What makes Lisp macros so special?

Question

Reading Paul Graham's essays on programming languages one would think that Lisp macros are the only way to go. As a busy developer, working on other platforms, I have not had the privilege of using Lisp macros. As someone who wants to understand the buzz, please explain what makes this feature so powerful.

Please also relate this to something I would understand from the worlds of Python, Java, C# or C development.

Durban

By the way, there is a LISP-style macro processor for C# called LeMP: ecsharp.net/lemp ... JavaScript also has one called Sweet.js: sweetjs.org – Olcott

@Olcott Does sweetjs even work these days? – Deaf

I haven't used it but the most recent commit was six months ago... good enough for me! – Olcott

Answer 1

To give the short answer, macros are used for defining language syntax extensions to Common Lisp or Domain Specific Languages (DSLs). These languages are embedded right into the existing Lisp code. Now, the DSLs can have syntax similar to Lisp (like Peter Norvig's Prolog Interpreter for Common Lisp) or completely different (e.g. Infix Notation Math for Clojure).

Here is a more concrete example:
Python has list comprehensions built into the language. This gives a simple syntax for a common case. The line

divisibleByTwo = [x for x in range(10) if x % 2 == 0]

yields a list containing all even numbers between 0 and 9. Back in the Python 1.5 days there was no such syntax; you'd use something more like this:

divisibleByTwo = []
for x in range( 10 ):
   if x % 2 == 0:
      divisibleByTwo.append( x )

These are both functionally equivalent. Let's invoke our suspension of disbelief and pretend Lisp has a very limited loop macro that just does iteration and no easy way to do the equivalent of list comprehensions.

In Lisp you could write the following. I should note this contrived example is picked to be identical to the Python code not a good example of Lisp code.

;; the following two functions just make equivalent of Python's range function
;; you can safely ignore them unless you are running this code
(defun range-helper (x)
  (if (= x 0)
      (list x)
      (cons x (range-helper (- x 1)))))

(defun range (x)
  (reverse (range-helper (- x 1))))

;; equivalent to the python example:
;; define a variable
(defvar divisibleByTwo nil)

;; loop from 0 upto and including 9
(loop for x in (range 10)
   ;; test for divisibility by two
   if (= (mod x 2) 0) 
   ;; append to the list
   do (setq divisibleByTwo (append divisibleByTwo (list x))))

Before I go further, I should better explain what a macro is. It is a transformation performed on code by code. That is, a piece of code, read by the interpreter (or compiler), which takes in code as an argument, manipulates and the returns the result, which is then run in-place.

Of course that's a lot of typing and programmers are lazy. So we could define DSL for doing list comprehensions. In fact, we're using one macro already (the loop macro).

Lisp defines a couple of special syntax forms. The quote (') indicates the next token is a literal. The quasiquote or backtick (`) indicates the next token is a literal with escapes. Escapes are indicated by the comma operator. The literal '(1 2 3) is the equivalent of Python's [1, 2, 3]. You can assign it to another variable or use it in place. You can think of `(1 2 ,x) as the equivalent of Python's [1, 2, x] where x is a variable previously defined. This list notation is part of the magic that goes into macros. The second part is the Lisp reader which intelligently substitutes macros for code but that is best illustrated below:

So we can define a macro called lcomp (short for list comprehension). Its syntax will be exactly like the python that we used in the example [x for x in range(10) if x % 2 == 0] - (lcomp x for x in (range 10) if (= (% x 2) 0))

(defmacro lcomp (expression for var in list conditional conditional-test)
  ;; create a unique variable name for the result
  (let ((result (gensym)))
    ;; the arguments are really code so we can substitute them 
    ;; store nil in the unique variable name generated above
    `(let ((,result nil))
       ;; var is a variable name
       ;; list is the list literal we are suppose to iterate over
       (loop for ,var in ,list
            ;; conditional is if or unless
            ;; conditional-test is (= (mod x 2) 0) in our examples
            ,conditional ,conditional-test
            ;; and this is the action from the earlier lisp example
            ;; result = result + [x] in python
            do (setq ,result (append ,result (list ,expression))))
           ;; return the result 
       ,result)))

Now we can execute at the command line:

CL-USER> (lcomp x for x in (range 10) if (= (mod x 2) 0))
(0 2 4 6 8)

Pretty neat, huh? Now it doesn't stop there. You have a mechanism, or a paintbrush, if you like. You can have any syntax you could possibly want. Like Python or C#'s with syntax. Or .NET's LINQ syntax. In end, this is what attracts people to Lisp - ultimate flexibility.

