# Some trouble with macros

**URL:** <https://users.rust-lang.org/t/some-trouble-with-macros/54390>\
**Category:** help\
**Created:** [January 19, 2021, 8:53pm UTC](https://users.rust-lang.org/t/some-trouble-with-macros/54390 "2021-01-19T20:53:32Z")\
**Posts on this page:** 3\
**Page:** 1

<div class="post-metadata">

**Author:** ![noob\_rustician](https://avatars.discourse-cdn.com/v4/letter/n/a5b964/32.png) [@noob\_rustician](https://users.rust-lang.org/u/noob_rustician)\
**Post date:** [January 19, 2021, 8:53pm UTC](https://users.rust-lang.org/t/some-trouble-with-macros/54390/1 "2021-01-19T20:53:33Z")

</div>

So here we go. I have this code

```rust
macro_rules! list {
    ($x:expr) => (FunList::new($x));
    ($x:expr, $($y:expr),+) => (
        add($x, list!($($y),+))
    );
}

fn main() {
    let a = list!(1, 2, 3, 4, 5);
    print(&a);
}  

struct FunList<T> {
    val: T,
    next: Option<Box<FunList<T>>>,
}

impl<T> FunList<T> {
    fn new(val: T) -> FunList<T> {
        FunList { val, next: None }
    }
}
fn add<T>(val: T, next: FunList<T>) -> FunList<T> {
    FunList {
        val,
        next: Some(Box::from(next)),
    }
}
fn print<T>(list: &FunList<T>)
where T: std::fmt::Display {
    print!("{}, ", list.val);
    match &list.next {
        None => {},
        Some(val) => print(val.as_ref())
    };
}

```

And it works good and all but my inner dump\_programmer wanted to try to change macro to this just for fun

```rust
macro_rules! list {
    ($x:expr) => (FunList::new($x));
    ($($y:expr),+, $x:expr) => (
        add($x, list!($($y),+))
    );
}

```

I was sure that it will work, but it doesn't and give me this

```rust
error: local ambiguity: multiple parsing options: built-in NTs expr ('x') or expr ('y').
 --> src/main.rs:9:22
  |
9 | let a = list!(1, 2, 3, 4, 5);
  | ^

error: aborting due to previous error

error: could not compile `test_crate`

To learn more, run the command again with --verbose.

```

And I'm interested if I can extract the last element from elements sequence in macro and if yes how and what I made wrong in my example.

---

<div class="post-metadata">

**Author:** ![H2CO3](https://avatars.discourse-cdn.com/v4/letter/h/b5ac83/32.png) [@H2CO3](https://users.rust-lang.org/u/H2CO3)\
**Post date:** [January 19, 2021, 9:07pm UTC](https://users.rust-lang.org/t/some-trouble-with-macros/54390/2 "2021-01-19T21:07:29Z")

</div>

It all boils down to [left-recursion vs. right-recursion](https://stackoverflow.com/questions/32123003/difference-between-left-right-recursive-left-right-most-derivation-precedence/32132367#32132367) in parsing. If parsing proceeds from left to right (which it mostly does in Rust), then `($y:expr),+, $x:expr` has multiple possible interpretations (as the error message tells you). In particular, `$y` can be bound to 1 or more comma-separated expressions, so `$y = 1, $x = 2`, `$y = (1, 2), $x = 3`, `$y = (1, 2, 3), $x = 4`, etc. are all valid parses of the macro input.

* * *

If you have tried implementing a recursive descent parser for a context-free grammar before, you might be aware how naïve recursion leads to infinite recursion when a production rule in the grammar is specified left-recursively. This is a similar (although not identical) issue – in general, left recursion is problematic when trying to parse eagerly from left to right.

> [@noob\_rustician](#):
>
> And I'm interested if I can extract the last element from elements sequence in macro and if yes how

You'll have to recurse until there is only a single element. [Something like this.](https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=77489a3b2e32c0e2e02568d24a22d301)

---

<div class="post-metadata">

**Author:** ![system](https://sea1.discourse-cdn.com/flex019/user_avatar/users.rust-lang.org/system/32/51372_2.png) [@system](https://users.rust-lang.org/u/system)\
**Post date:** [April 19, 2021, 9:07pm UTC](https://users.rust-lang.org/t/some-trouble-with-macros/54390/3 "2021-04-19T21:07:33Z")

</div>

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