Design an Interface

Built on the "Design It Twice" principle from "A Philosophy of Software Design": your first idea is unlikely to be the best one. Generate several radically different design options, then compare.

Workflow

1. Gather requirements

Before designing, understand:

• [ ] What problem does this module solve?
• [ ] Who calls it? (other modules, external users, tests)
• [ ] What are the key operations?
• [ ] Are there any constraints? (performance, compatibility, existing patterns)
• [ ] What should be hidden internally vs. exposed?

Ask: "What should this module do? Who will use it?"

2. Generate designs (parallel sub-agents)

Launch 3+ sub-agents simultaneously via the 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 the designs

Show each design with:

1. Interface signature — types, methods, parameters
2. Usage examples — how the calling code actually uses it
3. What it hides — internal complexity left under the hood

Present the designs sequentially so the user can absorb each approach before comparing.

4. Compare the designs

After showing all options, compare them by:

• Interface simplicity: fewer methods, simpler parameters
• Generality vs. specialization: flexibility vs. focus
• Implementation efficiency: does the shape allow efficient internals?
• Depth: a small interface that hides significant complexity (good) vs. a large interface with a thin implementation (bad)
• Easy to use correctly vs. easy to misuse

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

5. Synthesize

Often the best design combines ideas from several options. Ask:

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

Evaluation criteria

From "A Philosophy of Software Design":

Interface simplicity: fewer methods, simpler parameters — easier to learn and use correctly.

Generality: capable of handling future scenarios without changes. But beware of over-generalization.

Implementation efficiency: does the interface shape allow an efficient implementation? Or does it force awkward internals?

Depth: a small interface hiding significant complexity = a deep module (good). A large interface with a thin implementation = a shallow module (avoid).

Anti-patterns

• Don't let sub-agents produce similar designs — insist on radical differentiation
• Don't skip the comparison — the value is in the contrast
• Don't implement — this is only about interface shape
• Don't evaluate by implementation effort