Why does buildInputs accept a list of lists, but behaves differently?

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1

For a long time, I’ve used the following:

File: requirements.nix

let
  pkgs = import
    (fetchTarball "https://github.com/NixOS/nixpkgs/archive/65fae659e31.tar.gz")
    { };
  unstable = import
    (fetchTarball "https://github.com/NixOS/nixpkgs/archive/65fae659e31.tar.gz")
    { };

in with import <nixpkgs> {}; {
  inherit pkgs;
  inherit unstable;

  production_dependencies = [
    pkgs.python3
    pkgs.openssl
    pkgs.postgresql
  ];
  development_dependencies = [
  ];
  contributing_dependencies =
    [];

}

And then have a shell.nix file with:

let requirements = import ./requirements.nix;

in with requirements.pkgs;

mkShell {
  buildInputs = [
    requirements.production_dependencies
    requirements.development_dependencies
  ];
  shellHook = ''
    source $(npm bin)/setup_environment.sh
  '';
}

This seems to work. But then for unrelated issues, I need to install the pyscopg python package using pip from source. And it was erroring out with cryptic openssl errors missing. Through a ton of testing, this somehow does work (actually appending the lists):

let requirements = import ../requirements.nix;

in with requirements.pkgs;

mkShell {
  buildInputs = requirements.production_dependencies
    ++ requirements.development_dependencies;
  shellHook = ''
    source $(npm bin)/setup_environment.sh
  '';
}

Notice I’m properly concatenating the lists instead of creating a list of lists.

So… my question:

Why does a list of lists even work? And when it does, what’s causing it to fail with openssl stuff? Really hard to pin down.

Thanks for any help.

2

lists of lists only work by accidents and due to nix not having real strict type checks. *Inputs are expected to have lists of packages, so you need to concat them.