Why macros?

:: Racket, Clojure, Hacker School

Yesterday a couple people asked me, “How and why do you use macros in a Lisp like Racket or Clojure?”.

I gave answers like:

  • The compiler can do a search-and-replace on your code.

  • You can make DSLs.

  • They’re an “API for the compiler”.

Although all true, I wasn’t sure I was getting the full idea across.

Worse, one time Peter Seibel was within earshot. Although I don’t know if he heard my explanation, I imagined him biting his tongue and politely remembering the “well, actually” rule. :)

Later I remembered Matthias Felleisen boiling down macros into three main categories:

  1. Binding forms. You can make your own syntax for binding values to identifiers, including function definition forms. You may hear people say, in a Lisp you don’t have to wait for the language designers to add a feature (like lambda for Java?). Using macros you can add it yourself. Binding forms is one example.

  2. Changing order of evaluation. Something like or or if can’t really be a function, because you want it to “short-circuit” — if the first test evaluates to true, don’t evaluate the other test at all.

  3. Abstractions like domain specific langagues (DSLs). You want to provide a special language, which is simpler and/or more task-specific than the full/raw Lisp you’re using. This DSL might be for users of your software, and/or it might be something that you use to help implement parts of your own program.

Every macro is doing one of those three things. Only macros can really do the first two, at all1. Macros let you do the last one more elegantly.

I think the preceding is a better answer. However, maybe it’s still not the best way to get people from zero to sixty on, “Why macros?”.2

Maybe the ideal is a “teachable moment” — facing a problem that macrology would solve.3 That’s also good because you really really really don’t want to use a macro when a normal function would suffice. So the goal isn’t to get people so enthusiastic about macros that they go forth in search of nails to which to apply that new hammer. Macros often aren’t the right approach. But once in a while, they are the bestest approach ever.

  1. A language like Haskell can choose lazy evaluation, and implement if as a function. I’m saying that only a macro can futz with whatever the default evaluation order is, be it eager or lazy. 

  2. Although I wrote a guide called Fear of Macros, it’s (a) specific to Racket macros and (b) much more about the “how” than the “why”. 

  3. Certainly that’s my own optimal learning situation, as opposed to getting answers or solutions before I have the questions or problems.