CODEWITHSUNDEEP
rustmacrosmetaprogramming
Makogan
Makogan

Reputation: 9622

Why does a trailing comma prevent this macro from compiling?

I am trying to learn macros, to that effect I made this example:

macro_rules! Reflect
{
($name:ident {$($field_name:ident : $field_type:ty), *}) =>
{
  
    struct $name
    {
        $($field_name : $field_type), *
 
    }
 
};
}

Reflect!(
    Test
    {
        field : usize,
        field2 : usize,
    }
);

This exact syntax generates:

    |
183 | macro_rules! Reflect
    | -------------------- when calling this macro
...
202 |     }
    |     ^ no rules expected this token in macro call

But if I take out the last coma after field2 I can compile no problem. That makes no sense to me, the matching pattern has a comma at the end, why is it not always looking for a trailing comma?

Upvotes: 1

Views: 427

Answers (2)

Kevin Reid
Kevin Reid

Reputation: 43872

The way to write a macro which allows a trailing comma, but does not require it, is:

$($field_name:ident : $field_type:ty),* $(,)?

That is: a comma-separated repetition, followed by an optional comma.

Upvotes: 3

Makogan
Makogan

Reputation: 9622

Apparently one must do this instead:

macro_rules! Reflect
{
($name:ident {$($field_name:ident : $field_type:ty,) *}) =>
{
  
    struct $name
    {
        $($field_name : $field_type), *
 
    }
 
};
}

Notice the comma moved.

Upvotes: 0

Related Questions