We succeeded in running the Smalltalk-76 language on our vintage Xerox Alto; this blog post gives a quick overview of the Smalltalk environment. One unusual feature of Smalltalk is you can view and modify the system’s code while the system is running. I demonstrate this by modifying the scrollbar code on a running system.
Smalltalk is a highly-influential programming language and environment that introduced the term “object-oriented programming” and was the ancestor of modern object-oriented languages. The Alto’s Smalltalk environment is also notable for its creation of the graphical user interface with the desktop metaphor, icons, scrollbars, overlapping windows, popup menus and so forth. When Steve Jobs famously visited Xerox PARC, the Smalltalk GUI inspired him on how the Lisa and Macintosh should work.
Be sure to read the comments after the article itself, since it includes comments and clarifications from none other than Alan Kay himself.
A few years ago I was exploring new (to me) programming languages and looked into Smalltalk. I found a modern implementation called Squeak, and the ability to modify code during execution was one of the most interesting aspects of the system.