Differences between nix develop and nix build

[khazaddum]

This may be a very niche question, but here it goes. I have a haskell-flake project that has a FFI that is connected using the settings._.custom field.

If I run nix build, it succeeds. If I run nix develop and then cabal test, one unit test fails. This particular unit test is related to a new addition to the FFI to handle that particular test. It is as if the cabal test build/linking is using the old version of the FFI.

I have done a few tests, even changing the FFI so it's intentionally broken, and every time, nix build gives the expected result, cabal test passes all unit tests (that should fail now) except the new one. It seems as if the devShell is frozen with an old version of the FFI.

I tried bumping up the version of the FFI and put a version constraint in the .cabal file under pkgconfig-depends, same result.

If I use cabal repl and run this unit test manually, it succeeds and produces the expected result per the latest version of the FFI.

The real problem I'm facing is that when I use this flake as an input to a different flake, I'm seeing the issue for binaries built with this dependency. For that flake, nix build and cabal repl show the functionality of the updated FFI. But running the binary errors out and so does running cabal test.