[RFC 184] versioned flakes references

rfcs/0183-versioned-flakes.md

@@ -0,0 +1,55 @@
+---
+feature: versioned-flakes
+start-date: 2024-11-25
+author: domenkozar
+co-authors: None
+shepherd-team: None
+shepherd-leader: None
+related-issues: None
+---
+## Summary
+
+Introduce a standardized versioning schema for Nix flakes using [SemVer](https://semver.org/),
+following lessons learned from [RFC 144](https://github.com/NixOS/rfcs/pull/144/files).
+
+Adds to flakes:
+
+- `version` attribute to `inputs` in the top-level of `flake.nix` to specify version of the input to be resolved.
+- An attribute `version` in the top-level of `flake.nix`.
+
+## Motivation
+
+Currently, Flakes lack a formal versioning mechanism, leading to potential duplication of dependencies and inefficient evaluations.
+
+By adopting a versioning schema similar to Rust's Cargo system, we can utilize a SAT solver to manage dependencies more effectively, minimizing redundancy and improving performance.
+
+## Explanation
+
+- **Version Schema:** Implement a versioning system for flakes that follows SemVer conventions (e.g., `1.0.0`, `2.1.3`).
+- **Input Versioning:** Allow flake inputs to specify version constraints (e.g., `inputs.nixpkgs.version = "^3.0.0";`) following [Rust Caret Requirement Syntax]([https://doc.rust-lang.org/cargo/reference/specifying-dependencies.html#version-requirement-syntax](https://doc.rust-lang.org/cargo/reference/specifying-dependencies.html#caret-requirements)).
+- **Dependency Resolution:** Integrate a SAT solver to resolve dependencies based on specified versions, ensuring compatibility and reducing duplication.
+
+### Examples
+
+**Defining a Flake with a Version:**
+
+```nix
+{
+  version = "1.0.0";
+
+  inputs = {
+    nixpkgs.url = "github:NixOS/nixpkgs";
+    nixpkgs.version = "^1.0.55";
+  };
+
+  outputs = { self, nixpkgs }: { /* ... */ };
+}
+```
+
+### Implementation
+
+Nix would resolve versions as part of the evaluation, shipping a SAT version resolver.
+
+`flake-compat` wouldn't support version resolving.
+
+[flakestry.dev](https://github.com/flakestry/flakestry.dev) would provide an HTTP API for resolving by preevaluating flakes.