aposd-simplifying-complexity

Error Reduction Hierarchy

The best way to deal with exceptions is to define errors out of existence.

Priority order: Define out → Mask → Aggregate → Crash (app-level only)

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Pull Complexity Downward

Decision Procedure

Before adding complexity to an interface (new parameters, new exceptions, new caller responsibilities):

1. Is this complexity closely related to the module's existing functionality?
   NO  → Should it be pulled into a DIFFERENT module?
         YES → Identify correct module, pull there
         NO  → Leave in place (may be inherent to caller's domain)
   YES → Continue

2. Will pulling down simplify code elsewhere in the application?
   NO  → Do not pull down (no benefit)
   YES → Continue

3. Will pulling down simplify the module's interface?
   NO  → Do not pull down (risk of leakage)
   YES → Pull complexity down

All three conditions must be YES to pull down.

Critical Constraint: Pulling down UNRELATED complexity creates information leakage. If the complexity isn't intrinsic to the module's core abstraction, it doesn't belong there—find the right home or leave it with the caller.

Configuration Parameters

| Situation | Wrong Approach | Right Approach | |-----------|---------------|----------------| | Uncertain what value to use | Export parameter | Compute automatically | | Different contexts need different values | Export parameter | Use reasonable default, expose only for exceptions | | Policy decision unclear | Let user decide | Make a decision and own it |

Configuration parameters represent incomplete solutions. Every parameter pushes complexity to every user/administrator. Prefer dynamic computation over static configuration.

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Error Reduction Hierarchy

Apply in order of preference:

| Priority | Technique | How It Works | Example | |----------|-----------|--------------|---------| | 1 | Define out | Change semantics so error is impossible | unset(x) = "ensure x doesn't exist" (not "delete existing x") | | 2 | Mask | Handle at low level, hide from callers | TCP retransmits lost packets internally | | 3 | Aggregate | Single handler for multiple exceptions | One catch block in dispatcher handles all NoSuchParameter | | Special | Crash | Print diagnostic and abort (app-level only) | malloc failure in non-recoverable contexts |

Note on "Crash": This is NOT level 4 of a hierarchy—it's a special case for truly unrecoverable errors in application code. Libraries should NEVER crash; they expose errors for callers to decide.

Error Reduction Decision Procedure

When facing an exception handling decision:

1. Can semantics be redefined to eliminate the error condition?
   YES → Define out of existence
   NO  → Continue

2. Can exception be handled at low level without exposing?
   YES → Mask
   NO  → Continue

3. Can multiple exceptions share the same handling?
   YES → Aggregate
   NO  → Continue

4. Is error rare, unrecoverable, and non-value-critical?
   YES → Just crash (app-level only)
   NO  → Must expose (exception information needed outside module)

When NOT to Apply Hierarchy

| Exception Case | Why | What to Do Instead | |----------------|-----|-------------------| | Security-critical errors | Aggregating auth errors loses security-relevant distinctions | Keep distinct types for audit/logging | | Retry-differentiated errors | Callers need different retry strategies per error type | Expose type info for retry decisions | | Silent data loss risk | Define-out can mask user errors, complicate debugging | Fail fast for essential data errors | | Library code | Callers should decide crash policy, not library | Expose errors; let app-level code crash |

Validation Gates

| Technique | Gate Question | |-----------|---------------| | Define out | Does anyone NEED to detect this error case? | | Mask | Does the caller have ANY useful response to this error? | | Aggregate | Do callers handle these errors identically? | | Crash | Is this (a) application-level code, (b) truly unrecoverable, AND (c) crash acceptable? |

Define-Out Appropriateness Test

Before defining an error out of existence, verify it's an incidental error (safe) not an essential error (must fail fast):

| Question | If YES → | If NO → | |----------|----------|---------| | Would this state occur in normal, correct operation? | Safe to define out | Fail fast | | Can the caller proceed meaningfully with the "defined out" state? | Safe | Expose error | | Does the user/system have another way to detect this condition if needed? | Safe | Consider exposing |

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Obviousness Techniques

Three Ways to Make Code Obvious

| Technique | How | When to Use | |-----------|-----|-------------| | Reduce information needed | Abstraction, eliminate special cases | Design-level changes | | Leverage reader knowledge | Follow conventions, meet expectations | Incremental improvements | | Present explicitly | Good names, strategic comments | When other techniques insufficient |

Obviousness Test

If a code reviewer says your code is not obvious, it is not obvious—regardless of how clear it seems to you.

Common Obviousness Problems

| Problem | Why Nonobvious | Fix | |---------|----------------|-----| | Generic containers (Pair, Tuple) | getKey() obscures meaning | Define specific class with named fields | | Event-driven handlers | Control flow hidden | Document invocation context | | Type mismatches | List declared, ArrayList allocated | Match declaration to allocation | | Violated expectations | Code doesn't do what reader assumes | Document or refactor to meet expectations |

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Output Gate

Before presenting simplified code, produce a technique analysis table — this is the evidence that the hierarchy was applied:

| Error Condition | Technique | Gate Check | Reasoning |
|-----------------|-----------|------------|-----------|
| [each error]    | [1-4]     | [PASS/FAIL]| [why]     |

Proceed only when the following are true:

• Technique analysis table present for each error condition
• Complexity moved to fewer places (not just relocated)
• Interfaces are simpler than before
• Callers do less work than before
• Error handling is consolidated or eliminated
• Reader needs less context to understand

If a criterion legitimately cannot be satisfied, present the code with the failed criterion and reason stated.

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Principle Conflict Resolution

| Conflict | Resolution Heuristic | |----------|---------------------| | Define Out vs Fail Fast | Define out for incidental errors. Fail fast for essential errors. | | Mask vs Explicit Handling | Mask when caller has no useful response. Expose when caller's response differs. | | Aggregate vs Specific Messages | Aggregate the HANDLING, preserve specificity in the MESSAGE. | | Pull Down vs Single Responsibility | Only pull down complexity RELATED to module's core purpose. | | Obviousness vs Brevity | When define-out creates non-obvious behavior, add explanatory comment. | | Simplify vs Performance | Prefer simplicity unless profiling proves performance-critical. |

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Red Flags

| Red Flag | Symptom | Transformation | |----------|---------|----------------| | Scattered exceptions | Same error handled in many places | Aggregate to single handler | | Configuration explosion | Many parameters exported | Compute automatically, provide defaults | | Caller doing module's work | Logic outside that belongs inside | Pull complexity down | | Over-defensive code | Checks for impossible conditions | Define errors out | | Generic containers | Pair<X,Y> obscures meaning | Create named structure | | Comment-dependent understanding | Code unreadable without comments | Refactor for obviousness | | Masked error without observability | Applying Mask or Define-out but no logging, metrics, or alternate signal when the error actually occurs | Every masked error needs an observability escape hatch (log, metric, health check) so operators can detect when masking hides a real problem |

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Detailed checklists: Read(${CLAUDE_SKILL_DIR}/checklists.md)

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Chain

| After | Next | |-------|------| | Simplification done | Skill(code-foundations:aposd-verifying-correctness) — verify interface simplified |