Design An Interface

Design an Interface

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.

Workflow

1. Gather Requirements

Before designing, understand:

• What problem does this module solve?
• Who are the callers? (other modules, external users, tests)
• What are the key operations?
• Any constraints? (performance, compatibility, existing patterns)
• What should be hidden inside vs exposed?

Ask: "What does this module need to do? Who will use it?"

2. Generate Designs (Parallel Sub-Agents)

Spawn 3+ sub-agents simultaneously using Task tool. Each must produce a radically different approach.

Prompt template for each sub-agent:

Design an interface for: [module description]

Requirements: [gathered requirements]

Constraints for this design: [assign a different constraint to each agent]
- Agent 1: "Minimize method count - aim for 1-3 methods max"
- Agent 2: "Maximize flexibility - support many use cases"
- Agent 3: "Optimize for the most common case"
- Agent 4: "Take inspiration from [specific paradigm/library]"

Output format:
1. Interface signature (types/methods)
2. Usage example (how caller uses it)
3. What this design hides internally
4. Trade-offs of this approach

3. Present Designs

Show each design with:

1. Interface signature - types, methods, params
2. Usage examples - how callers actually use it in practice
3. What it hides - complexity kept internal

Present designs sequentially so user can absorb each approach before comparison.

4. Compare Designs

After showing all designs, compare them on:

• Interface simplicity: fewer methods, simpler params
• General-purpose vs specialized: flexibility vs focus
• Implementation efficiency: does shape allow efficient internals?
• Depth: small interface hiding significant complexity (good) vs large interface with thin implementation (bad)
• Ease of correct use vs ease of misuse

Discuss trade-offs in prose, not tables. Highlight where designs diverge most.

5. Synthesize

Often the best design combines insights from multiple options. Ask:

• "Which design best fits your primary use case?"
• "Any elements from other designs worth incorporating?"

Evaluation Criteria

From "A Philosophy of Software Design":

Interface simplicity: Fewer methods, simpler params = easier to learn and use correctly.

General-purpose: Can handle future use cases without changes. But beware over-generalization.

Implementation efficiency: Does interface shape allow efficient implementation? Or force awkward internals?

Depth: Small interface hiding significant complexity = deep module (good). Large interface with thin implementation = shallow module (avoid).

Anti-Patterns

• Don't let sub-agents produce similar designs - enforce radical difference
• Don't skip comparison - the value is in contrast
• Don't implement - this is purely about interface shape
• Don't evaluate based on implementation effort

Source: mattpocock/skills → skills/deprecated/design-an-interface/SKILL.md