nix-playgraound/notes/039-haskell-map-set-modeling.md

Haskell Map and Set Modeling

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

---

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.

---

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.

---

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.

---

4. Commands to Try

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