A Solution to the Modularity Problem for Programming Languages: 2023 Benjamin Franklin Medal in Computer and Cognitive Science presented to Barbara H. Liskov, Ph.D.

Early programming methodologies proved inadequate for the reliable implementation of the large programs enabled by rapidly growing computer memories, resulting in prohibitively long times for software development. Attempts to solve this problem by limiting control structure primitives within programming languages or by imposing external design methodologies to enforce ill-defined notions of modularity proved unsuccessful. Barbara Liskov's definition of abstract data types in 1973 and her embodiment of this notion in the programming language CLU provided a major step toward solving this problem. The key idea was to define the fundamental building blocks of programs as a set of abstractions, each of which resulted from the combination of a data type with all the operations that needed to be performed on the data type. Data abstraction is now fundamental to all modern programming languages. A second key innovation of Liskov's was what she called "behavioral subtyping" for object-oriented programming, now known as the Liskov Substitution Principle (LSP). The LSP formally defines when objects of subclasses can be safely substituted for the parent class, a crucial requirement of bug-free programming within this programming paradigm. In addition, Liskov has made several key contributions to distributed computer systems, including the first use of atomic actions for successful concurrency and providing a practical solution to the problem of Byzantine faults.