# Lib Outputs

This note covers `42-lib-outputs/`, which exposes a `lib` attrset with pure data and helper functions that other outputs can reuse.

---

## 1. Why `lib` Is Useful

Only some output names have special meaning to the `nix` CLI. A flake can still expose arbitrary attrs for consumers.

`lib` is a common convention for that. It is a good place for:

- pure helper functions,
- derived lookup tables, and
- reusable domain data.

This example keeps the `lib` attrset small so the distinction is clear: the flake exports logic, not just build targets.

---

## 2. What the Example Exports

The `lib` output contains:

- `rolloutPlan`, a small list of pure data,
- `ownerByService`, derived from that list, and
- `renderStep` plus `renderPlan`, which turn the data into text.

The package output then prints `self.lib.renderPlan`, so the runnable artifact is built from the same exported helper values that outside consumers can evaluate.

---

## 3. Why the Check Uses `self.lib`

The check reaches back through the public output path:

```nix
apiOwner = self.lib.ownerByService.api;
workerLine = self.lib.renderStep (builtins.elemAt self.lib.rolloutPlan 1);
```

That matters because it proves the helper data is not only local implementation detail in a `let` block. It is part of the flake interface.

---

## 4. Commands to Try

```bash
cd 42-lib-outputs

nix flake show
nix eval .#lib.ownerByService.api --raw
nix build
./result/bin/show-lib-plan
nix flake check
```