08 Apr 2014
tagged with: lisp, emacs, talks
We’ll go over the basic features of Emacs Lisp, including an introduction to higher-order functions, but not including macros.
Most of the Emacs standard library. This tutorial covers the features of the language, but it doesn’t address how we manipulate buffers, regions, and so on.
We’ll also be making passing references to macros, but we won’t be exploring them in any detail.
Emacs can be thought of as a giant REPL. Like
and other REPLs, it Reads in an expression, Evaluates it,
Prints the results, and Loops to read the next expression.
In Emacs’ case, the REPL is started when we launch the Emacs application. Every time we interact with the editor, we’re just executing some code in the context of that REPL. Every keystroke, every mouse click, and every menu action correspond to evaluating an expression in this REPL.
.emacs file is just the first code that’s executed
by that REPL after it starts.
Don’t worry if this doesn’t make too much sense now; you’ll get it soon. =)
To execute this code as we go along, you’ll first want to open up a
new buffer with
C-x b and name it whatever you’d like. Run
emacs-lisp-mode to put it into the correct mode.
Paste code snippets into this buffer, place your cursor (called
“point” in Emacs lingo) right after the expression, and hit
C-e. That’ll evaluate the expression in the Emacs REPL. The results
of the expression will appear in the echo area in the bottom-left
corner of your window.
Atoms are the simplest objects in Emacs Lisp. They evaluate to themselves, so evaluating these objects will just return themselves.
We have integers:
Lisp expressions are either atoms or applications of functions (or macro expressions, technically, but that’s outside the scope of this article).
To call a function, we wrap it all in parentheses, like so:
(+ 1 2 3)
That calls the function
+ on the arguments
When evaluating a function, Lisp first evaluates the arguments to the function, the applies the function to those arguments. This means that the arguments to a function can themselves be function calls:
(+ (* 2 3) (/ 8 4))
In this example, the interpreter first executes
(* 2 3) and
4), which evaluate to
2 respectively. These values become
the arguments to
+, so the entire expression evaluates to
Lisp is really interested in lists; it’s the LISt Processing language, after all.
Let’s try to evaluate a list:
(1 2 3)
When we execute that, we get the error:
Invalid function: 1. That’s
reasonable: the interpreter is trying to execute the function
3, but of course
1 isn’t really a function.
If we want to refer to a list without trying to evaluate it, we can
wrap it in a call to the
quote function. In the same way that
quoting a word in English refers to the word and not the thing it
represents, quoting an expression in Lisp refers to the list
itself. To say that another way, quoting an expression treats it as
data, not as code.
(quote (1 2 3))
And boom! We get
(1 2 3), just like we wanted.
It turns out that quoting is something we do pretty often in Lisp, so there’s a bit of handy syntax to make it easier.
'(1 2 3)
'(1 2 3) evaluates to
(1 2 3), too, and we can generally use
quote interchangeably. We’ll be using
' from here on out,
because it’s terser.
We can quote nested lists, too:
'(1 2 (3 4 5) (6 (7))) ; => (1 2 (3 4 5) (6 (7)))
We can also quote a list that looks like a function call:
'(+ 1 2 3) ; => (+ 1 2 3)
This just evaluates to the four-element list
(+ 1 2 3). Notice that
nothing is evaluated inside the quoted expression.
We can also create a list with the
(list 1 2 3) ; => (1 2 3)
Now that we know how to refer to whole lists, we can start
manipulating them. The simplest function is
car, which just returns
the first element of the list.
(car '(1 2 3)) ; => 1
cdr returns the list minus the first element.
(cdr '(1 2 3)) ; => (2 3)
Why are these functions called
cdr? It’s just an
historical accident, but you can read the whole story here. They’re
basically vestigial. We’d probably name these functions
rest if we were rewriting Emacs from scratch today, but because
they’re already used so widely, it’s just something you’ll need to
memorize. Sorry. =(
Let’s try evaluating the empty list.