Unload

+1 for implementing list comprehension in Lisp, because why not? – Nebraska

@Nebraska Actually LISP already has a list comprehension macro in the standard library : (loop for x from 0 below 10 when (evenp x) collect x), more examples here. But indeed, loop is "just a macro" (I actually re-implemented it from scratch some time ago) – Anabelle

I know it's pretty unrelated, but I'm wondering about syntax and how the parsing actually works... Say I call lcomp that way ( changing the thirs item from "for" to "azertyuiop" ) : (lcomp x azertyuiop x in (range 10) if (= (% x 2) 0)) will the macro still work as expected ? Or is the "for" parameter used in the loop so that it must be the string "for" when called ? – Synthesize

@Synthesize So, loop is actually another macro and for is a special symbol used in that macro. I could have easily defined the macro without the loop macro. However, if I had, the post would've been much longer. The loop macro and all its syntax is defined in CLtL. – Unload

This is very beautiful and works in Emacs Lisp also (only lisp I have on my machine). But to answer the question @Synthesize is asking, the value or expression passed to for in the macro is not evaluated. In the call to loop, the for is its literal value; otherwise would have been written as ,for. Same with in. – Quickly

Ok I think I got your point, for is a symbol, and some code inside the macro will check that the third symbol is for. – Synthesize

One thing I'm confused about with other language's macros, is that their macros are limited by the host language's syntax. Can Lispy macros interpret non-Lispy syntax. I mean like imagine creating a haskell like syntax (no paranthese) and interpreting it using Lisp macros. Is this possible, and what are the pros/cons of using macros compared to using a lexer and parser directly? – Rawlinson

@Rawlinson Simple answer, yes, lisp macros are often used to create Domain Specific Languages. The host language invariably imposes some limits to the syntax you can use in a macro. For example, you obviously cannot use a comment syntax as an active component within your macro. – Unload

@Synthesize I just tested it using elisp, you can change "for" freely and it works without a hitch: (lcomp x hey x in (range 50) if (= (mod x 2) 0)) – Regin

Ohhhh thank you for your test ! So indeed, it's just used as a symbol and a placeholder. I mean to improve clarity. – Synthesize

DSL appears to have a specific meaning for Java programmers, concise types, without custom language syntax. At least that's what I gathered from reading Bettini's Xtext and Xtend manual. Maybe better to say new language to prevent it from being confused with whatever DSL is meant to be. – Byplay

Maybe it is nitpicking, but can you actually exactly get the sytax of the python example? like "[x for x in range(10) if x % 2 == 0]" and not "(lcomp x for x in (range 10) if (= (mod x 2) 0))" – Logogram

@Logogram That stuff between square brackets wouldn't be understood properly as an s-expression, but you could do something like (defun expand-list-comp (str) ...) (eval (read-from-string (expand-list-comp (read-line)))) to process before the input even gets to the reader and macros get expanded. But that's a lot like doing what Vatine says about preprocessing using C instead of lisp. – Donn

Answer 2

You will find a comprehensive debate around lisp macro here.

An interesting subset of that article:

In most programming languages, syntax is complex. Macros have to take apart program syntax, analyze it, and reassemble it. They do not have access to the program's parser, so they have to depend on heuristics and best-guesses. Sometimes their cut-rate analysis is wrong, and then they break.

But Lisp is different. Lisp macros do have access to the parser, and it is a really simple parser. A Lisp macro is not handed a string, but a preparsed piece of source code in the form of a list, because the source of a Lisp program is not a string; it is a list. And Lisp programs are really good at taking apart lists and putting them back together. They do this reliably, every day.

Here is an extended example. Lisp has a macro, called "setf", that performs assignment. The simplest form of setf is

  (setf x whatever)

which sets the value of the symbol "x" to the value of the expression "whatever".

Lisp also has lists; you can use the "car" and "cdr" functions to get the first element of a list or the rest of the list, respectively.

Now what if you want to replace the first element of a list with a new value? There is a standard function for doing that, and incredibly, its name is even worse than "car". It is "rplaca". But you do not have to remember "rplaca", because you can write

  (setf (car somelist) whatever)

to set the car of somelist.

