# Haskell Transformer Stacks

This note covers `37-haskell-transformer-stack/`, which composes `ReaderT`, `ExceptT`, and `Writer` in one rollout workflow.

---

## 1. Why a Stack Helps Here

The rollout workflow needs three independent capabilities:

- configuration from an environment,
- explicit business failures, and
- ordered audit output.

This example puts them together directly:

```haskell
type App = ReaderT Env (ExceptT RolloutError (Writer [String]))
```

That is the core teaching point. The effect requirements live in one concrete stack, while the workflow stays readable.

---

## 2. What the Workflow Actually Does

The stack is not there for decoration. The rollout code:

- reads cluster and policy settings,
- rejects restricted or oversized production rollouts, and
- records each successful step in an audit log.

That gives each transformer a concrete job.

---

## 3. Why This Complements the Earlier Examples

Earlier notes introduced these pieces separately:

- `ReaderT` and `Except` in `10-haskell-effects/`, and
- `Writer` in `31-haskell-writer-audit/`.

This example shows the next practical step: combining them when one workflow needs all three.

---

## 4. Commands to Try

```bash
cd 37-haskell-transformer-stack

nix develop
cabal run
cabal run -- api:production:4
cabal test

nix build
./result/bin/mini-stack api:production:4

nix run . -- api:production:4
nix flake check
```