nix-playgraound/notes/019-formatter-and-checks.md

Formatter and Checks

This note covers 16-formatter-and-checks/, which exposes formatter.<system> and a small checks.<system>.* derivation that uses the same formatter.

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1. Why These Outputs Matter

Two flake outputs are especially practical in day-to-day use:

• formatter.<system>, which powers nix fmt, and
• checks.<system>.*, which powers nix flake check.

This example keeps both outputs small so the wiring is obvious.

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2. The Important Connection

The flake points formatter.<system> at pkgs.nixfmt, then reuses that same formatter in a check:

formatter.${system} = pkgs.nixfmt;

That shows the main idea: a formatter output is just another derivation, so checks can use it too.

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3. Commands to Try

cd 16-formatter-and-checks

nix fmt sample.nix
nix flake show
nix flake check