As already mentioned, many languages can be used interactively, although, as conveniently, they can be used so that they are slightly different. The interactive environment that I am most familiar with and the one that I found among the most suitable of all the free interactive programming environments that I tried (not that I tried all of them) is Slime, a mode for emacs that allows you to interact with running shared Lisp, and can also be used with Clojure, a Lisp for the JVM.
If Lisp is not your cup of tea, then many Smalltalk environments are worth mentioning. One of the interesting features of many Smalltalk systems is that they provide almost all of the code that the system implements in the programming environment — if you want you to be able to view or even rewrite parts of the programming environment when you use it, just like you will write new code. In fact, the line between the system provided to you and the code you write is pretty blurry. Squeak is an interesting, free Smalltalk, and Cincom offers an evaluation version of its commercial Smalltalk, which is an excellent IMHO environment.
In any case, if you are interested in playing with interactive environments, you can do worse than playing with these two, although, of course, there are many other systems that allow interactive programming to one degree or another.
T duncan smith
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