What is really happening here is that "setf" is a macro. At compile time, it examines its arguments, and it sees that the first one has the form (car SOMETHING). It says to itself "Oh, the programmer is trying to set the car of somthing. The function to use for that is 'rplaca'." And it quietly rewrites the code in place to:

  (rplaca somelist whatever)

Businesslike

setf is a nice illustration of the power of macros, thanks for including it. – Fend

I like the highlight ..because the source of a Lisp program is not a string; it is a list.! Is it the main reason why LISP macro is superior than most of others' due to its parentheses? – Recluse

@Recluse I suppose so: books.google.fr/… suggests you are right. – Businesslike

Answer 3

Common Lisp macros essentially extend the "syntactic primitives" of your code.

For example, in C, the switch/case construct only works with integral types and if you want to use it for floats or strings, you are left with nested if statements and explicit comparisons. There's also no way you can write a C macro to do the job for you.

But, since a lisp macro is (essentially) a lisp program that takes snippets of code as input and returns code to replace the "invocation" of the macro, you can extend your "primitives" repertoire as far as you want, usually ending up with a more readable program.

To do the same in C, you would have to write a custom pre-processor that eats your initial (not-quite-C) source and spits out something that a C compiler can understand. It's not a wrong way to go about it, but it's not necessarily the easiest.

Rayfordrayle

+! for the paragraph starting "But, since a lisp macro is..." as that clarified the whole subject more than just about anything else! – Ernestineernesto

Answer 4

Lisp macros allow you to decide when (if at all) any part or expression will be evaluated. To put a simple example, think of C's:

expr1 && expr2 && expr3 ...

What this says is: Evaluate expr1, and, should it be true, evaluate expr2, etc.

Now try to make this && into a function... thats right, you can't. Calling something like:

and(expr1, expr2, expr3)

Will evaluate all three exprs before yielding an answer regardless of whether expr1 was false!

With lisp macros you can code something like:

(defmacro && (expr1 &rest exprs)
    `(if ,expr1                     ;` Warning: I have not tested
         (&& ,@exprs)               ;   this and might be wrong!
         nil))

now you have an &&, which you can call just like a function and it won't evaluate any forms you pass to it unless they are all true.

To see how this is useful, contrast:

(&& (very-cheap-operation)
    (very-expensive-operation)
    (operation-with-serious-side-effects))

and:

and(very_cheap_operation(),
    very_expensive_operation(),
    operation_with_serious_side_effects());

Other things you can do with macros are creating new keywords and/or mini-languages (check out the (loop ...) macro for an example), integrating other languages into lisp, for example, you could write a macro that lets you say something like:

(setvar *rows* (sql select count(*)
                      from some-table
                     where column1 = "Yes"
                       and column2 like "some%string%")

And thats not even getting into Reader macros.

Hope this helps.

Piercing

I think it should be: "(apply && ,@exprs) ; this and might be wrong!" – Parvenu

@svante - on two counts: first, && is a macro, not a function; apply only works on functions. second, apply take a list of arguments to pass, so you want one of "(funcall fn ,@exprs)", "(apply fn (list ,@exprs)" or "(apply fn ,@exprs nil)", not "(apply fn ,@exprs)". – Tung

(and ... will evaluate expressions until one evaluates to false, note that side-effects generated by the false evaluation will take place, only the subsequent expressions will be skipped. – Editorial

Answer 5

I don't think I've ever seen Lisp macros explained better than by this fellow: http://www.defmacro.org/ramblings/lisp.html

Kreindler

Especially if you have a Java/XML background. – Perinephrium

What a joy to read this lying on my couch on a Saturday afternoon! Very clearly written and organized. – Apparitor

God bless you and the author. – Siddur

It's a long article but well worth reading - a lot of it is pre-amble which can be boiled down to - 1) Lisp S-expressions can represent code or data just like XML can, and 2) macros do not eagerly evaluate their inputs like functions do, and so can manipulate inputs as s-expression structures of code or data. The mind-blown moment is when something as mundane as a "todo list" representation can be weaponized as code simply by implementing a macro that can treat the todo data structure as code inputs into an item macro. That is not something you'd consider in most languages and is cool. – Avigation

Answer 6

Since the existing answers give good concrete examples explaining what macros achieve and how, perhaps it'd help to collect together some of the thoughts on why the macro facility is a significant gain in relation to other languages; first from these answers, then a great one from elsewhere:

... in C, you would have to write a custom pre-processor [which would probably qualify as a sufficiently complicated C program] ...

