TODO: These items will probably be moved to GitHub Issues.
There are a few issues in this project that would be wonderful to improve, but are probably fine for most contexts:
- the license report may often be incomplete.
- For Haskell, automation of Cabal2nix is fragile.
The following discussion assumes some deeper knowledge of using Nix and Nixpkgs.
We find runtime dependencies by looking through the generated artifact in
/nix/store, and finding all further references within /nix/store. This
doesn’t tell us anything about compile-time dependencies. Also, /nix/store
doesn’t contain license information.
To match the detected dependencies with license information, we do a heuristic crawl through Nixpkgs tree, starting with the derivation for our built artifact. Sometimes we don’t find what we want.
Also, this license report is currently limited to runtime dependencies (which is the common case for most inquiries). Compile-time dependencies explodes to a much larger set, and offers more challenges due to how some Nix expressions use string-interpolation.
Note, the accuracy of the report is only as good as the information in
Nixpkgs. For instance, gmp is currently listed as GPL-licensed, when it’s
actually dual-licensed with both GPL and LGPL.
Hopefully this report is still useful, provided you understand the caveats.
Because we use Cabal2nix to build a Haskell derivation, which we then import
in the same run of nix-build, we’re essentially bolting on a macro system
into Nix, which can cause some problems Most of these problems relate to
caching in /nix/store.
However, for the simple case of just getting a build to work, the technique works as you would expect.
If you really want to manually create default.nix files in your Haskell
projects with the cabal2nix command, you can. If a default.nix is found,
it will be used instead by this project.