# Haskell foldMap Summaries

This note covers `29-haskell-foldmap-summary/`, which summarizes deployment events with one `foldMap` pass into a monoidal report value.

---

## 1. Why `foldMap` Fits This Problem

Each deployment event contributes a small piece of information:

- one deployment count,
- maybe one production count,
- some replica count,
- maybe one failed service,
- some owners,
- maybe one cancellation flag, and
- one candidate for the largest rollout.

Those pieces all combine associatively, which makes the problem a good fit for monoids.

The example expresses that directly:

```haskell
summarizeEvents :: [DeploymentEvent] -> DeploymentSummary
summarizeEvents = foldMap summarizeEvent
```

---

## 2. What the Summary Type Encodes

`DeploymentSummary` is not one big counter. It is a record of smaller monoidal fields:

- `Count` for totals,
- `Set String` for failed services and owners,
- `CancelledSeen` for a yes-or-no flag, and
- `LargestReplicas` for the maximum rollout size.

That is the main teaching point: when each field has a lawful combine operation, the whole summary can combine cleanly too.

---

## 3. Why This Is Better Than a Manual Loop

The example could have used an explicit left fold and updated every field by hand inside the loop.

`foldMap` is cleaner here because it splits the work into two precise ideas:

1. how one event becomes a summary fragment, and
2. how summary fragments combine.

That makes the aggregation easier to extend without tangling the parsing and report logic.

---

## 4. Commands to Try

```bash
cd 29-haskell-foldmap-summary

nix develop
cabal run
cabal run -- api:production:succeeded:3:platform,security worker:staging:failed-db-lock:1:ops ui:production:cancelled:2:frontend
cabal test

nix build
./result/bin/mini-summary api:production:succeeded:3:platform,security worker:staging:failed-db-lock:1:ops ui:production:cancelled:2:frontend

nix run . -- api:production:succeeded:3:platform,security worker:staging:failed-db-lock:1:ops ui:production:cancelled:2:frontend
nix flake check
```