[mikelevins]

Common Lisp supports stopping execution of a function in the middle, redefining the function, changing the values of all accessible dynamic and lexical variables, adding new variable definitions, redefining the types of the values, and then resuming execution from the start of the redefined function, or from any other accessible control point.

It also supports writing condition handlers that can do all of the above under program control, rather than under human control. In other languages you would say "exception handlers", but Common Lisp conditions include representations of situations and responses that exceptions typically don't.

These facilities, along with things like a debugger, a tracer, support for built-in documentation, a compiler, and a disassembler, are part of the language standard, not the development environment.

Common Lisp is a language designed from the ground up for writing programs by interacting with them as they run. There aren't very many other languages like that. Smalltalk is one of them.