# Flake `system`

Oldest flake user issues
- #3487 Custom flake checkers
- flake-parts?
- #3602 follows behavior for flakes used as inputs
- Unsolved, 9 comments
- #3733 Confusing default update lock file behavoir
- Appears solved
- #3811 flake: command to build all the derivations
- Solved with flake check
- **#3843 provide builtins.currentSystem as a input to flakes**
- Unsolved, 48 comments
- **Better solution possible**

We can't have such a protracted experimental stage and not improve the flake schema despite the issues.

## Basic Solution

- Solution for unusable rare `system` values

- Incidentally, make `flake-utils` `forAllSystems` redundant

- No semantic compromise

- Close to the current schema

### Schema

- Add `systems :: ListOf String` (or `AttrsOf {}`)

- Add `perSystem :: ({ system :: String } -> Any) -> FlakeAttrs'`

- `FlakeAttrs`:
- `packages.<name>`
- `checks.<name>`
- ...

but not `overlays`, `nixosConfigurations`, etc

### `call-flake.nix`

- Improve the flake attributes

- Generate `packages.<system>` etc using `systems` and `perSystem` if both are defined
- Generate `perSystem` if any of `packages` etc is defined, but not `perSystem`

- Wrap `perSystem` to memoize it, at least for the keys in `systems`

## Convenient Solution

- `perSystem :: FlakeInputs' -> FlakeAttrs'`

- `FlakeInputs'`:

- For each input `${name} = flake.perSystem system`

- `self = top.self // perSystem system` where `top` is the top level flake output arguments (aka inputs)

- `system`

# FIXME

- System is more complicated than a string

# Refs

- https://github.com/NixOS/nix/issues/3843#issuecomment-1150111304