The Polymorphic Programming Language (PPL) was developed in 1969 at Harvard University by Thomas A. Standish. It is an interactive, extensible language with a base language similar to the language APL.
- Standish, Thomas A. (August 1969). Christensen; Shaw (eds.). "Some features of PPL – a Polymorphic Programming Language". ACM SIGPLAN Notices (Proceedings of Extensible Language Symposium ed.). ACM Digital Library: Association for Computing Machinery. 4 (8): 20–26. doi:10.1145/1115858.1115864. Retrieved 2018-06-23.
- Chambers, John M. (November 1994). "Evolution of the S Language". Archived from the original (PostScript) on 2007-04-09. Retrieved 2009-05-06.