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----
-title: "Lets Learn Nix: Reproducibility"
-date: 2020-03-17T12:06:47Z
-draft: true
----
-
-I am dedicating this page to defining and disambiguating some terminology. I
-think it is important to use these terms precisely, so it may be worthwhile to
-memorize these definitions and ensure that you are clarifying the discourse
-rather than muddying it.
-
-## Terms
-
-- repeatable build:
-- reproducible build:
-- deterministic build:
-- pure function:
-- impure function:
-- idempotent function:
-
-TODO(wpcarro): Consistently and deliberately use reproducible and
-deterministic.
-
-## Repeatable vs. Reproducible
-
-Is NixOS reproducible? Visit [@grhmc][who-grhmc]'s website,
-[r13y.com](https://r13y.com), to find out.
-
-At the time of this writing, 1519 of 1568 (i.e. 96.9%) of the paths in the
-`nixos.iso_minimal.x86_64-linux` installation image are reproducible.
-
-## What hinders reproducibility?
-
-Timestamps.
-
-If package A encodes a timestamp into its build artifact, then we can
-demonstrate that package A is *not reproducible* simply by building it at two
-different times and doing a byte-for-byte comparison of the build artifacts.
-
-## Does Nix protect developers against non-determinism
-
-Yes. But not entirely. How?
-
-## Deterministic Nix derivation
-
-```nix
-{ pkgs ? import <nixpkgs> {}, ... }:
-
-with pkgs;
-
-stdenv.mkDerivation {
-  name = "reproducible";
-  phases = [ "buildPhase" ];
-  buildPhase = "echo reproducible >$out";
-}
-```
-
-## Non-deterministic Nix derivation
-
-We can introduce some non-determinism into our build using the `date` function.
-
-```nix
-# file: /tmp/test.nix
-{ pkgs ? import <nixpkgs> {}, ... }:
-
-with pkgs;
-
-stdenv.mkDerivation {
-  name = "non-reproducible";
-  phases = [ "buildPhase" ];
-  buildPhase = "date >$out";
-}
-```
-
-Then run...
-
-```shell
-$ nix-build /tmp/test.nix
-$ nix-build /tmp/test.nix --check --keep-failed
-```
-
-## How do you test reproducibility?
-
-We can use `cmp` to compare files byte-for-byte. The following comparison should
-fail:
-
-```shell
-$ echo foo >/tmp/a
-$ echo bar >/tmp/b
-$ cmp --silent /tmp/{a,b}
-$ echo $?
-```
-
-And the following comparison should succeed:
-
-```shell
-$ echo hello >/tmp/a
-$ echo hello >/tmp/b
-$ cmp --silent /tmp/{a,b}
-$ echo $?
-```
-
-## Reproducible vs. deterministic
-
-Reproducible builds *are* deterministic builds and deterministic build
-
-## Deterministic, Reproducible, Pure, Idempotent, oh my
-
-- A pure function has no side-effects.
-
-- An idempotent function can be executed more than once with the same arguments
-  without altering the side-effects.
-
-- A deterministic function ensures that
-
-## Deterministic vs. Reproducible
-
-I can check if a build is reproducible using [these tools][wtf-repro-tools].
-
-[wtf-repro-tools]: https://reproducible-builds.org/tools/
-[who-grhmc]: https://twitter.com/grhmc