c011abs/design-an-interface
SkillGenerate multiple radically different interface designs for a module using parallel sub-agents. Use when user wants to design an API, explore interface options, compare module shapes, or mentions "design it twice".
SKILL.md
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Based on "Design It Twice" from "A Philosophy of Software Design": your first idea is unlikely to be the best. Generate multiple radically different designs, then compare.
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From "A Philosophy of Software Design":
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**Interface simplicity**: Fewer methods, simpler params = easier to learn and use correctly.
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**General-purpose**: Can handle future use cases without changes. But beware over-generalization.
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**Implementation efficiency**: Does interface shape allow efficient implementation? Or force awkward internals?
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**Depth**: Small interface hiding significant complexity = deep module (good). Large interface with thin implementation = shallow module (avoid).
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- Don't let sub-agents produce similar designs - enforce radical difference
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- Don't skip comparison - the value is in contrast
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- Don't implement - this is purely about interface shape
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- Don't evaluate based on implementation effort
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Before designing, understand:
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- [ ] What problem does this module solve?
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- [ ] Who are the callers? (other modules, external users, tests)
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- [ ] What are the key operations?
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- [ ] Any constraints? (performance, compatibility, existing patterns)
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- [ ] What should be hidden inside vs exposed?
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Ask: "What does this module need to do? Who will use it?"
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Spawn 3+ sub-agents simultaneously using Task tool. Each must produce a **radically different** approach.
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```
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Prompt template for each sub-agent:
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Design an interface for: [module description]
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Requirements: [gathered requirements]
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Constraints for this design: [assign a different constraint to each agent]
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- Agent 1: "Minimize method count - aim for 1-3 methods max"
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- Agent 2: "Maximize flexibility - support many use cases"
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- Agent 3: "Optimize for the most common case"
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- Agent 4: "Take inspiration from [specific paradigm/library]"
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Output format:
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1. Interface signature (types/methods)
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2. Usage example (how caller uses it)
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3. What this design hides internally
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4. Trade-offs of this approach
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```
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Show each design with:
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1. **Interface signature** - types, methods, params
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2. **Usage examples** - how callers actually use it in practice
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3. **What it hides** - complexity kept internal
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Present designs sequentially so user can absorb each approach before comparison.
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After showing all designs, compare them on:
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- **Interface simplicity**: fewer methods, simpler params
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- **General-purpose vs specialized**: flexibility vs focus
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- **Implementation efficiency**: does shape allow efficient internals?
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- **Depth**: small interface hiding significant complexity (good) vs large interface with thin implementation (bad)
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- **Ease of correct use** vs **ease of misuse**
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Discuss trade-offs in prose, not tables. Highlight where designs diverge most.
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Often the best design combines insights from multiple options. Ask:
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- "Which design best fits your primary use case?"
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- "Any elements from other designs worth incorporating?"