adr: 0001-condense-information-around-nix-3

adr/0001-condense-information-around-nix-3.md

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+# Condense information around Nix 3.0
+
+- Status: informational
+- Date: 2022-03-11
+
+## Context
+
+We have a lot of feedback and arguments,
+from different people,
+in different sources (comments, issues, discourse),
+around a Nix 3.0 release,
+principally around Flakes.
+
+Being able to draw a short,
+consistent mental model of such feedback
+would allows us to have a clear understanding of the why.
+
+Once we understand the why of everything,
+it's easy to shape an implementation.
+
+# Criticism of current design (Nix 2.x, default configuration)
+
+> "Reproducible builds and deployments" - https://nixos.org
+
+Nix builds and deployments can be reproducible,
+provided you avoid common temptations - see below.
+
+However, Nix expressions are not reproducible,
+which in practice extends to either:
+
+- Making the build not reproducible.
+- Making it unnecessarily hard to reproduce the build.
+
+This problem originates from the following mechanisms:
+
+- Env vars access (builtins.getEnv).
+- Current time access (builtins.currentTime).
+- Fetchers infrastructure works without a hash (builtins.fetch\*, etc).
+- Arbitrary file system access
+  outside of the concept of a project
+  (e.g. `~/.config/nixpkgs/config.nix`).
+- `NIX_PATH`.
+
+  - Depends on the environment.
+  - Cannot be pinned easily.
+  - Differs across machines.
+  - Is not easily portable.
+  - Open the door for ad-hoc hacks.
+  - Is imperative instead of declarative.
+  - Cannot be shipped as part of the expression.
+
+# Goals
+
+## Making Nix expressions reproducible
+
+### Enable purity and reproducibility by default
+
+- No access to current time (hermeticity).
+- Fetchers require a hash (reproducibility).
+- No access to env vars (hermeticity).
+- Add a `--impure` flag as a escape hatch
+  (same as you can disable sandbox if you wish).
+  This eases migration.
+
+### Remove NIX_PATH and make sharing of code a thing
+
+- Define a concept of project (hermeticity is relative)
+  - No access to arbitrary places in the FS outside of it.
+- Turn release.nix into a real, recognized, documented interface and schema (discoverability).
+- Define an official interface for importing code from other people as input (composability & extensibility).
+
+# Pending to do
+
+- Analyze the 551 comments of [RFC0049](https://github.com/NixOS/rfcs/pull/49).
+
+- Explain how Flakes fit into the model and solve this problem once and for all.
+
+- Have some stats about what the more controversial points are
+  since this can bring some light into what is really important,
+  niche, worked around, or postponed, etc.