Skill: 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