Questions to answer:

• Why should I care about equality?
• How do I properly implement equality?
• Do I need to care about coercion?

Methods

• #==
• #eql?
• #equal?
• #===
• #hash

Not quite related to equality itself:

• #<=> and Comparable
• #deconstruct and #deconstruct_keys

Rules

• For equality
  • Reflexivity
  • Symmetry
  • Transitivity. If 15 USD = 15, and 15 EUR = 15, then it must follow that 15 USD = 15 EUR, which is certainly not right.
• If two objects are eql?, then their hashes must be equal as well.

Examples

• Entity: Employee type with id and name.
  • Name can change but that does not change equality. Equality is based only on ID.
  • Alternatively, other fields can be taken into account for equality. But even then, not all. A Page might change identity if its content changes, but wouldn’t change when view_count changes.

• Value types, coercible: Path type (coercible into String), or Point type (coercible into array)

• Value types, non-coercible: Money

Further reading

• Kevin Newton: Ruby type conversion
• Ruby 3.1 documentation on Object#== etc