# Haskell Algebraic Data Types

This note covers `08-haskell-adt/`, which models a build plan with sum types, a record type, and pattern matching.

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## 1. Why This Example Matters

Haskell programs often start by turning vague strings into precise domain types.

This example does that with:

- `Target` as a sum type,
- `Mode` as a sum type,
- `Output` as a sum type, and
- `BuildPlan` as a product type with record fields.

That is one of the most important intermediate Haskell habits: model the domain first, then write functions over the constructors.

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## 2. Pattern Matching in Two Places

The example uses pattern matching in both parsing and behavior:

- `parseTarget`, `parseMode`, and `parseOutput` turn strings into constructors, and
- `describePlan` matches on the `BuildPlan` value to decide what to print.

That shows two common styles:

- pattern matching on one constructor at a time in small helper functions, and
- pattern matching on a whole record value when several fields matter together.

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## 3. Commands to Try

```bash
cd 08-haskell-adt

nix develop
cabal run
cabal run -- executable release quiet
cabal test

nix build
./result/bin/mini-plan executable release quiet

nix run . -- executable release quiet
nix flake check
```