Single Purpose Variables

Single Purpose Variables

Overview

Each variable should represent exactly ONE thing. No reusing for different purposes. No hidden meanings.

Core principle: If variable represents count sometimes and error other times, use two variables.

Baseline Violation: Hybrid Coupling

From baseline, agents use special values to indicate errors:

❌ Hybrid coupling (baseline):

def process_file_pages(filename):
    try:
        pages_processed = 0  # Count (integer purpose)
        # ... processing ...
        return pages_processed
    except:
        return -1  # Error flag (boolean purpose as -1)

Problem: pages_processed represents TWO things:

• Non-negative integer = page count
• -1 = error occurred

This is hybrid coupling: Variable moonlights as different type.

✅ Separate concerns:

def process_file_pages(filename):
    try:
        pages_processed = 0
        # ... processing ...
        return (True, pages_processed)  # Success, count
    except Exception as e:
        return (False, str(e))  # Failure, error message

Or raise exception:

def process_file_pages(filename):
    # Let exceptions propagate - no hybrid variable needed
    pages_processed = 0
    # ... processing (raises on error) ...
    return pages_processed  # Always a count, never an error

Common Hidden Meanings

❌ What agents naturally do:

page_count = 15  # Number of pages
page_count = -1  # Wait, now it means error!

customer_id = 1234  # Customer number
customer_id = 500001  # Wait, > 500000 means delinquent (subtract 500000)!

bytes_written = 1024  # Bytes written
bytes_written = -5  # Wait, negative means disk drive number!

✅ Separate variables:

page_count = 15
processing_failed = True  # Separate boolean for error state

customer_id = 1234
is_delinquent = False  # Separate boolean for status

bytes_written = 1024
disk_drive = 5  # Separate variable for drive number

Legitimate Variable Reuse

Good reuse (same purpose, same meaning):

# ✅ GOOD: total_sales used for multiple related calculations
total_sales = sum(sales)
average = total_sales / len(sales)  # Same value, same meaning
percentage = (total_sales / target) * 100  # Same value, same meaning

Bad reuse (different purposes):

# ❌ BAD: temp reused for unrelated purposes
temp = sqrt(b*b - 4*a*c)  # Discriminant
root1 = (-b + temp) / (2*a)
# ...
temp = root1  # Now reused for swapping (different purpose!)
root1 = root2
root2 = temp

✅ Separate variables:

discriminant = sqrt(b*b - 4*a*c)  # Clear purpose
root1 = (-b + discriminant) / (2*a)
# ...
old_root = root1  # Clear purpose (swapping)
root1 = root2
root2 = old_root

Quick Reference

Red Flags

• Variable represents different types (integer sometimes, boolean as -1)
• Special values have hidden meanings (-1, 0, null mean different things)
• Reusing temp, result, value for unrelated purposes
• Code comments explain "if X then it means Y, else Z"
• Must remember what value currently means

Fix: Create separate variable with clear name for each purpose.

Real-World Impact

From Code Complete:

• Hybrid coupling creates confusion
• Even if clear to you, won't be to others
• Extra variable costs nothing, clarity is priceless

From baseline:

• Agent used -1 to indicate error in count variable (hybrid coupling)

With this skill: Separate variables for separate purposes.

Integration with Other Skills

For naming clarity: See skills/naming-variables - each purpose needs its own well-named variable