# Flake Apps

This note covers `39-flake-apps/`, which exposes runnable `apps.<system>` outputs instead of relying only on packages.

---

## 1. What an App Output Is

An app output is a small attrset with a type and a program path:

```nix
apps.${system}.default = {
  type = "app";
  program = "${reportPackage}/bin/show-rollout-report";
};
```

`nix run` looks for this schema. The program must already exist in the Nix store, so apps usually point at a package output.

---

## 2. Why This Example Still Defines Packages

The runnable programs are built with `writeShellApplication`, then exposed in two ways:

- `packages.<system>.report` and `packages.<system>.promote` build the scripts, and
- `apps.<system>.default` and `apps.<system>.promote` tell `nix run` which program to execute.

That separation keeps the build logic in packages and the command-line entry points in apps.

---

## 3. What the Example Demonstrates

This example keeps the data a little richer than a one-line hello script:

- one release train,
- three rollout waves, and
- one named app for the next promotion step.

That makes it clear that app outputs are just pointers to real programs. They do not replace packages or change how the program is built.

---

## 4. What the Check Verifies

`nix flake check` does not run apps automatically, so the example adds a check that executes both app program paths and verifies their output.

That is the pattern to remember: expose the app through `apps`, then add an explicit check if you want CI to prove the command works.

---

## 5. Commands to Try

```bash
cd 39-flake-apps

nix flake show
nix run
nix run .#promote
nix flake check
```