Comment by reikonomusha

Something like SLIME isn't essential to execute Lisp code, you're right, but it is essential in order to understand the efficiencies afforded by interactive development, which may as well be one of the pillars of the language, since "garbage collection" and "an object system" aren't in and of themselves differentiators anymore*. Using something like SLIME also takes a lot of the pains that people have with Lisp away, namely balancing parens and indenting code correctly. People who do the "I'll use my own editor" approach to beginning Lisp usually write things that look like:

    (defun myfun(x)
        (let (x)
             (setq x 5)
              (when (eq x 6)
                  (print "6")
             )
         )
     )

Which is absolutely not what Lisp code should look like. Emacs-and-kin don't outright stop that, but the defaults are such that it's less likely.

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* Of course, technically, CLOS is something to behold. But you won't sell someone on Lisp because it can do "OOP".