In classical Smalltalk, there is no difference between development time and runtime. Everything, including system code modification, needs to be done in runtime

In classical Smalltalk, there is no difference between development time and runtime. Everything, including system code modification, needs to be done in runtime. There is no difference between a running program and a development environment so writing a program in Smalltalk is a process of self-modification of the development environment to extend it to provide the functionality you want. So, for Smalltalk, it is essential to provide tools to allow that like objects identity swapping, migration of living instances to a new class definition, objects enumeration etc. The basic conceptual idea behind it is to look at Smalltalk as a living organism that is able to grow and evolve without the need for restarts/bootstrap. You may try Pharo (https://pharo.org/features https://pharo.org/features) to experience it yourself.