Book: Chapter 1.2 - Reason for the `println!` macro

[rpjohnst]

The first paragraph that mentions macros describes their syntax and defers to an appendix for more information, but does not mention why someone might use a macro in Rust:

The second important part is println!. This is calling a Rust macro, which is how metaprogramming is done in Rust. If it were calling a function instead, it would look like this: println (without the !). We'll discuss Rust macros in more detail in Appendix E, but for now you just need to know that when you see a ! that means that you’re calling a macro instead of a normal function.

In situations like this, I always appreciate at least a hand-wavy explanation of why someone might do something like making println! a macro rather than a function. (In this case, so format strings can be processed at compile time, which normal functions can't do.) Otherwise I'm left unsure what to expect when dealing with these ! things- I only know something different is going on.

[seanjensengrey]

A macro! can optionally evaluate its arguments.

Say one wanted to implement a function lazy_or and call it via

lazy_or(a(),b(),c())

Under eager evaluation, the functions a, b and c would all be evaluated before the call to lazy_or. But with a macro, the generated code can decide whether to call functions b or c.

[rpjohnst]

Another point from Reddit: macros can be variadic while functions cannot. This one is probably even more relevant to the println! example than the compile-time processing.

[carols10cents]

Thanks! We actually haven't revised Chapter 1 heavily yet, I'll make sure we take care of this when we get there!

[monuszko]

I would also appreciate at least a one sentence explanation. This is a pretty widely asked question:

https://www.reddit.com/r/rust/comments/4qor4o/newb_question_why_is_println_a_macro/
https://github.com/rust-lang/rust/issues/17190#issuecomment-55372530

meanwhile macros are not found in "common language features", they don't even have a chapter, they're delegated to an appendix which leads me to believe they're an advanced language feature. This makes me either google the question immediately or forget about it forever.