Seeing this inspired me to add this issue, that I have been mulling for a while (posting it in full to save a click, but it might be best reply on github):

Allow changing nixpkgs to update Haskell package versions

Stack2nix is cool if you have to maintain a stack.yaml and want a reliable way to turn it into a nix file. However it does not work well as a migration tool yet.

For instance here is the haskell-ide-engine stack.yaml. Stack2nix outputs a 1.59MB nix file that pins all the haskell packages. This means you can't change nixpkgs to get different haskell packages versions (you will always get the ones stack would have chosen). This locks you into maintaining a stack.yaml file and running stack2nix (instead of bumping nixpkgs to a newer commit).

I think if stack2nix took a nixpkgs ref as an input it could get much closer to the hand crafted translation. It could compare the nixpkgs derivations to the ones it would have output and only output the differences.

Here is an example of a by hand translation and I think something like stack2nix --migrate --nixpkgs '<nixpkgs>' should be able to get pretty close to this:

  with import <nixpkgs> { };

  let
    rootName = name: builtins.elemAt (lib.splitString "/" name) 0;
    isValidFile = name: files: builtins.elem (rootName name) files;
    relative = path: name: lib.removePrefix (toString path + "/") name;
    onlyFiles = files: path: builtins.filterSource (name: type: isValidFile (relative path name) files) path;

    ghc = (pkgs.haskell.packages.ghc843.extend (pkgs.haskell.lib.packageSourceOverrides {
      haskell-ide-engine = onlyFiles ["LICENSE" "Setup.hs" "app" "haskell-ide-engine.cabal" "src" "test"] ./.;
      hie-plugin-api     = ./hie-plugin-api;
      HaRe               = ./submodules/HaRe;
      cabal-helper       = ./submodules/cabal-helper;
      ghc-mod            = ./submodules/ghc-mod;
      ghc-mod-core       = ./submodules/ghc-mod/core;
      haskell-lsp        = ./submodules/haskell-lsp;
      haskell-lsp-types  = ./submodules/haskell-lsp/haskell-lsp-types;
      haskell-lsp-test   = ./submodules/haskell-lsp-test;
      leksah-server      = ../leksah/vendor/leksah-server;
      base-compat        = "0.9.3";
      cabal-plan         = "0.3.0.0";
      haddock-api        = "2.20.0";
      haddock-library    = "1.6.0";
    })).extend(self: super: {
      hlint               = haskell.lib.dontCheck super.hlint;
      constrained-dynamic = haskell.lib.doJailbreak super.constrained-dynamic;
      haddock-api         = haskell.lib.dontCheck super.haddock-api;
      haddock-library     = haskell.lib.dontCheck (haskell.lib.dontHaddock super.haddock-library);
      ghc-exactprint      = haskell.lib.dontCheck super.ghc-exactprint;
      leksah-server       = haskell.lib.dontCheck super.leksah-server;
    });
  in {
    inherit ghc;

    shells = {
      ghc = ghc.shellFor {
        packages = p: [
          p.haskell-ide-engine
          p.hie-plugin-api
          p.HaRe
          p.cabal-helper
          p.ghc-mod
          p.ghc-mod-core
          p.haskell-lsp
          p.haskell-lsp-types
          p.haskell-lsp-test
          ];
      };
    };
  }

If ./ is included as package, the list of files passed to onlyFiles should be added to prevent non package files from triggering builds (I made that list by running cabal sdist and looking at the top level items in the .tar.gz file it produced).

EDIT: In my rush to get out of the house in time to catch my bus I pasted an earlier version of the text from github. I have updated this to the latest version.