# Haskell Newtypes and Smart Constructors

This note covers `09-haskell-newtype/`, which models validated user input with `newtype`, record fields, and smart constructors.

---

## 1. Why `newtype` Matters

Plain `Text` values do not tell you what they represent. A user name and an email address could both be `Text`, even though they mean different
things.

This example wraps those concepts explicitly:

- `UserName`,
- `Email`, and
- `Registration`.

That lets the rest of the code depend on validated domain types instead of raw input.

---

## 2. Smart Constructors

The module exposes constructor functions like `mkUserName` and `mkEmail`, which return `Either String ...`.

That is the main idea:

- invalid input is rejected at the boundary,
- successful validation returns a domain type, and
- the rest of the program works with trusted values.

This is a common intermediate Haskell pattern because it pushes validation close to the edge of the program.

---

## 3. Commands to Try

```bash
cd 09-haskell-newtype

nix develop
cabal run
cabal run -- learner learner@example.com
cabal test

nix build
./result/bin/mini-registration learner learner@example.com

nix run . -- learner learner@example.com
nix flake check
```