—Vatine

Talk to anyone that's mastered C++ and ask them how long they spent learning all the template fudgery they need to do template metaprogramming [which is still not as powerful].

—Matt Curtis

... in Java you have to hack your way with bytecode weaving, although some frameworks like AspectJ allows you to do this using a different approach, it's fundamentally a hack.

—Miguel Ping

DOLIST is similar to Perl's foreach or Python's for. Java added a similar kind of loop construct with the "enhanced" for loop in Java 1.5, as part of JSR-201. Notice what a difference macros make. A Lisp programmer who notices a common pattern in their code can write a macro to give themselves a source-level abstraction of that pattern. A Java programmer who notices the same pattern has to convince Sun that this particular abstraction is worth adding to the language. Then Sun has to publish a JSR and convene an industry-wide "expert group" to hash everything out. That process--according to Sun--takes an average of 18 months. After that, the compiler writers all have to go upgrade their compilers to support the new feature. And even once the Java programmer's favorite compiler supports the new version of Java, they probably ''still'' can't use the new feature until they're allowed to break source compatibility with older versions of Java. So an annoyance that Common Lisp programmers can resolve for themselves within five minutes plagues Java programmers for years.

—Peter Seibel, in "Practical Common Lisp"

Coles

Answer 7

A lisp macro takes a program fragment as input. This program fragment is represented a data structure which can be manipulated and transformed any way you like. In the end the macro outputs another program fragment, and this fragment is what is executed at runtime.

C# does not have a macro facility, however an equivalent would be if the compiler parsed the code into a CodeDOM-tree, and passed that to a method, which transformed this into another CodeDOM, which is then compiled into IL.

This could be used to implement "sugar" syntax like the for each-statement using-clause, linq select-expressions and so on, as macros that transforms into the underlying code.

If Java had macros, you could implement Linq syntax in Java, without needing Sun to change the base language.

Here is pseudo-code for how a lisp-style macro in C# for implementing using could look:

define macro "using":
    using ($type $varname = $expression) $block
into:
    $type $varname;
    try {
       $varname = $expression;
       $block;
    } finally {
       $varname.Dispose();
    }

Hexastyle

Now that there actually is a Lisp-style macro processor for C#, I would point out that a macro for using would look like this ;) – Olcott

Answer 8

Think of what you can do in C or C++ with macros and templates. They're very useful tools for managing repetitive code, but they're limited in quite severe ways.

• Limited macro/template syntax restricts their use. For example, you can't write a template which expands to something other than a class or a function. Macros and templates can't easily maintain internal data.
• The complex, very irregular syntax of C and C++ makes it difficult to write very general macros.

Lisp and Lisp macros solve these problems.

• Lisp macros are written in Lisp. You have the full power of Lisp to write the macro.
• Lisp has a very regular syntax.

Talk to anyone that's mastered C++ and ask them how long they spent learning all the template fudgery they need to do template metaprogramming. Or all the crazy tricks in (excellent) books like Modern C++ Design, which are still tough to debug and (in practice) non-portable between real-world compilers even though the language has been standardised for a decade. All of that melts away if the langauge you use for metaprogramming is the same language you use for programming!

Venola

Well, to be fair, the problem with C++ template metaprogramming is not that the metaprogramming language is different, but that it's horrid -- it was not so much designed as discovered in what was intended to be a much-simpler template functionality. – Charlatanry

@Brooks Sure. Emergent features are not always bad. Unfortunately, in a slow moving committee driven language, it's difficult to fix them when they are. It's a shame so much of C++'s modern useful new features are written in a language few can even read, and there is a huge gap between the average programmer and the "high priest". – Venola

@downvoter: if there's something wrong with my answer, please leave a comment so we can all share the knowledge. – Venola

Answer 9

I'm not sure I can add some insight to everyone's (excellent) posts, but...

Lisp macros work great because of the Lisp syntax nature.

