Assignment 2. Lisp and Python scripting

Exercise 2.1: Navigating through Emacs source code

The basic idea here is to get a mental model of how Emacs works by looking at a bit of its keybindings and source code.

Start up a fresh Emacs with a *scratch* buffer.

To warm up, compute (2^{521 − 1}) × (2⁵²¹ − 1) (i.e., 2**(521 - 1) * (2**521 - 1)) in the *scratch* buffer, by using the expt and other functions. This is the 13th perfect number, discovered in 1952.

Use Emacs to determine how many bits it would take to represent this number in base-2 notation (not counting any sign bit), by writing a Lisp expression that yields the number of bits as an integer.

Type M-: and use it to compute (2^{521 − 1}) × (2⁵²¹ − 1).

Get a list of keybindings by typing C-h b.

Look for two keybindings: C-h k and M-SPC. C-h k stands for “Type Control-h, then ‘k’.” M-SPC is “Meta Space”; on good keyboards you can get this by holding down Alt while hitting the space bar, but you may need to type “Esc” and then follow by hitting the space bar. We will examine these two keybindings in more detail.

Type C-h k C-h k and describe what happens and why. (This should relate to the C-h b output mentioned previously.)

Type C-h k M-SPC and describe what happens and why. (This should also relate.)

Try out M-SPC on some sample text with a lot of white space, to see how it works.

Visit the source code for the function that implements M-SPC, by going to its help and clicking (or typing RET) on its source file name.

Notice how M-SPC is implemented in terms of a more-general function, which does not have a keybinding. Use M-: to execute this more-general function on a buffer, such that the function changes the buffer's contents.

Similarly, use M-x to execute the more-general function on a buffer.

Exercise 2.2: Scripting Emacs

Use the Emacs command M-x what-line and see what it does.

M-x what-line simply tells you what line you are on, not how many lines are in the buffer. Design and implement a command M-x gps-line that acts like M-x what-line except that it says "Line 27/106" in contexts where M-x what-line would merely say "Line 27"; here, it’s assumed the buffer has 106 lines. Do this by using C-h f to get help about what-line, navigating through that help to find its source code, putting a copy of the source code into a new file gps-line.el, editing that file, loading it into Emacs, and then executing your new command.

When counting all the lines in a buffer, simply count the number of newline characters that it contains. This means that if a buffer does not end in a newline, you should not count the characters after the last newline to be part of another line. In particular, an empty buffer has zero lines.

Test your function on buffers that do not end in newline. Your function should be able to say things like "Line 1/0" and "Line 3/2".