Hi,

I have the following use case, but it seems like niv (currently) does not support it.

Here is the scenario:

• I want to add https://github.com/darktable-org/darktable to niv
  • but only adding it as type=tarball does not allow me to fetch submodules, therefore the sha256 is incorrect (differs from the one with submodules)
  • using type=git and the unstable nix version as described in fetchGit and submodules #58 would work, but as I understand it, that means that niv update is a no-op on the json and everytime I import from sources.nix it will download the repo again (which takes a lot time)

What I want is a json entry that also has the sha256 set, such that the evaluation of sources.nix does not download the huge git repo everytime.

As far as I see, the problem is that the builtins.fetchgit does not support adding a sha256, so this would require fetchFromGitHub which is not builtin. I could change my sources.nix to add this, which is nice, but what I don't get is that niv update works and updates the package...

Can you confirm my observations? What would be a good way to add this behavior to the code? It seems like we could add a case on type in:

and for example use nix-prefetch-git for a github type?