Bizarrely, this evaluates to
nil! In Lisp,
nil and the empty list
are equivalent. They’re literally the same thing. You can append
nil to create lists, and you can test for the empty list
in conditionals (where it’s falsy).
The empty list evaluates to itself:
When we take the
cdr of a one-element list, we get the empty list,
null determines if a value is
()). This is
especially useful in recursive functions, where we often use the empty
list as a base case.
We’ve seen how to decompose lists with
cdr. The reverse
operation, building up a list, is accomplished with
cons (think “construct”) takes an expression and a list. It
returns a new list whose
car is the result of the expression and
cdr is the list.
(cons 1 '(2 3))
'(2 3) we created the list
'(1 2 3).
consing a value onto an empty list creates a list containing that
(cons "foo" '()) ; => ("foo")
We can chain these
cons calls together to build up a list from
(cons 1 (cons 2 (cons 3 '())))
Under the hood, the
list function is just a series of
This technique is very common when designing recursive functions. We
often recurse through a list using
cdr and/or build up a
There’s also a handy
append function for joining two lists to create
a new one:
(append '(1 2) '(3 4))
This just creates a new list,
(1 2 3 4).
A fun exercise (after you’ve read the next couple sections) might be
implementing your own version of
Like virtually every other language, Emacs Lisp has variables. Emacs
has many variables that are already defined and in use; for example,
path-separator contains a string that separates file paths in the
In my system, evaluating
Trying to evaluate an undefined variable raises an error:
I got the error
Symbol's value as variable is void: some-list. That
just means that the symbol
some-list doesn’t point to a
variable. Let’s fix that.
(set 'some-list '(1 2 3))
We assign variables with
set takes the name of a variable
(quoted, so it’s not evaluated) and a value, and sets the variable to
that value. So, if we try to evaluate
some-list ; => '(1 2 3)
Unsurprisingly, we set variables fairly often. A long time ago, Lisp
programmers decided that having to quote the name of the variable was
a bit of a hassle, so they added
setq as an alias.
implicitly quotes the first variable, so it’s a bit more
convenient. In practice, you’ll rarely see
set; most folks favor
setq, so I’ll be using that exclusively from here on.
(setq my-list '(foo bar baz))
my-list ; => (foo bar baz)
“But Harry,” you might justly complain, “I thought you said that the
arguments to the function would be evaluated before the function was
executed! How does
setq automatically quote that variable name?
It’s just a function, right?”
The answer is that
setq actually isn’t just a function.
an example of a macro. Macros are a bit like functions, but they
also make it possible to manipulate their arguments before they get
executed—in this case, the
setq macro implicitly wraps a
around the first argument. Macros are a big topic that’s way outside
the scope of this tutorial, but I think they’re just awesome, so I’d
highly recommend checking them out when you get a chance. They’re one
of the things that make Lisp really fun to work with.
Now, back to variables.
setq defines a variable globally. Defining all of our variables
globally is shoddy practice, so we also have locally-scoped
variables. These are defined with a
let also does
some delayed-evaluation trickery. Its first argument is a list of
two-element pairs. Each pair contains a variable name and its initial
Every subsequent argument to
let is evaluated after those variables
have been defined.
(let ((a 1) (b 5)) (+ a b))
Evaluating the expression above should yield
a was bound to
b was bound to
5, and we summed them. The return value of a
let expression is the return value of the last expression in it (in
(+ a b)).
To prove to ourselves that
b were only defined in the scope
let expression, let’s try evaluating
Symbol's value as a variable is void: a. So that’s good;
isn’t defined any more.
Sometimes we want to define a few variables in a
let expression, and
have the value of one depend on the value of another.
allow this (you could think of the values in a
let expression as
being bound in parallel), but a variant,
(let* ((a 3) (b (+ a 5))) (+ a b))
This should return
a was bound to
b was bound
Why wouldn’t we always just use
let*? Heck, given that we have
let*, why have
let at all?
