nix-playgraound/notes/010-haskell-adts.md

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

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