# Haskell Type Classes and Custom Instances

This note covers `11-haskell-typeclasses/`, which defines a custom type class and several instances that share one rendering interface.

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## 1. Why Type Classes Matter

Type classes let you describe a capability independently from any one data type.

This example introduces:

- `Renderable` as a capability,
- `Environment`, `BuildState`, and `Deployment` as domain types, and
- `DeploymentReport` as a wrapper type for a whole collection.

Each type gets its own `Renderable` instance, but all of them can be used through the same `render` function.

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## 2. Why the Instances Are Separate

Each instance decides how that type should appear:

- `Environment` renders short environment names,
- `BuildState` renders status text, and
- `Deployment` combines those smaller renderings into one message.

That shows the core value of type classes: behavior is attached per type, while the calling code can stay generic.

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## 3. Commands to Try

```bash
cd 11-haskell-typeclasses

nix develop
cabal run
cabal run -- production failed 2
cabal test

nix build
./result/bin/mini-render production failed 2

nix run . -- production failed 2
nix flake check
```