Lint-ConstantReassignment.md

Pattern: Constant reassignment

Issue: -

Description

Constant reassignment in Ruby is problematic:

In Ruby, when you reassign a constant, you get a warning message because it goes against several important programming principles:

1. Violation of immutability - Constants are meant to represent values that shouldn't change during program execution. When you reassign them, you break this contract, making your code less predictable and harder to reason about.

MAX_USERS = 100
MAX_USERS = 200  # warning: already initialized constant MAX_USERS

2. Thread safety concerns - If multiple threads are accessing a constant and one thread reassigns it, it can lead to race conditions and unpredictable behavior.

3. Performance implications - Ruby optimizes constant lookups because it assumes they won't change. Reassignment can impact these optimizations.

4. Code maintainability - When other developers see a constant, they expect it to remain constant. Reassignment makes the code harder to understand and maintain.

Instead of reassigning constants, you should:

• Use variables if the value needs to change
• Use frozen objects to ensure true immutability
• Consider using configuration objects or environment variables for values that might need to change

# Better approach
SETTINGS = { max_users: 100 }.freeze  # Can't be modified

Examples

# bad
X = :foo
X = :bar

# bad
class A
  X = :foo
  X = :bar
end

# bad
module A
  X = :foo
  X = :bar
end

# good - keep only one assignment
X = :bar

class A
  X = :bar
end

module A
  X = :bar
end

# good - use OR assignment
X = :foo
X ||= :bar

# good - use conditional assignment
X = :foo
X = :bar unless defined?(X)

# good - remove the assigned constant first
class A
  X = :foo
  remove_const :X
  X = :bar
end

Further Reading

• RuboCop - Lint/ConstantReassignment