# Pastebin LOndShpJ
{ nixpkgs ? import <nixpkgs> {} }:
let
  haskellPackages = nixpkgs.haskell.packages.ghc8101.override {
    overrides = self: super: {
      # Sometimes this doesn't work, if you're asking for really new
      # packages, because nixpkgs points at a specific revision of
      # all-cabal-hashes, a list of every package on hackage.
      # In those cases, you want to use `super.callHackageDirect`.
      # https://github.com/NixOS/nixpkgs/blob/f5b6ea126f0f85cd2126984607e95151daf9a859/pkgs/development/haskell-modules/make-package-set.nix#L184
      #
      # Although microlens is found by callHackage, here's the
      # equivalent so you know how it works:
      #
      # microlens = super.callHackageDirect {
      #   pkg = "microlens";
      #   ver = "0.4.11.2";
      #   # replace this with the hash nix tells you it actually found.
      #   sha256 = "0000000000000000000000000000000000000000000000000000";
      # } {};
      microlens = super.callHackage "microlens" "0.4.11.2" {};
    };
  };
in
haskellPackages.microlens