Nix shell with pinned packages

shell.nix

with import <nixpkgs> {};
let
  # a function that takes a descriptive name and a hash of nixpkgs-unstable to pin
  # and returns the package from the channel at that version
  pinned = name: hash:
    let pinnedPkgs =
      import
        (builtins.fetchGit {
          # Descriptive name to make the store path easier to identify
          # ...
          # Or maybe we don't need to do it? Say, we pin multiple packages from the same hash... Don't know :)
          name = "pinned-${name}";
          url = "https://github.com/NixOS/nixpkgs/";
          ref = "refs/heads/nixpkgs-unstable";
          rev = hash;
        })
        { };
    in builtins.getAttr name pinnedPkgs;

  # let's say I want to have cabal-install 3.2.0.0.
  # I find an appropriate hash in https://lazamar.co.uk/nix-versions/ and pin it from that hash
  cabal-install = (pinned "cabal-install" "2c162d49cd5b979eb66ff1653aecaeaa01690fcc");

in mkShell {

  shellHook = ''
    export FOO="bar";
    '';

  buildInputs = [
    haskell.compiler.ghc94
    haskell-language-server
    hlint

    cabal-install
  ];
}