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(emacs-lisp-intro.info)rotate-yk-ptr arg
Passing the argument
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The hard part of `yank' is understanding the computation that
determines the value of the argument passed to `rotate-yank-pointer'.
Fortunately, it is not so difficult as it looks at first sight.
What happens is that the result of evaluating one or both of the
`if' expressions will be a number and that number will be the argument
passed to `rotate-yank-pointer'.
Laid out with comments, the code looks like this:
(if (listp arg) ; if-part
0 ; then-part
(if (eq arg '-) ; else-part, inner if
-1 ; inner if's then-part
(1- arg)))) ; inner if's else-part
This code consists of two `if' expression, one the else-part of the
other.
The first or outer `if' expression tests whether the argument passed
to `yank' is a list. Oddly enough, this will be true if `yank' is
called without an argument--because then it will be passed the value of
`nil' for the optional argument and an evaluation of `(listp nil)'
returns true! So, if no argument is passed to `yank', the argument
passed to `rotate-yank-pointer' inside of `yank' is zero. This means
the pointer is not moved and the first element to which
`kill-ring-yank-pointer' points is inserted, as we expect. Similarly,
if the argument for `yank' is `C-u', this will be read as a list, so
again, a zero will be passed to `rotate-yank-pointer'. (`C-u' produces
an unprocessed prefix argument of `(4)', which is a list of one
element.) At the same time, later in the function, this argument will
be read as a `cons' so point will be put in the front and mark at the
end of the insertion. (The `P' argument to `interactive' is designed
to provide these values for the case when an optional argument is not
provided or when it is `C-u'.)
The then-part of the outer `if' expression handles the case when
there is no argument or when it is `C-u'. The else-part handles the
other situations. The else-part is itself another `if' expression.
The inner `if' expression tests whether the argument is a minus
sign. (This is done by pressing the <META> and `-' keys at the same
time, or the <ESC> key and then the `-' key). In this case, the
`rotate-yank-pointer' function is passed `-1' as an argument. This
moves the `kill-ring-yank-pointer' backwards, which is what is desired.
If the true-or-false-test of the inner `if' expression is false
(that is, if the argument is not a minus sign), the else-part of the
expression is evaluated. This is the expression `(1- arg)'. Because
of the two `if' expressions, it will only occur when the argument is a
positive number or when it is a negative number (not just a minus sign
on its own). What `(1- arg)' does is decrement the number and return
it. (The `1-' function subtracts one from its argument.) This means
that if the argument to `rotate-yank-pointer' is 1, it is reduced to
zero, which means the first element to which `kill-ring-yank-pointer'
points is yanked back, as you would expect.
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