pinned-nixpkgs

Pinned Nixpkgs

The basic pillar of any nix expression is nixpkgs. Nixpkgs is a nix expression providing the whole infrastructure necessary to actually build a package.

Pinning

You've probably seen derivations starting like:

{ nixpgs }:
...

or

let

    nixpkgs = import <nixpkgs> {};

...

Both of these approaches expect nixpkgs to be supplied from their environment, making the whole derivation non-reproducible. The moment the environment changes its nixpkgs, the result of our expression will be completely different.

NOTE

Here we describe a technique where we let the Nix evaluator fetch a pre-specified commit of Nixpkgs. An alertnative using git submodules is described in nixops tutorial.

To prevent this, we need to use a technique called nixpkg pinning. It's nothing really scary - you simply specify an exact nixpkg commit in your expression and manually fetch it. From version 2.0 nix has a handy function called builtins.fetchGit that allows us to do just that!

IMPORTANT!

Nixpkgs also provides a fetcher function called fetchgit.

DO NOT USE THAT ONE ! We will not go into explaining the difference for simplicity's sake.

# release.nix
let

    nixpkgs = import (import ./pinned-nixpkgs.nix) {};

in

    nixpkgs

In the next chapters we will modify release.nix to apply modifications to nixpgs.

# pinned-nixpkgs.nix
builtins.fetchGit {
    url = "https://github.com/NixOS/nixpkgs.git";
    rev = "98922fecef2b52002745c258a4a4b78c3c52a24b";
    ref = "master";
}

With this we can run:

nix-shell -p '(import ./release.nix).haskellPackages.ghcWithPackages (pkgs: [ pkgs.lenses pkgs.aeson ])'

Which will throw us into a prompt where we have ghc available with lenses and diagrams preinstalled. It is important to emphasize that the two above files will always provide us with the exact same version of ghc, lenses and diagrams no matter the system, time or other circumstances under which the nix-shell command will be run.

So the output of the following command in the above provided shell should be the same even if run years from now (see this to understand why it might differ).

ghc-pkg list
/nix/store/g1pp002k9ba0almdfdn4jdkfz81yr62s-ghc-8.6.4-with-packages/lib/ghc-8.6.4/package.conf.d
    Cabal-2.4.0.1
    aeson-1.4.2.0
    array-0.5.3.0
    attoparsec-0.13.2.2
    base-4.12.0.0
    base-compat-0.10.5
    binary-0.8.6.0
    bytestring-0.10.8.2
    containers-0.6.0.1
    deepseq-1.4.4.0
    directory-1.3.3.0
    dlist-0.8.0.6
    filepath-1.4.2.1
    ghc-8.6.4
    ghc-boot-8.6.4
    ghc-boot-th-8.6.4
    ghc-compact-0.1.0.0
    ghc-heap-8.6.4
    ghc-prim-0.5.3
    ghci-8.6.4
    hashable-1.2.7.0
    haskeline-0.7.4.3
    hpc-0.6.0.3
    integer-gmp-1.0.2.0
    integer-logarithms-1.0.3
    lenses-0.1.8
    libiserv-8.6.3
    mtl-2.2.2
    old-locale-1.0.0.7
    parsec-3.1.13.0
    pretty-1.1.3.6
    primitive-0.6.4.0
    process-1.6.5.0
    random-1.1
    rts-1.0
    scientific-0.3.6.2
    stm-2.5.0.0
    tagged-0.8.6
    template-haskell-2.14.0.0
    terminfo-0.4.1.2
    text-1.2.3.1
    th-abstraction-0.2.11.0
    time-1.8.0.2
    time-locale-compat-0.1.1.5
    transformers-0.5.6.2
    unix-2.7.2.2
    unordered-containers-0.2.9.0
    uuid-types-1.0.3
    vector-0.12.0.2
    xhtml-3000.2.2.1

In the next chapter we will generate Nix expressions for our monorepo packages.