What does Barbara Liskov say the Liskov substitution principle actually means?

Barbara Liskov says the principle is about behavior, not shared code: a subclass has to behave like its superclass whenever it is used in a place where the superclass is expected. She arrived at it because Smalltalk-era papers wanted subclass types related to parent types but could not say how, since they described classes by their implementation differences.

• Smalltalk inheritance let a subclass borrow and tweak its parent's implementation; CLU's clusters had no inheritance.
• Liskov thought of modules through their specifications, which let her define the relationship by meaning.

Source: our own transcription of the episode. The full conversation is on Ryan Peterman (2026-10-28). Read the whole episode summary →

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