Use when a loop accumulates into a variable, when combining a collection into one value, when the same tree walk keeps being written again, or when a combining law matters.
Install
npx skills add yutna/functional-design-skills --skill functional-folding-over-datanpx skills add yutna/functional-design-skills --skill functional-folding-over-data -a claude-code -gnpx skills add yutna/functional-design-skills --skill functional-folding-over-data -a codex -ggemini skills install https://github.com/yutna/functional-design-skills.git --path plugin/skills/functional-folding-over-datagh skill install yutna/functional-design-skills functional-folding-over-datagit 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-folding-over-data ~/.claude/skills/functional-folding-over-data # Claude Code
cp -R /tmp/yutna__functional-design-skills/plugin/skills/functional-folding-over-data ~/.agents/skills/functional-folding-over-data # Codex, Cursor, Gemini CLI, Copilot, OpenCode