Barbara Liskov

Liskov led development of the CLU language at MIT through the 1970s. It introduced iterators, exception handling and — most importantly — abstract data types, the idea that a type is defined by what you can do with it rather than how it is stored. That is encapsulation before the word existed.

The substitution principle, formulated with Jeannette Wing in 1994, says a subtype must be usable anywhere the base type is expected without changing the program's correctness. It is the L in SOLID and the most frequently violated of the five. She was among the first women in the United States to earn a doctorate in computer science.