about lisp editors - via "Common Lisp - A Gentle Introduction to Symbolic Computation"

gistfile1.txt

    The most frequently occurring errors in LISP are parenthetical errors. It
    is thus almost imperative to employ some sort of counting or pairing 
    device to check parentheses every time that a function is changed. 
    — Elaine Gord, “Notes on the debugging of LISP programs,” 1964.

  The above quote was written 25 years ago, when Lisp programs were typed
on punched cards. Today, of course, we use interactive editors. Lisp editors
are not ordinary text editors: They “understand” the syntax of Lisp programs.
On my machine, whenever I type a right parenthesis, the editor flashes the
corresponding left parenthesis for me. This keeps me from making a
“parenthetical error” when entering Lisp expressions. Another one of my 
editor’s jobs is to automatically indent every line as I type it. If a 
function definition takes several lines, it will be indented in a neat and 
orderly format that is easy to read.

  Some of the earliest Lisp books were written before anyone thought of 
systematically indenting programs to make them readable. A program that would
have been written this way back then:

             (defun long-function (some-list) (cons
             (third some-list) (list (second (some-list)
             (fourth some-list) (first some-list))))

would today be automatically indented to look like this:

                 (defun long-function (some-list)
                   (cons (third some-list)
                         (list (second some-list)
                               (fourth some-list)
                               (first some-list))))

  There are two more things a good Lisp editor provides. One is an easy way
to evaluate expressions while editing. You can position the cursor (or mouse)
on a function definition, hit a few keys, and that function definition will be
evaluated without ever leaving the editor. The second thing a good editor
provides is rapid access to online documentation. If I want to see the
documentation for any Lisp function or variable, I can call it up with just a
few keystrokes. The editor also provides online documentation about itself.