Use fetchFromGitHub instead of fetchgit when the source is a GitHub clone URL

[taktoa]

Pretty self-explanatory. Should be very easy to implement, and will save some bandwidth for anyone who uses cabal2nix on GitHub repos (since, IIRC, it will only download the revision tarball, not the whole git repo).

It would also be nice if cabal2nix gave a warning when an SSH clone URL is given, since those are impure (they depend on the SSH keys in your home directory).

[taktoa]

BTW, I might take a crack at this at some point, but I'm busy right now, so I figured I'd at least put up an issue.

[Fresheyeball]

What about those of us who are not using github? Can this be made compatible?

[taktoa]

Then it should fall back to fetchgit? I didn't say "use fetchFromGitHub always"...

[domenkozar]

@Fresheyeball it's possible to use shallow clone (--depth 1) on revision with some remotes if they have uploadpack.allowReachableSHA1InWant=true set (github does).

But the optimization should only be used for github.com urls.

[domenkozar]

Depends on NixOS/nixpkgs#21732

[domenkozar]

Besides bandwidth, time would be spent for any package set projects like stack2nix, hackage2nix, stackage2nix, etc.

[domenkozar]

I've looked into how to implement this in cabal2nix.

fetchFromGitHub is really just a small wrapper around fetching a zip file.

Prefetching would be: nix-prefetch-url --unpack https://github.com/NixOS/nixpkgs/archive/f40f2f21c03bec1cabd2a59c1863659e8c4d60dc.zip. Generated Nix expression would use fetchFromGitHub { ... }.

cabal2nix currently just guesses what prefetcher to use, by trying all of them and using the first one to succeed.

I'd propose we add a flag like --fetcher and support github as one of the options, as I'm afraid any automatic guessing within cabal2nix is going to be hard to get right.

Thoughts? /cc @peti

[peti]

Personally, I am rather unhappy about the current "guessing code" and would love to replace it with a more deterministic solution that chooses the appropriate src value based on the command-line configuration.

[nh2]

See input-output-hk/stack2nix#130 for more problems relating to this

[danwdart]

Hmm, is there some convertor if one uses lots of these then?