There are some purity arguments—the implementation of
let is quite
let*’s is significantly more complex—but that’s not
really a compelling reason unless you’re a language designer.
When possible, you should use
let because it’s
intention-revealing. Other programmers will be able to read through
your code and immediately know that there are no dependencies between
the variable bindings. That makes reading code a bit easier, and other
programmers reading your code (including yourself, in a few months)
will appreciate it.
Okay, so. We can evaluate atoms, call functions, build and decompose lists, and bind variables globally or locally. We’re making some good progress.
Here’s the simplest function I can think of.
(defun say-hello () "hello!")
In this expression,
defun defines a function called
empty list indicates that it takes no arguments. Every subsequent
defun will be executed sequentially when the function is
called, and the return value of the last expression will be the return
value of the function. In this case, the body of the function is
just the string
Sound good? Let’s call this thing.
Sweet, it said
"hello!" Why hello, function! :D
Next, let’s try a function that takes an argument. How about
squareing a number?
(defun square (x) (* x x)) (square 2)
4. Lookin’ fine.
We might wonder, “What happens if we send something non-numeric to our function?” Well,
We get the error
Wrong type argument: number-or-marker-p, "uh-oh".
This is Lisp’s way of saying that some function (
*, in this case)
can only operate on numbers.
Lisp is a dynamic, interpreted language. It doesn’t have a strong static typing system like Haskell (or even Java!), so there’s no way for it to detect problems like this in advance. This actually gives us a lot of power, and it’s often terribly convenient, but it does mean that we can shoot ourselves in the foot sometimes.
(Languages like Ruby and Python often overcome these pitfalls with solid suites of unit tests. Lisp is just as testable as those languages, but it doesn’t have a strong testing culture. I’ll save that rant for another day.)
Let’s look at a more complex example. In 2D space, to determine the distance between two points we use the Euclidean distance metric (usually formulated as “the square root of the sum of the squares”). Suppose we have two points, (x1, y1) and (x2, y2). Let’s implement a function to compute the distance between them:
(defun distance (x1 y1 x2 y2) (sqrt (+ (square (- x2 x1)) (square (- y2 y1)))))
Here we’ve defined a function that takes four arguments and returns
the square root of the sum of the squares. Note that we used our
square function as well as the built-in
(distance 3 0 0 4)
Lo and behold, the distance between (3, 0) and (0, 4) is
We’ll be looking at three types of conditionals, from the simplest to the most general. First, though, we should talk about Boolean expressions.
In Emacs Lisp, every value is truthy except
nil and the empty list
"" and also both truthy. If we want to speak
directly about truth, we use
t, the equivalent of
in other languages. There’s no equivalent false value in Emacs Lisp;
we generally use
In summary: use
t (or anything else, really) for truth, and
The functions for and, or, and not are, reasonably enough,
not. No big shocks here.
or can both
take any number of arguments.
(and t "" 0 7) ; => 7 (or nil "foo" '() "bar") ; => "foo" (not nil) ; => t
Note in the above examples that
or return the first
argument that satisfies them.
or will return the first truthy value
nil, if nothing’s truthy), and
and will return the last
argument, if they’re all truthy (and
nil otherwise). You may be
tempted to use these return values to write very terse code, but I’d
advise you to resist that temptation. Overly clever code will only be
harder for you to understand later.
null, for example—just return
nil. Lisp programmers refer to such functions as predicates.
Predicates are usually (though not always) distinguished by being
suffixed by “
-p”. For example, when we tried to
square a string in
the last section, the interpreter claimed that our string didn’t
number-or-marker-p, which we can now deduce was a predicate
that tested the input of the
Now that we can write our own predicates, we can get to conditionals.
If we want to take an action only if a predicate is true, and
otherwise do nothing, we can use a
(when (= (+ 2 2) 4) "passed sanity check!")
2 + 2 = 4 and the world is sane.
