Actors were developed to unify and overcome limitations of previous important contributions including lambda calculus [Church 1932] (first-class procedures),

Actors were developed to unify and overcome limitations of previous important contributions including lambda calculus [Church 1932] (first-class procedures), Petri nets [Petri 1962] (true concurrency), semaphores [Dijkstra 1962-1963], packet switching [Baran 1964] (concurrent communication between computers using fixed size packets), capabilities [Dennis and van Horn 1966] (pointer security), Simula [Dahl and Nygaard 1967] (class hierarchy), and SmallTalk-72 [Goldberg and Kay 1976] (bit-mapped computer graphics and browser, the precursor of modern programming Integrated Development Environments). The lambda calculus could not express concurrency with the consequence that lambda calculus programs can be thousands of times slower than Actors. A Petri net cannot dynamically expand and has the limitation that tokens can miraculously disappear from different places, which is inefficient to implement. Capabilities have the limitation that pointers must be manipulated using indexes into capability lists thereby making programming awkward. Simula did not implement concurrency and did not have messages. SmallTalk-72 used a byte stream of program tokens thereby making it awkward for concurrency. See the following for more information: https://papers.ssrn.com/sol3/papers.cfm?abstract_id=3418003 https://papers.ssrn.com/sol3/papers.cfm?abstract_id=3418003