nix-playgraound/notes/032-haskell-foldmap-summary.md

Haskell foldMap Summaries

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

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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:

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

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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

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