Personally, I don’t use
when all that much, but it shows up
occasionally in my
.emacs configuration. For example, there are
certain things I only want to do if Emacs is running as a stand-alone
application (not in a terminal), and I find that
when is a nice fit
if evaluates a predicate, then evaluates one
expression if it was true, and the other if it was false. Let’s wrap
if in a function to try it out.
(defun evens-or-odds (n) (if (= 0 (% n 2)) "even!" "odd!")) (evens-or-odds 4) (evens-or-odds 3)
We’re probably unsurprised to learn that
if is great, but it’d be nice if we had an extended
if/elseif/…/elseif/else construct like we do in other
languages. And, of course, we do!
cond is a generalization of
that can match an arbitrary number of cases. It takes a collection of
lists, each of which starts with a Boolean expression. It runs through
each such expression searching for one that matches, then evaluates
the remaining elements in that list and returns the result. This
makes more sense as an example:
(defun pick-a-word (n) (cond ((= n 1) "bulbous") ((= n 2) "bouffant") ((= n 3) "beluga") (t "gazebo!"))) (pick-a-word 2) ; => "bouffant" (pick-a-word -72) ; => "gazebo!"
Notice that our last condition is just
t is always
truthy, we can use it to indicate an else or otherwise case. If
none of the other expressions are truthy,
t definitely will be.
Recursion is the process of calling a function that calls itself. It’s pretty trippy, but it’s extraordinarily useful and not too difficult to use once you get the hang of it.
A recursive function generally contains a conditional. One branch of the conditional involves somehow simplifying the problem – perhaps taking an element off a list or decrementing a number – then calling the function itself with that simplified quantity and using the results. The other branch is the base case, which kicks in when the thing being investigated is at its simplest – perhaps the list is empty, or the number had been reduced to 0.
I’m not going to go into any great depth with recursion, because it’s a technique with some complex implications. However, I think you should see a simple recursive function in Lisp. An implementation of the factorial function is the canonical example; I don’t dare buck tradition by demonstrating anything else.
(defun factorial (n) (if (< n 1) 1 (* n (factorial (- n 1))))) (factorial 5) ; => 120
In this case,
factorial calls itself repeatedly until the base case
kicks in when
n hits 0. The stack of function calls unwinds,
eventually returning the product
(* 5 (* 4 (* 3 (* 2 (* 1 1))))),
which evaluates to
"foo" is a string literal.
42 is an integer literal. In the same
way, anonymous functions are function literals.
Variables point to literal values. When we
(setq foo "bar"), we’re
pointing the symbol
foo to the literal value
"bar". Likewise, when
(defun baz () "quuz"), we’re pointing the symbol
baz to a
function. Without referencing
baz, how could we refer to the
baz points to?
Lisp uses a macro called
lambda to create function literals. The
name is a reference to the lambda calculus, which you emphatically
do not have to understand to wield Lisp perfectly effectively.
A note on vocabulary: most programmers use the terms function literal, anonymous function, and lambda or lambda function more or less interchangeably. Unless you’re a researcher working in programming language theory they mean essentially the same thing.
Okay, let’s define us a lambda:
(lambda (x) (* x x x))
Painless! This is a function that takes an argument and cubes it. Simple enough!
However, we notice that it didn’t really do anything. To execute a lambda, just use it like you’d use a function:
((lambda (x) (* x x x)) 5)
125. We created a lambda which was immediately evaluated and
We can also call a lambda with
(funcall (lambda (x) (* x x x)) 3) ; => 27
funcall takes a function and (optionally) some arguments, and calls
the function on those arguments. The function doesn’t even have to be
(funcall '+ 1 2) ; => 3
This is handy when the function’s stored in a variable; we’ll see that show up a little later.
So here’s a cool trick. Since lambdas are just function literals, and
since we can bind variable names to literals, we can replicate the
defun on our own!
(fset 'cube (lambda (x) (* x x x))) (cube 4)
64! Pretty awesome.
