aposd-designing-deep-modules

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.

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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.

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## 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.

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## 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" |

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## 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

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## 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.

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## 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 |

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## 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]

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## Chain

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