The Liskov Substitution Principle does more than you think
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The Liskov Substitution Principle (LSP) is commonly understood as just the 'L' in SOLID, applying only to inheritance hierarchies. But the original Liskov-Wing paper's core insight — that subtypes must have weaker or equal preconditions and stronger or equal postconditions — applies far more broadly. Any time you substitute one function or code block for another (including API versioning and backward compatibility), the same contract rules apply. Making a required parameter optional weakens preconditions (safe), while making an optional parameter required strengthens them (breaking). The post also connects this to Hyrum's Law: any observable behavior can become an implicit postcondition that someone depends on, meaning even 'strengthening' a postcondition by adding new output fields can break callers who relied on the absence of those fields.
