nix-playgraound/notes/013-haskell-typeclasses.md

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.

---

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.

---

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.

---

3. Commands to Try

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