# Haskell State

This note covers `25-haskell-state/`, which plans a sequence of deployments by threading a build counter and per-environment wave numbers through
`State`.

---

## 1. What the State Represents

The planner keeps two changing values:

- the next global build number, and
- the next rollout wave number for each environment.

That is a good fit for `State`, because each planned deployment needs to read the current values and write back updated ones.

---

## 2. Why This Stays Pure

The planning logic does not perform I/O. It just transforms a list of deployment requests into a list of planned deployments.

`State` keeps that transformation pure:

```haskell
planDeployment :: DeploymentRequest -> State PlannerState PlannedDeployment
```

The caller still gets a plain value at the end through `evalState`.

That is the important intermediate Haskell idea here: stateful logic does not have to mean mutable variables or `IO`.

---

## 3. What the Example Allocates

Each request receives:

- a build number that increments globally, and
- a wave number that increments separately per environment.

That makes the output more interesting than a single counter example, while still teaching one concept.

---

## 4. Commands to Try

```bash
cd 25-haskell-state

nix develop
cabal run
cabal run -- api:production:3 worker:staging:1 cache:production:2
cabal test

nix build
./result/bin/mini-state-planner api:production:3 worker:staging:1 cache:production:2

nix run . -- api:production:3 worker:staging:1 cache:production:2
nix flake check
```