Lisp is an extremely regular language (think of everything is a list); macros enables you to treat data and code as the same (no string parsing or other hacks are needed to modify lisp expressions). You combine these two features and you have a very clean way to modify code.

Edit: What I was trying to say is that Lisp is homoiconic, which means that the data structure for a lisp program is written in lisp itself.

So, you end up with a way of creating your own code generator on top of the language using the language itself with all its power (eg. in Java you have to hack your way with bytecode weaving, although some frameworks like AspectJ allows you to do this using a different approach, it's fundamentally a hack).

In practice, with macros you end up building your own mini-language on top of lisp, without the need to learn additional languages or tooling, and with using the full power of the language itself.

Lavatory

It's insightful comment, however, this idea that "everything is a list" may scare newcomers. to understand a list, you need to understand cons, cars, cdrs, cells. More precisely, Lisp is made of S-expressions, not lists. – Dorita

Answer 10

Lisp macros represents a pattern that occurs in almost any sizeable programming project. Eventually in a large program you have a certain section of code where you realize it would be simpler and less error prone for you to write a program that outputs source code as text which you can then just paste in.

In Python objects have two methods __repr__ and __str__. __str__ is simply the human readable representation. __repr__ returns a representation that is valid Python code, which is to say, something that can be entered into the interpreter as valid Python. This way you can create little snippets of Python that generate valid code that can be pasted into your actually source.

In Lisp this whole process has been formalized by the macro system. Sure it enables you to create extensions to the syntax and do all sorts of fancy things, but it's actual usefulness is summed up by the above. Of course it helps that the Lisp macro system allows you to manipulate these "snippets" with the full power of the entire language.

Lento

Your first paragraph is very clear to a Lisp outsider, which is important. – Ladysmith

You leave out too much. The whole aspect of pasting in code in your first paragraph is actually part of the problem. Everyone is actually forking code! Now as you want to fix or enhance that code that you've pasted all over, now you have to go and personally maintain innumerable copies, that keep forking themselves because that's now your standard practice. Lisp Macros stay clean, and you can fix them once for all, which is harder at first, but gets much easier. With copy & paste, it's easy up front, but keeps getting harder until the code collapses under its own complexity weight. – Mikelmikell

Also, don't underestimate what the simplicity get you. Lisp doesn't really have much syntax, and that help it stay out of your way when you're using macros to auto-generate code -- you don't have to jump through curly-brace hoops (C/C++/C#/Java) or indent things perfectly (Python). That's really hard to make sure that every line ends with a semicolon, when you have multiple layers of generation going on, and it gets easier with a lighter syntax load (it's not so taxing). – Mikelmikell

Also, Homoiconicity is a big win -- the idea that everything is the same shape is a benefit. The template language of C and C++ end up being a completely separate language, and can look very different and very cryptic. And don't even attempt multiple levels of code generation that way. It's not that template macros are not powerful -- they have their place -- but they feel like they've been bolted-on, an after-thought, not well-integrated, cumbersome, now a necessary evil going forward. Once your Lisp macros are in place, everything gets easier because you have a new atomic language to use. – Mikelmikell

Answer 11

While the above all explains what macros are and even have cool examples, I think the key difference between a macro and a normal function is that LISP evaluates all the parameters first before calling the function. With a macro it's the reverse, LISP passes the parameters unevaluated to the macro. For example, if you pass (+ 1 2) to a function, the function will receive the value 3. If you pass this to a macro, it will receive a List( + 1 2). This can be used to do all kinds of incredibly useful stuff.

• Adding a new control structure, e.g. loop or the deconstruction of a list

• Measure the time it takes to execute a function passed in. With a function the parameter would be evaluated before control is passed to the function. With the macro, you can splice your code between the start and stop of your stopwatch. The below has the exact same code in a macro and a function and the output is very different. Note: This is a contrived example and the implementation was chosen so that it is identical to better highlight the difference.

