Higher-order rust macro that assists in parsing by macro

split_comma.rs

/// Higher-order macro fold function for pre-parsing comma separated lists
///
/// The commas on KEYWORD lines can be changed to parse lists with any
/// separator. 

/// run a macro on a list of lists after splitting the input at commas
macro_rules! split_comma {
    // no defaults
    {$fun:ident <= $($item:tt)*} => {
        split_comma![$fun () () () <= $($item)*]
    };
    // give initial params to the function
    {$fun:ident ($($first:tt)*) <= $($item:tt)*} => {
        split_comma![$fun ($($first)*) () () <= $($item)*]
    };
    // give inital params and initial inner items in every group 
    {$fun:ident ($($first:tt)*) ($($every:tt)*) <= $($item:tt)*} => {
        split_comma![$fun ($($first)*) ($($every)*) ($($every)*) <= $($item)*]
    };
    // KEYWORD line
    // on non-final seperator, stash the accumulator and restart it
    {$fun:ident ($($first:tt)*) ($($every:tt)*) ($($current:tt)*) <= , $($item:tt)+} => {
        split_comma![$fun ($($first)* ($($current)*)) ($($every)*) ($($every)*) <= $($item)*] 
    };
    // KEYWORD line
    // ignore final seperator, run the function
    {$fun:ident ($($first:tt)*) ($($every:tt)*) ($($current:tt)+) <= , } => {
        $fun![$($first)* ($($current)*)] 
    };
    // on next item, add it to the accumulator
    {$fun:ident ($($first:tt)*) ($($every:tt)*) ($($current:tt)*) <= $next:tt $($item:tt)*} => {
        split_comma![$fun ($($first)*) ($($every)*) ($($current)* $next)  <= $($item)*] 
    };
    // at end of items, run the function
    {$fun:ident ($($first:tt)*) ($($every:tt)*) ($($current:tt)+) <= } => {
        $fun![$($first)* ($($current)*)] 
    };
    // if there were no items and no default, run with only initial params, if any
    {$fun:ident ($($first:tt)*) () () <= } => {
        $fun![$($first)*]
    };
}
// Macros passed to `split_comma` take optional initial parameters followed by a list of
// parenthesised items. These outer parentheses now separate what was originally separated
// by commas.
macro_rules! sum_vec {
    {$(($($item:tt)*))*} => { vec![$(0 $(+ $item )*, )*] };
    {debug $($item:tt)*} => { stringify![$($item)*].to_string() };
}
fn main(){
    let x = split_comma![sum_vec <= 1 2, 3 4, 5 6];
    println!("{:?}",x);
    // [3, 7, 11]
    let x = split_comma![sum_vec (debug) (hi) <= 4 5,7,,1 2 3,];
    println!("{:?}",x);
    // "( hi 4 5 ) ( hi 7 ) ( hi ) ( hi 1 2 3 )"
    let hi = 8;
    let x = split_comma![sum_vec () (hi) <= 4 5,7,,1 2 3];
    println!("{:?}",x);
    // [17, 15, 8, 14]
    let x = split_comma![sum_vec (debug) <= ];
    println!("{:?}",x);
    // ""
}