52 lines
1.2 KiB
Markdown
52 lines
1.2 KiB
Markdown
# Haskell Algebraic Data Types
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This note covers `08-haskell-adt/`, which models a build plan with sum types, a record type, and pattern matching.
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---
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## 1. Why This Example Matters
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Haskell programs often start by turning vague strings into precise domain types.
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This example does that with:
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- `Target` as a sum type,
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- `Mode` as a sum type,
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- `Output` as a sum type, and
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- `BuildPlan` as a product type with record fields.
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That is one of the most important intermediate Haskell habits: model the domain first, then write functions over the constructors.
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---
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## 2. Pattern Matching in Two Places
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The example uses pattern matching in both parsing and behavior:
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- `parseTarget`, `parseMode`, and `parseOutput` turn strings into constructors, and
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- `describePlan` matches on the `BuildPlan` value to decide what to print.
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That shows two common styles:
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- pattern matching on one constructor at a time in small helper functions, and
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- pattern matching on a whole record value when several fields matter together.
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---
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## 3. Commands to Try
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```bash
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cd 08-haskell-adt
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nix develop
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cabal run
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cabal run -- executable release quiet
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cabal test
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nix build
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./result/bin/mini-plan executable release quiet
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nix run . -- executable release quiet
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nix flake check
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```
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