nix-playgraound/notes/027-haskell-deriving.md

Haskell Deriving Strategies

This note covers 24-haskell-deriving/, which uses deriving stock and GeneralizedNewtypeDeriving to build useful behavior for release-planning types with very little manual code.

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1. What the Example Derives

The example derives three kinds of behavior:

• ordering and display for enums and records,
• enumeration for the environment list, and
• numeric and semigroup behavior for newtypes.

Those derived instances are then used directly in the program:

• allEnvironments comes from Enum and Bounded,
• sortedTargets relies on derived Ord, and
• totalFailureBudget relies on derived Num.

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2. Why the Constructor Order Matters

Derived ordering is not magical. It follows constructor order for sum types, and field order for product types.

That matters in this example:

• Priority lists Urgent before Standard and Background, so urgent work sorts first, and
• ReleaseTarget stores environment and priority before the service name, so the derived record ordering matches the intended rollout order.

This is the main teaching point: derived instances are only as good as the domain shape you give them.

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3. Why the Newtypes Are Useful

BatchName and FailureBudget are wrappers, but they still need behavior.

GeneralizedNewtypeDeriving lets the example reuse the underlying instances:

• BatchName derives Semigroup and Monoid, and
• FailureBudget derives Num and Ord.

That means the code can concatenate names and sum budgets without unpacking the wrappers everywhere.

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4. Commands to Try

cd 24-haskell-deriving

nix develop
cabal run
cabal test

nix build
./result/bin/mini-deriving

nix run
nix flake check