I asked this question in the IRC channel, but thought I'd document it here for my own as well as other's reference.

I have a shell.nix file that is taking a long time to build, in particular it seems to be building a HUnit package.

My shell.nix file looks like:

 sources ? import ./nix/sources.nix
, compiler ? "ghc865" } :
  niv = import sources.nixpkgs {
    overlays = [
      (_ : _ : { niv = import sources.niv {}; })
    ] ;
    config = {};
  pkgs = niv.pkgs;
  myHaskellPackages = pkgs.haskell.packages.${compiler}.override {
myHaskellPackages.callCabal2nix "moscoviumorange" (./.) {}

And the source's is pinned to (with niv):

    "niv": {
        "branch": "master",
        "description": "Easy dependency management for Nix projects",
        "homepage": "https://github.com/nmattia/niv",
        "owner": "nmattia",
        "repo": "niv",
        "rev": "88d6f20882b0422470acbcbf2d1b5f07e1d436f0",
        "sha256": "0wkvz4drnglmmdrz8q1i1yr2fqizpf96k1wq2rlhd8l8x1522izq",
        "type": "tarball",
        "url": "https://github.com/nmattia/niv/archive/88d6f20882b0422470acbcbf2d1b5f07e1d436f0.tar.gz",
        "url_template": "https://github.com/<owner>/<repo>/archive/<rev>.tar.gz"
    "nixpkgs": {
        "branch": "nixos-19.03",
        "description": "A read-only mirror of NixOS/nixpkgs tracking the released channels. Send issues and PRs to",
        "homepage": "https://github.com/NixOS/nixpkgs",
        "owner": "NixOS",
        "repo": "nixpkgs-channels",
        "rev": "775fb69ed73e7cf6b7d3dd9853a60f40e8efc340",
        "sha256": "1w068b0ydw4c26mcjiwlzdfqcdk3rrwmfx4hxzgfhfwcz2nmh3if",
        "type": "tarball",
        "url": "https://github.com/NixOS/nixpkgs-channels/archive/775fb69ed73e7cf6b7d3dd9853a60f40e8efc340.tar.gz",
        "url_template": "https://github.com/<owner>/<repo>/archive/<rev>.tar.gz"

Why is nix building a bunch of packages, when I assume they should be in the nixpkg's binary cache?

For example the output of nix-shell includes:

building '/nix/store/7011izw8f2xyvhqadrhnmapddyz61f29-HUnit-'...

We can search for the HUnit package in Hydra (for 19.03):


According to the above HUnit is built and would be in the binary cache.

We determine the store path with:

nix-store --query --binding out /nix/store/7011izw8f2xyvhqadrhnmapddyz61f29-HUnit-

We can compare the below with hydra: https://hydra.nixos.org/build/103222205#tabs-details

curl https://cache.nixos.org/ryr2qdms3n0qbj8d3l9pvs7ajz4dzav4.narinfo

404 means that path is NOT in the binary cache.

The answer turned out to be that I was referencing ghc to be ghc865 while hydra is probably building with ghc864 by default (for 19.03).

The above answers were originally from clever in the #nixos IRC channel, thank you!

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