# Aposd Designing Deep Modules

> Guides module and API design using APOSD principles: generates multiple design alternatives, compares them on information hiding and interface depth, and produces a documented design decision. For creating new module/API design; not for assessing existing designs (use aposd-reviewing-module-design) or routine-level design (use cc-routine-and-class-design).

- Skill: `ryanthedev/aposd-designing-deep-modules` (Agent Skill)
- Install (CLI): `npx skillmds@latest add ryanthedev/aposd-designing-deep-modules`
- Raw SKILL.md: https://api.skillmd.com/api/skills/ryanthedev/aposd-designing-deep-modules/raw
- Safety review: pending
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: Integrations & APIs
- Author: ryanthedev (https://skillmd.com/u/ryanthedev)
- Updated: 2026-09-17
- Page: https://skillmd.com/skills/ryanthedev/aposd-designing-deep-modules

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# Skill: aposd-designing-deep-modules

## Before Implementing — the design-it-twice gate

**Never implement your first design.** Generate 2-3 radically different approaches, compare them, then implement.

---

## Design-It-Twice Workflow

```
BEFORE implementing any module:

1. DEFINE - What are you designing? (class, API, service)
2. GENERATE - 2-3 RADICALLY different approaches
3. SKETCH - Rough outline each (important methods only, no implementation)
4. COMPARE - List pros/cons, especially ease of use for callers
5. EVALUATE - Is there a clear winner or hybrid?
6. VERIFY - Does chosen design pass depth evaluation?
7. IMPLEMENT - Only then write the code
```

**If none attractive:** Use identified problems to drive a new iteration of step 2.

---

## Depth Evaluation

| Metric | Deep (Good) | Shallow (Bad) |
|--------|-------------|---------------|
| Interface size | Few methods | Many methods |
| Method reusability | Multiple use cases | Single use case |
| Hidden information | High | Low |
| Caller cognitive load | Low | High |
| Common case | Simple | Complex |

**Exemplar:** Unix file I/O - 5 methods hide hundreds of thousands of lines of implementation.

---

## Three Questions Framework

Ask these when designing interfaces:

| Question | Purpose | Red Flag Answer |
|----------|---------|-----------------|
| "What is the simplest interface that covers all current needs?" | Minimize method count | "I need many methods" |
| "In how many situations will this method be used?" | Detect over-specialization | "Just this one situation" |
| "Is this easy to use for my current needs?" | Guard against over-generalization | "I need lots of wrapper code" |

---

## Information Hiding Checklist

When embedding functionality in a module:

- [ ] Data structures and algorithms stay internal
- [ ] Lower-level details (page sizes, buffer sizes) hidden
- [ ] Higher-level assumptions (most files are small) hidden
- [ ] No knowledge shared across module boundaries unnecessarily
- [ ] Common case requires no knowledge of internal details

---

## Generality Sweet Spot

**Target:** Somewhat general-purpose

| Aspect | Should Be |
|--------|-----------|
| Functionality | Reflects current needs |
| Interface | Supports multiple uses |
| Specialization | Pushed up to callers OR down into variants |

**Push specialization UP:** Top-level code handles specific features; lower layers stay general.

**Push specialization DOWN:** Define general interface, implement with device-specific variants.

---

## Red Flags

| Red Flag | Symptom | Fix |
|----------|---------|-----|
| **Shallow Module** | Interface complexity rivals implementation | Combine with related functionality |
| **Classitis** | Many small classes with little functionality each | Consolidate related classes |
| **Single-Use Method** | Method designed for exactly one caller | Generalize to handle multiple cases |
| **Information Leakage** | Same knowledge in multiple modules | Consolidate in single module |
| **Temporal Decomposition** | Structure mirrors execution order | Structure by knowledge encapsulation |
| **False Abstraction** | Interface hides info caller actually needs | Expose necessary information |
| **Granularity Mismatch** | Caller must do work that belongs in module | Move logic into module |
| **Silent Failure** | Module handles errors internally but gives callers no way to know something went wrong (no error return, no observable state change, no logging) | Errors are implementation details that can be hidden; failures are not — surface failure states even when hiding the mechanism |

---

## Mandatory Output Format

When designing, produce:

```
## Design: [Component Name]

### Approaches Considered
1. [Approach A] - [1-2 sentence description]
2. [Approach B] - [1-2 sentence description]
3. [Approach C] - [1-2 sentence description] (if applicable)

### Comparison
| Criterion | A | B | C |
|-----------|---|---|---|
| Interface simplicity | | | |
| Information hiding | | | |
| Caller ease of use | | | |
| [Domain-specific criterion] | | | |

### Choice: [A/B/C/Hybrid]
Rationale: [Why this wins, what's sacrificed]

### Depth Check
- Interface methods: [count]
- Hidden details: [list]
- Common case complexity: [simple/moderate/complex]
```


---

## Chain

| After | Next |
|-------|------|
| Design chosen | `Skill(code-foundations:cc-pseudocode-programming)` |