I previously wrote about the reasons to learn Emacs Lisp. That covered the why well enough, but the how is still another question.
There is plenty of information out there about Elisp. However, as I began to learn, I found that none of it took the form that I like when I learn something. I like to have a basic guide that covers everything that I need to know, and has pointers on where to find more information next. Exhaustive manuals are great for reference, but not good for teaching. I do not want to sit down and read a manual from front to back, and only then start to give it some meaning. I cannot remember all of that, so I need to use the manual as a refernce anyway. I can’t prioritize what I am reading without real world context, as most of it wont make a ton of sense until I have internalized some of the system.
All a hacker really needs to know is enough to get comfortable, and some guidance over the various gotchas on the platform. I personally prefer a “layered” approach to learning, which alternates between exploration, instruction, and creation. That is the style that I would like to try to follow.
So, I assume that you are a programmer, and that you know nothing about functional programming or Lisp. If you do not know how to program, then I suggest you look into document that would be more suited to you, An Introduction to Programming in Emacs Lisp This first post covers the very basics of lisp.
First of all, you need to know how to evaluate Emacs Lisp. For this tutorial, at first I suggest you use ielm, the interactive Emacs Lisp interpreter.
To get started, open Emacs. Type
M-x ielm, and then press
return. A new buffer should appear which looks like:
*** Welcome to IELM *** Type (describe-mode) for help. ELISP>
(message "hullo middle earth")
and press return. If your console looks something like below, you are read to continue.
*** Welcome to IELM *** Type (describe-mode) for help. ELISP> (message "hullo middle earth") "hullo middle earth" ELISP>
For now, you use ielm to experiment with Emacs Lisp.
Introduction to Lisp
This tutorial is specific to Emacs Lisp, but really is basically applicable to any lisp in general. As such, I will be referring to “Lisp” for a little while. If you have already done some Lisp programming, feel free to skip to the next section (Introduction to Emacs Lisp). You can always come back here to review.
In Lisp, an integer looks like this:
ELISP> 1 1 ELISP> 3 3 ELISP> 5435 5435
That is all you need to know for a long time.
Lisp strings are double quoted, only. No single quotes.
ELISP> "this is a string" "this is a string"
A symbol is a name for something. Just as names are essentially strings, so are symbols just essentially strings. Symbols are used to reference a variety of things within Lisp, espeically to values and functions.
ELISP> my-symbol *** Eval error *** Symbol's value as variable is void: my-symbol ELISP> 'my-symbol my-symbol
If you notice, the first expression gave an error. It recognized that “my-symbol” was a symbol, but it tried to evaluate it. Since “my-symbol” doesn’t refer to anything here, evaluating it doesn’t make sense.
Afterwards, I “quoted” my symbol with the single quotation mark and the symbol name. That meant just return the symbol, and do not evaluate it. We will cover quoting in a minute, but first, we need to talk about evaluation
Evaluation is the process of taking an expression and “evaluating” it.
We have already seen what happens when we evaluate a number, a string, a symbol, and a quoted symbol:
ELISP> 56 56 ELISP> "hullo" "hullo" ELISP> a-symbol *** Eval error *** Symbol's value as variable is void: a-symbol ELISP> 'a-symbol a-symbol
Evaluation is the process of taking an expression and figuring out what it “means”.
In Lisp, calling a function looks like:
(function-name arg1 arg2 arg3)
So, the following calls the function “+” with the arguments 1 and 2.
ELISP> (+ 1 2) 3
Thus, evaluating a list (a list is indicted by the parentheses) means “apply these arguments to this function”.
Sometimes it is convenient to not evaluate code. Lets imagine that you want to create a list of the first three numbers.
ELISP> (1 2 3) *** Eval error *** Invalid function: 1
That was an error because when emacs evaluates that statement, since it is a list, it thinks that this is a function call.
Of course, we can create a list using a regular function:
ELISP> (list 1 2 3) (1 2 3)
However, this gets unwieldly:
ELISP> (list (list 1) (list 2) (list 3)) ((1) (2) (3))
So, for convenience, Lisp gives the ability to “quote” an expression, preventing it from being evaluated. The easiest way to do that is with the single quotation mark, ‘.
ELISP> '((1) (2) (3)) ((1) (2) (3))
Quoting makes it much, much easier to reference things within your code.
Next time, we will talk about some specifics of Emacs Lisp, which should hopefully be enough to enable you to understand most Emacs Lisp that you come accross. But for now, this covers the essentials of a Lisp program, and you should already be equipped to “read” an Emacs Lisp program while relying heavily on a reference.