: /Users/mtolly/git/closed-lib
  extra-dep: true

This is because, ideally, I want the program's repo to be in a state to build without the add-on by default, and I don't want to have to keep the stack.yaml in a modified state in my local copy of the public repo.

But, as per the Stack documentation, it's not possible to specify project-specific options such as extra dependencies in the global Stack config. Is there another way I can do this? Or, do you have a better idea for how I can set this up?

[mgsloan]

Multiple configurations / transient configuration would support this nicely. See #1568

In practice, we just handle this by creating a docker image with the private dep already installed.

Ideally we'd be able to ignore the repo if it's not needed, but we don't know the name of the package stored in the repo till it's downloaded..