65 lines
1.7 KiB
Markdown
65 lines
1.7 KiB
Markdown
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# Haskell Deriving Strategies
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This note covers `24-haskell-deriving/`, which uses `deriving stock` and `GeneralizedNewtypeDeriving` to build useful behavior for release-planning
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types with very little manual code.
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---
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## 1. What the Example Derives
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The example derives three kinds of behavior:
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- ordering and display for enums and records,
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- enumeration for the environment list, and
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- numeric and semigroup behavior for newtypes.
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Those derived instances are then used directly in the program:
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- `allEnvironments` comes from `Enum` and `Bounded`,
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- `sortedTargets` relies on derived `Ord`, and
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- `totalFailureBudget` relies on derived `Num`.
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---
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## 2. Why the Constructor Order Matters
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Derived ordering is not magical. It follows constructor order for sum types, and field order for product types.
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That matters in this example:
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- `Priority` lists `Urgent` before `Standard` and `Background`, so urgent work sorts first, and
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- `ReleaseTarget` stores environment and priority before the service name, so the derived record ordering matches the intended rollout order.
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This is the main teaching point: derived instances are only as good as the domain shape you give them.
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---
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## 3. Why the Newtypes Are Useful
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`BatchName` and `FailureBudget` are wrappers, but they still need behavior.
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`GeneralizedNewtypeDeriving` lets the example reuse the underlying instances:
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- `BatchName` derives `Semigroup` and `Monoid`, and
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- `FailureBudget` derives `Num` and `Ord`.
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That means the code can concatenate names and sum budgets without unpacking the wrappers everywhere.
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---
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## 4. Commands to Try
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```bash
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cd 24-haskell-deriving
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nix develop
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cabal run
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cabal test
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nix build
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./result/bin/mini-deriving
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nix run
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nix flake check
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```
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