The first time I tried to do this I did something like
...). That doesn’t work. Emacs Lisp keeps variables and functions in
different namespaces (Emacs Lisp is a Lisp-2), so we have to use
And no, there’s isn’t a
As we’ve seen, we can pretty much treat functions as just another kind of variable. There’s no reason that we can’t pass them in as arguments to other functions. Functions that take other functions as arguments are commonly called higher-order functions, but otherwise there’s nothing special about them. We can easily write our own.
(defun transform-unless-zero (fn n) (if (= n 0) 0 (funcall fn n)))
0, we just return
0, but otherwise we apply the
n and return the result. Cool?
(transform-unless-zero (lambda (n) (+ 1 n)) 0) ; => 0 (transform-unless-zero (lambda (n) (+ 1 n)) 7) ; => 8
Higher-order functions aren’t terribly scary, and they’re really useful.
In fact, they’re so useful that they show up all over the place in
Lisp. One of the most common examples is
mapcar. It takes a function
and a list, applies the function to each element in the list, and
returns a new list of results.
(mapcar (lambda (n) (+ 1 n)) '(1 2 3 4)) ; => (2 3 4 5)
Pretty cool! Note that
mapcar doesn’t change the original list. If
it’d been stored in a variable, that variable’s value wouldn’t have
changed. So, for example:
(setq our-list '(1 2 3 4)) (mapcar (lambda (n) (+ 1 n)) '(1 2 3 4)) ; => (2 3 4 5) our-list ; => (1 2 3 4)
If the mapping had changed the variable, we’d have called it a destructive operation. We like to avoid destructive operations in Lisp (and many other languages), because they introduce side-effects that can make our code harder to reason about. (Incidentally, a function that operates deterministically with no side-effects is called referentially transparent. Impress your friends!)
We can also refer to functions by name, of course.
(mapcar 'upcase '("foo" "bar" "baz")) ; => ("FOO" "BAR" "BAZ")
It’s worth noting that applying
mapcar to an empty list will always
just return an empty list.
(mapcar 'upcase '()) ; => ()
remove-if-not is another handy higher-order function. It takes a
predicate and a list. It returns a new list that only contains those
items that satisfy the predicate.
(remove-if-not 'oddp '(0 1 2 3 4 5 6 7 8 9)) ; => (1 3 5 7 9)
It’s also not destructive, and it maintains the order of the items in the list.
Incidentally, if you find the name
remove-if-not a bit confusing
because it contains a double negative, you’re not the only one. I
still have to consciously think it through sometimes. Other languages
often refer to this function as
of which seem a bit clearer to me.
Okay, we’re in the home stretch. We’ve covered a lot of ground, so let’s pull it all together into a big, fancy example. We’re going to implement quicksort, a very clever recursive algorithm for efficiently sorting a list. Here’s how the algorithm works:
And here’s how we’d implement it in Emacs Lisp:
(defun qs (items) (if (null items) '() (let* ((pivot (car items)) (rest (cdr items)) (lesser (remove-if-not (lambda (x) (<= x pivot)) rest)) (greater (remove-if-not (lambda (x) (> x pivot)) rest))) (append (qs lesser) (list pivot) (qs greater)))))
This might look a little intimidating, but there’s actually nothing
new here. We’re defining a function called
qs that takes the list
items is empty, it returns an empty list, which is the
base case. Otherwise it defines a pivot, uses
greater lists, recursively calls
them, and joins the results together. Not really that bad, is it?
(qs '(3 5 7 8 4 2 5 7 0 8 4 6)) ; => (0 2 3 4 4 5 5 6 7 7 8 8)
Will ya look at that! It even works!
In real life, of course, we’d probably use the built-in
function. There’s no reason to build our own, and the built-in one is
almost certainly more efficient.
We just went from atoms to quicksort in under five thousand words. Not too shabby!
If you’d like to keep going with Emacs Lisp, there are a few resources I’d like to recommend:
I’ll also be writing a couple short blog posts in the next couple weeks about setting keybindings and using the built-in documentation. Keep an eye out! =)