Use when error handling dwarfs the happy path, when the same failure is handled in many places, or when an interface forces callers to handle rare cases.
Install
npx skills add yutna/functional-design-skills --skill functional-defining-errors-out-of-existencenpx skills add yutna/functional-design-skills --skill functional-defining-errors-out-of-existence -a claude-code -gnpx skills add yutna/functional-design-skills --skill functional-defining-errors-out-of-existence -a codex -ggemini skills install https://github.com/yutna/functional-design-skills.git --path plugin/skills/functional-defining-errors-out-of-existencegh skill install yutna/functional-design-skills functional-defining-errors-out-of-existencegit clone --depth 1 https://github.com/yutna/functional-design-skills /tmp/yutna__functional-design-skills
mkdir -p ~/.claude/skills ~/.agents/skills
cp -R /tmp/yutna__functional-design-skills/plugin/skills/functional-defining-errors-out-of-existence ~/.claude/skills/functional-defining-errors-out-of-existence # Claude Code
cp -R /tmp/yutna__functional-design-skills/plugin/skills/functional-defining-errors-out-of-existence ~/.agents/skills/functional-defining-errors-out-of-existence # Codex, Cursor, Gemini CLI, Copilot, OpenCode