2026-04-22 15:43:26 +02:00
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# Haskell Newtypes and Smart Constructors
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This note covers `09-haskell-newtype/`, which models validated user input with `newtype`, record fields, and smart constructors.
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---
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## 1. Why `newtype` Matters
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2026-04-22 15:44:05 +02:00
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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
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things.
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2026-04-22 15:43:26 +02:00
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This example wraps those concepts explicitly:
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- `UserName`,
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- `Email`, and
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- `Registration`.
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That lets the rest of the code depend on validated domain types instead of raw input.
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---
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## 2. Smart Constructors
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The module exposes constructor functions like `mkUserName` and `mkEmail`, which return `Either String ...`.
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That is the main idea:
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- invalid input is rejected at the boundary,
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- successful validation returns a domain type, and
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- the rest of the program works with trusted values.
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This is a common intermediate Haskell pattern because it pushes validation close to the edge of the program.
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---
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## 3. Commands to Try
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```bash
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cd 09-haskell-newtype
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nix develop
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cabal run
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cabal run -- learner learner@example.com
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cabal test
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nix build
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./result/bin/mini-registration learner learner@example.com
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nix run . -- learner learner@example.com
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nix flake check
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```
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