Looks like this is two-space indentation is a hard coded behavior of :set lisp mode which ignores shiftwidth.
We can trace this to the source code which contains a hard-coded amount += 2; increment statement.
It's that way for a good reason; it has come to be the predominant way of writing Lisp.
As I wrote this answer, I peeked at samples of source code the following projects (all Lisp or Lisp-like languages):
- Steel Bank Common Lisp (SBCL);
- Clozure Common Lisp (CCL);
- GNU Emacs;
- Clojure;
- GNU Guile;
- Racket;
- and GNU CLISP.
I did not spot a single example of anything but two-space indentation! With two-space indentation, you are in good/large company. You might as well get used to that; even if you somehow get Vim to follow your way, if you upstream anything into anyone's Lisp project, you will likely have to reformat.
Now, I have seen Lisp code using four-space indentation, like in very old code bases and some books.
By the way, sometimes you see if indented like this:
(if (condition)
(then)
(else))
This may happen where indentation is four spaces, but I'm referring situations when this is alignment, and not four space indentation. This formatting is, of course, standard for function arguments, but controversial for operators such as if. Some people like it that way in their code bases. For instance, this institution's randomly found coding style guide recommends this way of writing if, while also recommending two-space indentation.
Vim will do the above if you remove if from lispwords.
The :set lispwords=... parameter contains a comma-separated list of identifiers which follow operator-like indentation, meaning two spaces rather than function-like alignment: second and third lines align with argument. By default, if is found in lispwords.
Lisp mode also applies two space indentation to a function (i.e. form not listed in lispwords, if there is no argument):
(gobbledygook 1 2
2 3)
(gobbledygook
1)
That's also "canonical". Sometimes this comes in handy if you're trying to conform to some maximum line length; a function call running past the limit can sometimes be made to fit by moving all its arguments down, and just giving them two space indentation.
