eintr: kill-append function
The ‘kill-append’ function
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The ‘kill-append’ function looks like this:
(defun kill-append (string before-p &optional yank-handler)
"Append STRING to the end of the latest kill in the kill ring.
If BEFORE-P is non-nil, prepend STRING to the kill.
... "
(let* ((cur (car kill-ring)))
(kill-new (if before-p (concat string cur) (concat cur string))
(or (= (length cur) 0)
(equal yank-handler
(get-text-property 0 'yank-handler cur)))
yank-handler)))
The ‘kill-append’ function is fairly straightforward. It uses the
‘kill-new’ function, which we will discuss in more detail in a moment.
(Also, the function provides an optional argument called
‘yank-handler’; when invoked, this argument tells the function how to
deal with properties added to the text, such as bold or italics.)
It has a ‘let*’ function to set the value of the first element of the
kill ring to ‘cur’. (I do not know why the function does not use ‘let’
instead; only one value is set in the expression. Perhaps this is a bug
that produces no problems?)
Consider the conditional that is one of the two arguments to
‘kill-new’. It uses ‘concat’ to concatenate the new text to the CAR of
the kill ring. Whether it prepends or appends the text depends on the
results of an ‘if’ expression:
(if before-p ; if-part
(concat string cur) ; then-part
(concat cur string)) ; else-part
If the region being killed is before the region that was killed in the
last command, then it should be prepended before the material that was
saved in the previous kill; and conversely, if the killed text follows
what was just killed, it should be appended after the previous text.
The ‘if’ expression depends on the predicate ‘before-p’ to decide
whether the newly saved text should be put before or after the
previously saved text.
The symbol ‘before-p’ is the name of one of the arguments to
‘kill-append’. When the ‘kill-append’ function is evaluated, it is
bound to the value returned by evaluating the actual argument. In this
case, this is the expression ‘(< end beg)’. This expression does not
directly determine whether the killed text in this command is located
before or after the kill text of the last command; what it does is
determine whether the value of the variable ‘end’ is less than the value
of the variable ‘beg’. If it is, it means that the user is most likely
heading towards the beginning of the buffer. Also, the result of
evaluating the predicate expression, ‘(< end beg)’, will be true and the
text will be prepended before the previous text. On the other hand, if
the value of the variable ‘end’ is greater than the value of the
variable ‘beg’, the text will be appended after the previous text.
When the newly saved text will be prepended, then the string with the
new text will be concatenated before the old text:
(concat string cur)
But if the text will be appended, it will be concatenated after the old
text:
(concat cur string))
To understand how this works, we first need to review the ‘concat’
function. The ‘concat’ function links together or unites two strings of
text. The result is a string. For example:
(concat "abc" "def")
⇒ "abcdef"
(concat "new "
(car '("first element" "second element")))
⇒ "new first element"
(concat (car
'("first element" "second element")) " modified")
⇒ "first element modified"
We can now make sense of ‘kill-append’: it modifies the contents of
the kill ring. The kill ring is a list, each element of which is saved
text. The ‘kill-append’ function uses the ‘kill-new’ function which in
turn uses the ‘setcar’ function.
Info Catalog
eintr: last-command & this-command
eintr: copy-region-as-kill body
eintr: kill-new function