nix-playgraound/notes/011-haskell-newtypes.md

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

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