Currently, flakes are evaluated in pure mode and have no function arguments (other than the computed set of flake dependencies), so it's not possible to do things like:

• Use builtins.currentSystem.
• Access ~/.config/nixpkgs/{config.nix,overlays}.
• Access command line arguments passed via --arg.

Thus we need a controlled way to pass arguments to a flake. This must not compromise hermetic evaluation or the ability to aggressively cache evaluation results. For example, the arguments passed to a flake should be unambiguously hashable to produce a cache key (such that (flakeClosureHash, argumentsHash, attributeName) uniquely determines the evaluation result).

For replacing builtins.currentSystem, however, we don't necessarily have to rely on a function argument. Instead, a flake could simply provide packages/derivations for all platforms that it supports. This is feasible thanks to laziness. For example, the nixpkgs flake would provide attributes such as packages.x86_64-linux.hello and packages.x86_64-darwin.hello rather than a single packages.hello that depends on a function argument. (This is an approach familiar from Hydra jobsets.) The nix UI can select the appropriate set automatically (e.g. nix run nixpkgs.hello will select nixpkgs.provides.packages.$system.hello.)