49 lines
1.2 KiB
Markdown
49 lines
1.2 KiB
Markdown
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# Haskell Type Classes and Custom Instances
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This note covers `11-haskell-typeclasses/`, which defines a custom type class and several instances that share one rendering interface.
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---
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## 1. Why Type Classes Matter
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Type classes let you describe a capability independently from any one data type.
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This example introduces:
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- `Renderable` as a capability,
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- `Environment`, `BuildState`, and `Deployment` as domain types, and
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- `DeploymentReport` as a wrapper type for a whole collection.
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Each type gets its own `Renderable` instance, but all of them can be used through the same `render` function.
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---
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## 2. Why the Instances Are Separate
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Each instance decides how that type should appear:
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- `Environment` renders short environment names,
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- `BuildState` renders status text, and
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- `Deployment` combines those smaller renderings into one message.
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That shows the core value of type classes: behavior is attached per type, while the calling code can stay generic.
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---
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## 3. Commands to Try
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```bash
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cd 11-haskell-typeclasses
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nix develop
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cabal run
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cabal run -- production failed 2
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cabal test
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nix build
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./result/bin/mini-render production failed 2
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nix run . -- production failed 2
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nix flake check
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```
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