  (defmacro working-timer (b) 
    (let (
          (start (get-universal-time))
          (result (eval b))) ;; not splicing here to keep stuff simple
      ((- (get-universal-time) start))))
  
  (defun my-broken-timer (b)
    (let (
          (start (get-universal-time))
          (result (eval b)))    ;; doesn't even need eval
      ((- (get-universal-time) start))))
  
  (working-timer (sleep 10)) => 10
  
  (broken-timer (sleep 10)) => 0
  

Selectee

Btw, Scala has added macros to the language. While they lack the beauty of the Lisp macros because the language isn't homoiconic they are definitely worth looking into it and the abstract syntax trees they provider might be easier to use in the end. It's too early for me to say which macro system I prefer. – Selectee

"LISP passes the parameters unevaluated to the macro" finally an answer that says it plainly. but you forgot the second half of that: and the result of a macro is a transformed code that will be evaluated whole by the system in place of the original as if it were there in the first place (unless it is itself again a call to a macro, which will also get transformed, by that macro this time). – Pule

Answer 12

In short, macros are transformations of code. They allow to introduce many new syntax constructs. E.g., consider LINQ in C#. In lisp, there are similar language extensions that are implemented by macros (e.g., built-in loop construct, iterate). Macros significantly decrease code duplication. Macros allow embedding «little languages» (e.g., where in c#/java one would use xml to configure, in lisp the same thing can be achieved with macros). Macros may hide difficulties of using libraries usage.

E.g., in lisp you can write

(iter (for (id name) in-clsql-query "select id, name from users" on-database *users-database*)
      (format t "User with ID of ~A has name ~A.~%" id name))

and this hides all the database stuff (transactions, proper connection closing, fetching data, etc.) whereas in C# this requires creating SqlConnections, SqlCommands, adding SqlParameters to SqlCommands, looping on SqlDataReaders, properly closing them.

Frederic

Answer 13

One-liner answer:

Minimal syntax => Macros over Expressions => Conciseness => Abstraction => Power

---

Lisp macros do nothing more than writing codes programmatically. That is, after expanding the macros, you got nothing more than Lisp code without macros. So, in principle, they achieve nothing new.

However, they differ from macros in other programming languages in that they write codes on the level of expressions, whereas others' macros write codes on the level of strings. This is unique to lisp thanks to their parenthesis; or put more precisely, their minimal syntax which is possible thanks to their parentheses.

As shown in many examples in this thread, and also Paul Graham's On Lisp, lisp macros can then be a tool to make your code much more concise. When conciseness reaches a point, it offers new levels of abstractions for codes to be much cleaner. Going back to the first point again, in principle they do not offer anything new, but that's like saying since paper and pencils (almost) form a Turing machine, we do not need an actual computer.

If one knows some math, think about why functors and natural transformations are useful ideas. In principle, they do not offer anything new. However by expanding what they are into lower-level math you'll see that a combination of a few simple ideas (in terms of category theory) could take 10 pages to be written down. Which one do you prefer?

Recluse

Answer 14

I got this from the common lisp cookbook and I think it explained why lisp macros are useful.

"A macro is an ordinary piece of Lisp code that operates on another piece of putative Lisp code, translating it into (a version closer to) executable Lisp. That may sound a bit complicated, so let's give a simple example. Suppose you want a version of setq that sets two variables to the same value. So if you write

(setq2 x y (+ z 3))

when z=8 both x and y are set to 11. (I can't think of any use for this, but it's just an example.)

It should be obvious that we can't define setq2 as a function. If x=50 and y=-5, this function would receive the values 50, -5, and 11; it would have no knowledge of what variables were supposed to be set. What we really want to say is, When you (the Lisp system) see (setq2 v1 v2 e), treat it as equivalent to (progn (setq v1 e) (setq v2 e)). Actually, this isn't quite right, but it will do for now. A macro allows us to do precisely this, by specifying a program for transforming the input pattern (setq2 v1 v2 e)" into the output pattern (progn ...)."

If you thought this was nice you can keep on reading here: http://cl-cookbook.sourceforge.net/macros.html

Barnebas

It is possible to define setq2 as a function if x and y are passed by reference. I don't know if it is possible in CL, however. So for someone who don't know Lisps or CL in particular this is not very illustrative example IMO – Duckling

@Duckling CL argument passing is by value only (that's why it needs macros in the first place). some values are pointers though (like lists). – Pule

Answer 15

In python you have decorators, you basically have a function that takes another function as input. You can do what ever you want: call the function, do something else, wrap the function call in a resource acquire release, etc. but you don't get to peek inside that function. Say we wanted to make it more powerful, say your decorator received the code of the function as a list then you could not only execute the function as is but you can now execute parts of it, reorder lines of the function etc.

Invisible