Ln Design

Apply Ousterhout's "Design It Twice": generate 3+ radically different module shapes, compare on depth, and synthesize. The goal is deep modules — small interfaces hiding significant complexity. Do not implement; this is purely about the shape of the seam.

Use ln-design as the deepening pathway from ln-review: when review surfaces a shallow module or weak seam, explore alternative deepened module shapes here before routing to ln-scope or ln-refactor.

Input

The module or API boundary: $ARGUMENTS

Procedure

1. Gather requirements

Understand the problem, the callers, the key operations, constraints, and — crucially — what complexity should be hidden inside vs exposed. If this design follows an ln-review deepening candidate, start from that candidate's files, problem, possible direction, and benefits. Skip steps you already know the answer to.

Read memory/SPEC.md first when it exists. Use its lexicon for domain terms and respect its live assumptions, decisions, and invariants. Read memory/PLAN.md when the seam touches active or near-horizon work.

2. Generate designs (parallel sub-agents)

Spawn 3+ sub-agents simultaneously. Each must produce a radically different shape — enforce this by assigning divergent constraints:

• "Minimize method count — aim for 1–3 methods max"
• "Maximize flexibility — support many use cases"
• "Optimize for the most common case"
• "Take inspiration from [specific paradigm or library]"

Each agent returns: interface (types, methods, params, invariants, ordering constraints, error modes, required configuration, and performance characteristics), usage example, what it hides, seam / adapter strategy where relevant, and trade-offs.

3. Present and compare

Show each design sequentially, then compare in prose on:

• Depth (Ousterhout's depth test): small interface hiding significant complexity (good) vs large interface with thin implementation (bad)
• Locality: whether change, bugs, knowledge, and verification concentrate behind the seam
• Leverage: what callers get per fact they must learn about the interface
• Ease of correct use vs ease of misuse
• General-purpose vs specialized: flexibility vs focus
• Implementation efficiency: does the shape allow efficient internals?

Highlight where designs diverge most.

4. Synthesize

The best design often combines insights from multiple options. Ask which shape best fits the primary use case and whether elements from other designs are worth incorporating.

Output

Present the recommended module shape with rationale. If memory/SPEC.md exists, ensure names align with its lexicon.

Do not invent a standalone design document unless the user explicitly asks for one. Durable design choices reconcile back into memory/SPEC.md and memory/PLAN.md.

Routing

After choosing a design, present these options to the user (use tool-ask-question):

#	Label	Target	Why
1	Scope a slice	ln-scope	Design is chosen, define the first slice
2	Write a spec	ln-spec	Module needs a full spec before slicing
3	Grill it more	ln-grill	Design choice raised new questions

Recommended: 1

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Adapted from mattpocock/skills/design-an-interface.