Discussion on: Struct callback.

pub struct Callback<T> {
    f: Rc<dyn Fn(&str) -> T>,
}

impl<T> Callback<T> {
    /// Call this callback
    pub fn call(&self, token: &str) -> T {
        return (self.f)(token);
    }
}

impl<T> Clone for Callback<T> {
    fn clone(&self) -> Self {
        Self {
            f: Rc::clone(&self.f),
        }
    }
}

impl<T, F: Fn(&str) -> T + 'static> From<F> for Callback<T> {
    fn from(func: F) -> Self {
        Self { f: Rc::new(func) }
    }
}

struct ASTNode {
    id: GRAM,
    allow_next: Vec<GRAM>,
    foo: &'static str,
    matcher: Callback<bool>
}


// Usage

fn is_import_statement (token: &str) -> bool {    
    // TBA
    return true;
}

let x = ASTNode{
    id: GRAM::IMPORT_STATEMENT,
    foo: "hey",
    identify_statement_from_next: vec!(
        GRAM::L_C_BRACE,
        GRAM::L_P_BRACKET,
        GRAM::NAME,
        GRAM::ASTERISK
    ),
    matcher: Callback::from(is_import_statement)
};

x.matcher.call("@import");