62 lines
1.6 KiB
Markdown
62 lines
1.6 KiB
Markdown
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# Haskell Generic JSON
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This note covers `38-haskell-generic-json/`, which derives JSON instances generically for a release manifest and rollout strategy.
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---
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## 1. Why This Exists Next to the Manual JSON Example
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`27-haskell-aeson-roundtrip/` defines JSON instances by hand so the wire format stays fully explicit.
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This example shows the contrasting approach:
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- derive `Generic`,
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- configure Aeson options once, and
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- let `genericToJSON` and `genericParseJSON` do the repetitive instance work.
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That is useful because both styles show up in real Haskell codebases.
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---
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## 2. What the Generic Options Control
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The example is not just `deriving anyclass`.
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It still configures the shape:
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- field labels drop the `manifest` prefix, and
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- the rollout strategy sum type uses a tagged object representation.
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That is the important teaching point. Generic deriving can still produce an intentional JSON format when you provide the right options.
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---
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## 3. When This Tradeoff Makes Sense
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Generic deriving reduces boilerplate when:
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- your Haskell fields already describe the desired structure closely, and
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- you want the encoder and decoder to stay in sync with minimal manual code.
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It is less appropriate when the wire format needs heavy customization. That contrast is exactly why it is useful to pair this example with the manual
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JSON example earlier in the track.
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---
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## 4. Commands to Try
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```bash
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cd 38-haskell-generic-json
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nix develop
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cabal run
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cabal run -- api production 3 platform,security stable
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cabal test
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nix build
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./result/bin/mini-generic-json api production 3 platform,security stable
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nix run . -- api production 3 platform,security stable
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nix flake check
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```
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