# Haskell NonEmpty Waves

This note covers `32-haskell-nonempty-waves/`, which models rollout plans with `NonEmpty` so every plan is guaranteed to have at least one wave.

---

## 1. Why `NonEmpty` Matters

A rollout plan with zero waves is not meaningful.

Using `[Wave]` would force every caller to handle an impossible-but-representable empty case. This example makes that impossible state unrepresentable
instead:

```haskell
planWaves :: NonEmpty Wave
```

That means the rendering code can safely ask for the first and last wave without defensive checks.

---

## 2. What the Example Builds

The example turns a release job into:

- one wave for a rolling rollout, or
- two waves for a canary rollout.

Both cases still share the same output type. That is the real teaching point: `NonEmpty` lets you preserve list-like behavior while tightening the
domain guarantee.

---

## 3. Why This Is Better Than a Runtime Check

You could build a plain list and reject `[]` later.

That would move the invariant into comments and runtime branches. `NonEmpty` pushes the guarantee into the type itself, which is more precise and
easier to trust.

---

## 4. Commands to Try

```bash
cd 32-haskell-nonempty-waves

nix develop
cabal run
cabal run -- api:production:canary:20:6
cabal test

nix build
./result/bin/mini-waves api:production:canary:20:6

nix run . -- api:production:canary:20:6
nix flake check
```