# Haskell Map and Set Modeling

This note covers `36-haskell-map-set-modeling/`, which models release approval policy directly with `Map` and `Set`.

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## 1. Why These Structures Deserve Their Own Example

Several earlier examples already use `containers`, but only as support code.

This example makes the data structures themselves the teaching point:

- `Map` for service ownership and environment access rules, and
- `Set` for required, supplied, missing, and unexpected approver groups.

That is a practical step up from list-based toy models.

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## 2. What the Policy Computation Shows

The report logic uses set operations directly:

- intersection for required approvers that are valid in the environment,
- difference for missing approvers, and
- difference again for unexpected approvers.

That makes the policy behavior compact and declarative. The code describes the relationships instead of manually looping over lists.

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## 3. Why This Is Good Domain Modeling

Ownership, access grants, and approval groups are not “just lists”.

They have semantics:

- service names map to owner teams,
- environments map to allowed teams, and
- approver groups should not contain duplicates.

Using `Map` and `Set` makes those semantics explicit in the type choices.

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## 4. Commands to Try

```bash
cd 36-haskell-map-set-modeling

nix develop
cabal run
cabal run -- api:production:platform,security
cabal test

nix build
./result/bin/mini-access-policy api:production:platform,security

nix run . -- api:production:platform,security
nix flake check
```