The let Expression in insert-buffer

After ensuring that the variable buffer refers to a buffer itself and not just to the name of a buffer, the insert-buffer function continues with a let expression. This specifies three local variables, start, end, and newmark and binds them to the initial value nil. These variables are used inside the remainder of the let and temporarily hide any other occurrence of variables of the same name in Emacs until the end of the let.

The body of the let contains two save-excursion expressions. First, we will look at the inner save-excursion expression in detail. The expression looks like this:

(save-excursion
  (set-buffer buffer)
  (setq start (point-min) end (point-max)))

The expression (set-buffer buffer) changes Emacs's attention from the current buffer to the one from which the text will copied. In that buffer, the variables start and end are set to the beginning and end of the buffer, using the commands point-min and point-max. Note that we have here an illustration of how setq is able to set two variables in the same expression. setq's first argument is set to the value of its second and its third argument is set to the value of its fourth.

After the body of the inner save-excursion is evaluated, the save-excursion restores the original buffer, but start and end remain set to the values of the beginning and end of the buffer from which the text will be copied.

The outer save-excursion expression looks like this:

(save-excursion
  (inner-save-excursion-expression
     (go-to-new-buffer-and-set-start-and-end)
  (insert-buffer-substring buffer start end)
  (setq newmark (point)))

The insert-buffer-substring function copies the text into the current buffer from the region indicated by start and end in buffer. Since the whole of the second buffer lies between start and end, the whole of the second buffer is copied into the buffer you are editing. Next, the value of point, which will be at the end of the inserted text, is recorded in the variable newmark.

After the body of the outer save-excursion is evaluated, point and mark are relocated to their original places.

However, it is convenient to locate a mark at the end of the newly inserted text and locate point at its beginning. The newmark variable records the end of the inserted text. In the last line of the let expression, the (push-mark newmark) expression function sets a mark to this location. (The previous location of the mark is still accessible; it is recorded on the mark ring and you can go back to it with C-u C-SPC.) Meanwhile, point is located at the beginning of the inserted text, which is where it was before you called the insert function.

The whole let expression looks like this:

(let (start end newmark)
  (save-excursion
    (save-excursion
      (set-buffer buffer)
      (setq start (point-min) end (point-max)))
    (insert-buffer-substring buffer start end)
    (setq newmark (point)))
  (push-mark newmark))

Like the append-to-buffer function, the insert-buffer function uses let, save-excursion, and set-buffer. In addition, the function illustrates one way to use or. All these functions are building blocks that we will find and use again and again.

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