[CalChris]

That was just a trivial example. Yes, you could use a const which would be difficult to share across files. Moreover, if you wanted to do something equally textual

  #define PASS_VERBOSE if (flag_verbose && first_pass) printf

you just might be able to but only with a completely different tool.

A macro processor is not of the language; it is above the language.

[lobster_johnson]

A "macro" in C is not the same thing as a "macro" in Rust. By your logic, Erlang processes aren't processes because they aren't kernel processes, or Go packages aren't packages because they don't use the Java package naming convention. Nobody owns the exclusive right to define the words we use.

[kibwen]

> A macro processor is not of the language; it is above the language.

Then feel free to use the C preprocessor with Rust, it works just as well. :P Just like it does with Python, and Java, and...

[kazinator]

The C preprocessor can only be used with Python as long as you don't do anything multi-line:

   #define whatever(param) \
   foo: \
      bar \
      baz

I made a preprocessor some 18 years ago that could be used with Python.

Wayback Machine:

https://web.archive.org/web/20000815202258/http://users.foot...

[int_19h]

Better yet, use m4.

[steveklabnik]

> which would be difficult to share across files.

It's path::to::wherever::PI. That's it. Just like any other item. If you want to use only PI in your code, you'd use 'use', like any other name.

Your second example is something better suited to a macro, it's true.

I _think_ what you're getting at here is that you only want text substitution? I think we will have to agree to disagree if that's true :)

[CalChris]

Well, given that you have only written two macros in your years of Rust, I would strongly encourage you or the language ergonomics initiative to openly question why this is so. Clearly, Rust has a clever approach but I'm questioning whether it is in fact a usable approach. Too much solution for not enough problem.

Yes, I do like textual substitution. Guilty. This is a common old school low level paradigm. Still, the underlying language and its compiler exist below to enforce the rules on any atrocities I commit with my macros.

[int_19h]

If you want textual substitution, you are not limited by anything that Rust offers. You can incorporate any existing text preprocessor - of which there's a multitude, and at least cpp and m4 are in pretty much any Unix system - into your compile pilelines. By the very definition of textual substitution, it is completely orthogonal to the meaning of the output (in this case, Rust code).

The kind of macros Rust implements, on the other hand, are the kind that have to be language-specific, because they deal with the syntax tree of that particular language. They also enable many things that are outright impossible with text substitution, such as hygienic macros.

[naasking]

> Clearly, Rust has a clever approach but I'm questioning whether it is in fact a usable approach. [...] Yes, I do like textual substitution.

These two statements are not consistent. Textual substitution is not usable. The pitfalls with it have been well documented for decades, and yet the same problems persist. Why on earth would you want to perpetuate the list of problems caused by such a facility?

Syntactic macros are absolutely superior to textual substitution. You can take issue with Rust's currently limited support for macros, but to propose textual substitution as a viable and more usable alternative is simply absurd.

[steveklabnik]

What I'm saying is, I personally find Rust expressive enough to never need macros, and so their ease of use, to me personally, is not an issue either way. (Well, other than the import bit, I do care about that.)

Those who do use and write them heavily are the ones actually involved in the macros 2.0 effort. There are certainly flaws.

[a_t48]

You know what I like? Sensible error messages, which you tend to lose if you use macros for anything complex (disclaimer: I am very guilty here)

[tedmielczarek]

I very rarely write macros despite having written thousands of lines of Rust, but they are super useful on occasion. Often in Rust something that you might have used preprocessor macro tricks to do in C can be done without macros at all. At other times, Rust macros will let you do things that would be impossible to express in C.

[tatterdemalion]

You want textual macros. Rust doesn't have this feature, it has a syntactic macro feature inspired by the Lisp family. Sorry.