[ashton314]

I have a hard time trusting the CL macros I write because of unexpected interactions with the context that I use them in. While it is the case that CL macros are more powerful than the R6RS macros-by-example system, Racket’s system (and some other newer languages that have adopted things pioneered by Scheme and Racket, such as Elixir) give you hygienic macros without sacrificing expressive power.

I want my macros to be easy to write correctly. That can only happen when the system has proper hygiene.

[codr7]

They are not unexpected though.

If you put your own symbols into generated code you better have a damn good idea what you're doing.

It's a power tool.

[ashton314]

Table saws were only improved by the addition of an emergency stop to prevent people from maiming themselves. Power tools don’t have to be dangerous.

If you are wanting to introduce variable capture you better be really explicit about when you want it.

If there’s no hygiene, I have to know everything about how the macro is implemented in order to trust it and use it confidently. Might be fine for small shorthand, but that won’t scale. You need non-leaky abstractions to build on them.

Racket’s `syntax-parse` and “syntax parameters” show that you can have it both ways: procedural macros that are hygienic by default, but with an explicit escape hatch when you do want to introduce new bindings into the macro call site. It also gives you much much better errors.

CL macros are about as dangerous as malloc/free, but without years of experience and tools like Valgrind to debug. They’re hard to trust and get right.

Racket macros are like GC/affine typing: everything is correct by construction.

[codr7]

I've used table saws, with and without protection. They're all dangerous as fck, because removing the chance of getting hurt means converting it to a completely different kind of tool. Chain saws, same thing. Power tools.

Of course we want them to be as safe as possible, but that's a different discussion. All attempts I've seen so far have dropped functionality to get there.

[ashton314]

> All attempts I've seen so far have dropped functionality to get there.

Well, then I recommend you take a look at Racket's macro system: Racket gives you hygienic macros without any loss of power. (It's actually more powerful and expressive than CL macros.)

[codr7]

So they allow you to introduce arbitrary symbols in expansions? Doesn't that mean we're back to regular CL macros?

[ashton314]

The way you phrased that suggests you're only familiar with CL-style macros, where arguments to macros are nested lists of symbols a function or variable is known by it's name (a symbol) and nothing more.

Racket's model is much more sophisticated and powerful. The input to a macro in Racket is a syntax object [1], which combines the CL-like quoted expression with additional source and lexical binding information. This means that in Racket, unlike CL, a variable is not just it's name—it's also all this other information. Racket uses scope sets to track binding information in a sane and hygienic manner across different macros and functions.

So, if you want to introduce an identifier that the macro caller can interact with (note: I said an identifier—you can introduce any symbols you want but they'll be different identifiers because their scope sets will be different) you need to explicitly state that you would like to create an identifier with a particular scope set. [4]

But that's the old, dumpy, clunky way of doing things. Thanks to recent research, we have much better ways of introducing identifiers in a sane, hygienic way. Gregg Hendershott's excellent "Fear of Macros" walks through making the `aif` macro using syntax parameters [3] which let you cleanly introduce new bindings. (See the paper "Keeping it Clean with Syntax Parameters" he's linked to in his post.)

So, in short, no, we're not back to regular CL macros because Racket prevents us from accidental variable capture but gives us an easy way to do 99.999% of the use cases for breaking hygiene (syntax parameters) and then one more way (`datum->syntax`) just in case we really need to do something out of the ordinary. In either way, Racket lets you express your intent with macros better and more precisely than CL.

[1]: Racket syntax objects: https://docs.racket-lang.org/guide/stx-obj.html

[2]: Syntax model, scope sets: https://docs.racket-lang.org/reference/syntax-model.html

[3]: "Fear of Macros", writing the `aif` macro: https://www.greghendershott.com/fear-of-macros/Syntax_parame...

[4]: Reddit thread on making an unhygienic macro: https://www.reddit.com/r/Racket/comments/tpgaa4/is_there_a_w...