A few tips for writing macros in Clojure

http://blog.altometrics.com/2016/04/a-few-tips-for-writing-macros-in-clojure/

by dwighttk • 10 years ago

49 43 10 years ago

blog.altometrics.com

jwr• 10 years ago

Excellent tips. I'd advise most inexperienced programmers (say under 10 years of lisp) to stick to tip #1. You very rarely need a macro in Clojure.

cageface• 10 years ago

If it's true that you rarely need macros then isn't the awkwardness of s-expr syntax a very high price to pay just to have access to them?

aaron-santos2• 10 years ago

Sure. The syntax is distracting at first, but in my experience after a few weeks I saw past it. It's a non-issue for me at this point and I dearly miss it when writing Scala on another project.

jdminhbg• 10 years ago

I'd say it's more true that you rarely need to write a macro than it is that you rarely need to use them. Lots of very useful things across the language, from builtins like the threading macros (->, ->>) to libaries like Compojure, are built on macros.

bkirkbri• 10 years ago

I've found s-expressions to be one of my favorite things about Clojure. I understand that many people dislike them, and that many people get used to them, but I loved the syntax from the get go.

reitzensteinm• 10 years ago

I have a pet theory that someone will put a Python skin on Clojure and have significant success with it.

andrewingram3• 10 years ago

This is interesting, but also a little discouraging. Coming from a background of Python and JavaScript, and using a little bit of Elixir (which also supports macros) lately, the power of macros for defining DSLs is the main draw for me.

As an example, I've been using GraphQL quite heavily for the last 8 months, primarily with JavaScript and i've found it to be a fairly tedious experience. I tried out an Elixir GraphQL library that's heavily based on macros and was almost immediately more productive - despite not even knowing the language.

Similarly, coming up with a nice API for working with forms seems to be pretty much impossible with JavaScript. I've evaluated dozens, and not one comes close to the simplicity of Django's forms (which is only possible thanks to Python's metaclass magic).

Having sampled the power of languages with macros, I'm naturally drawn to Clojure, simply because it has a very decent JavaScript implementation. I'm sure there are other very good reasons for using the language, but right now macros are the motivation.

I'd like to understand why one would need 10 years of lisp experience before starting to author their own macros, it feels like a philosophy that's just going to stop people trying out lisp.

asimuvPR3• 10 years ago

It's OK to be discouraging to a certain degree. It helps stop people from cooking up "clever" solutions.

bendyorke• 10 years ago

The article says that you shouldn't write them unless you need to, not that you shouldn't write them. He lists quite a few scenarios where they should be used, one of which is DSLs. :)

Also, given that Clojure is a lisp, I think you'll find you need macros left often than a language like elixir. They're still incredibly powerful, but you can do so much in Clojure without them that you would otherwise need a macro to do.

jcadam• 10 years ago

I've recently (about two weeks ago) picked up Clojure for use on a side project after getting frustrated with trying to use Haskell for something non-trivial -- not smart enough for Haskell, apparently :D

Haven't messed with a LISP language since college but clojure has grown on me considerably. I already like it more than Scala... though I did almost immediately find myself reaching for a monad library (cats seems good so far) :) Haven't felt the need to write my own macros yet, but certainly many of the libraries I'm using (compojure) make heavy use of them.

Too bad my day job is all Java, all the time :( My continual requests to use Scala have been shot down over the last couple of years -- "We want something maintainable, where am I going to find Scala devs when you leave?" So obviously Clojure isn't going to fly :)

spion2• 10 years ago

That seems off by about 1 order of magnitude...

AimHere• 10 years ago

Lisp is getting on a bit, but even the greyest of Lisp hackers haven't quite racked up a hundred years of Lisp...

nonrecursive• 10 years ago

I'm going to be contrary and suggest that, when you're first learning a Lisp, write a macro every time you think "hey this might be a good place to write a macro" - and then try to achieve the same behavior with a function. There's no better way to learn when macros are useful and when they over-complicate things.

frou_dh3• 10 years ago

In my home-made lisp, a macro can be an anonymous first-class value.

e.g. this is early in the stdlib and is what sets up the ability to write 'foo as a shorthand for (quote foo):

    (extend-parser "'"
                   (mac (x) (list (quote quote) x)))

Haven't yet contorted myself into wanting to map a macro value over a list, though!

emidln2• 10 years ago

This is true in emacs lisp too, although the syntax for it is a bit wonky:

    ;; increment as a macro, anonymously
    (= ((macro lambda (x) `(+ 1 ,x)) 10) 11)

frou_dh3• 10 years ago

Oh, good. It's not a completely stupid notion then. Someone on another site told me an anonymous macro is a contradiction in terms.

I have a diagnostics toggle (SIGUSR2) that traces what's happening with macros for debugging. Can you think of a better way to convey it? http://i.imgur.com/kFzEvd6.png

thomasvarney723• 10 years ago

I've heard it said that Clojure's macro system is most similar to Common Lisp's macro system and that Racket (and Scheme) style macros are more powerful. Can anyone compare the differing approaches?

mej10• 10 years ago

It is more similar to CL's macro system in that it doesn't force you to write hygienic macros -- though I think CL's macro system is actually more powerful than Scheme's since it does let you write unhygienic macros.

Clojure doesn't allow user defined _reader macros_, which means it is less flexible than either CL or Scheme. Basically, you can't really define your own syntax parser in Clojure, you are limited to what the Clojure reader can already parse. In CL and Scheme you don't have this restriction.

soegaard• 10 years ago

Scheme has historically been used as a research vehicle for macros. When the fifth revised specification of Scheme (known as R5RS) was written there were no consensus on the best practices for writing a system supporting unhygienic macros.

Since then both R6RS and R7RS has support for unhygienic macros.

Note that there are two important properties of the macro systems of modern Scheme implementations: hygiene and referential transparency. The second property is often overlooked.

The standard syntax-rules macro system of R5RS Scheme is hygienic

    If a macro transformer inserts a binding for an identifier (variable or keyword), 
    the identifier will in effect be renamed throughout
    its scope to avoid conflicts with other identifiers.

and referentially transparent

    If a macro transformer inserts a free reference to 
    an identifier, the reference refers to the binding 
    that was visible where the transformer was specified,
    regardless of any local bindings that may surround
    the use of the macro.

The first property (automatically renaming of identifiers) is prevents common errors. This property is easy to hack around in macro systems without automatic renaming. The macro writer simply generates a new identifier (gensym) each time a new identifier is needed.

The second one referential transparency is the killer feature. Names inserted by a macro refers to binding where the macro is defined. This means that the macro writer has control over the meaning of the binding of identifiers in the expansion. In other words: a user of a macro can not by accident rebind or assign values to identifiers inserted by a macro use.

When referential transparency is not supported, then one must be careful to load libraries in the right order:

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A few tips for writing macros in Clojure

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