`macro_rules` that "captures" outter variable fails to compile if used inside a function, but not if used inside a method

[japaric]

STR:

#![feature(macro_rules)]

macro_rules! check_len {
    () => { xs.len() == 0 }
}

struct Foo;

impl Foo {
    fn bar(self, xs: Vec<uint>) -> bool {
        // GOOD
        check_len!()

        // GOOD
        //xs.len() == 0
    }
}

fn baz(xs: Vec<uint>) -> bool {
    // BAD
    check_len!()

    // GOOD
    //xs.len() == 0
}

fn main() { }

Output:

$ rustc foo.rs
foo.rs:4:13: 4:15 error: unresolved name `xs`.
foo.rs:4     () => { xs.len() == 0 }
                     ^~
foo.rs:3:1: 5:2 note: in expansion of check_len!
foo.rs:21:5: 25:2 note: expansion site
error: aborting due to previous error

Expansion:

$ rustc --pretty expanded foo.rs
#![feature(macro_rules)]
#![feature(phase)]
#![no_std]
#![feature(globs)]
#[phase(plugin, link)]
extern crate std;
extern crate native;
use std::prelude::*;


struct Foo;

impl Foo {
    fn bar(self, xs: Vec<uint>) -> bool {

        // GOOD
        xs.len() == 0

        // GOOD
        //xs.len() == 0
    }
}

fn baz(xs: Vec<uint>) -> bool {

    // BAD
    xs.len() == 0

    // GOOD
    //xs.len() == 0
}

fn main() { }

Version:

$ rustc --version
rustc 0.11.0 (4f120e6bafe971452adfede158a7957b00562a4e 2014-07-07 08:16:29 +0000)

P.S. I got the impression that we are moving towards macros that can't "capture" outter variables (like xs in this case) for "hygienic" reasons. Is that correct? And, if that's the case, may I ask what is this "hygiene" about? (Is a concept related to compilers (only)?)

[brson]

cc @jbclements

[jbclements]

Okay, let's see.

First things first: you're right that methods are currently different from functions; that's because hygiene
isn't yet complete for methods. I hope to finish that later today :).

Next, Hygiene. The basic idea is that in general, macros such as your check_len are fragile; specifically, a programmer who comes to this code later and edits the baz function might change the name of the xs argument, and all uses of xs inside that function. That programmer will probably be unhappy and confused when the code stops working. The basic idea behind hygiene is that bindings have lexical scope, even in the presence of macros.

In this instance, the easiest fix would simply be to put the definition of the check_len macro inside of the function itself. Alternatively, you could have the macro accept the name xs explicitly. I'd be happy to show you some example code, if you like.

Thanks for your question!

[japaric]

Next, Hygiene. The basic idea is that in general, macros such as your check_len are fragile; specifically, a programmer who comes to this code later and edits the baz function might change the name of the xs argument, and all uses of xs inside that function. That programmer will probably be unhappy and confused when the code stops working.

That makes sense.

The basic idea behind hygiene is that bindings have lexical scope, even in the presence of macros.

With these hygiene rules, would it be possible for the compiler to catch these kind of errors in the macro definiton? Because, right now, all errors point to the macro expansions. If you used a bad macro ten times, you'll get ten errors - it'll be better (less noisy) if the compiler pointed directly at the culprit: the macro definition.

the easiest fix would simply be to put the definition of the check_len macro inside of the function itself

In that case, I would end with two equal macro definitions: one in bar() and another in baz(). Right?

Alternatively, you could have the macro accept the name xs explicitly.

Something like this?

macro_rules! check_len {
    ($collection:ident) => { $collection.len() == 0 }
}

@jbclements Thanks for your answer!

(Should I leave this issue open until the method hygiene patch lands, or just close it?)

[jbclements]

Re: catching the errors in the definition: the problem here is that there are things that macros don't know about their contexts. There are certain macros that can be used as patterns or as expressions or as statements; the compiler doesn't know how it'll be used until it plugs it in. Another way of saying that: this is still an area of active research!

You can go ahead and close this now, if you like. Naturally, you can also wait, if you prefer that.

[japaric]

@jbclements 👍
Fixed in #15537. Closing.