如何使用rust和pomelo在backus naur形式中表示if-else语句？

Question

我试图在rust中使用pomelo来解析一种脚本语言，但在该语言的backus-naur表示中遇到了障碍，它无法像我定义的那样解析if-else语句。例如，如果它要解析语句

if (1) {
  return;
}

解析器将会成功，但是如果我向代码中添加一个else语句，比如

if (1) {
 return;
} else {
 return;
}

它将在else标记处抛出一个语法错误，具体地说，我得到的输出是

gettin If
gettin LParen
gettin Num(1)
gettin RParen
gettin LBrace
gettin Return
gettin Semicolon
gettin RBrace
gettin Else
should not execute this
thread 'main' panicked at 'called `Result::unwrap()` on an `Err` value: "syntax Some(Else)"', src/main.rs:122:24
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

在查看bnf表单时，我似乎不太清楚为什么这会失败。我很抱歉这个最小的可重现示例的大量代码，但我不知道bnf表单中错误的确切原因，因此我不知道如何压缩它以产生相同的错误。

extern crate pomelo;
use pomelo::pomelo;
pomelo!{
   %error String;
   %syntax_error{
       Err(format!("syntax {:?}",token))
   }
   %parse_fail {
        "Giving up.  Parser is hopelessly lost...".to_string()
   }
   %token #[derive(Debug)] pub enum Token {};
   //define types to make the compiler happy
   %type Ident String;
   %type Type String;
   %type Num i64;
   %type String String;
   %type expr Vec<String>;
   %type expr_list Vec<Vec<String>>;
   %type stmt Vec<String>;
   %type input Option<Vec<Vec<String>>>;
   %type block Vec<Vec<String>>;
   %type decl_list Vec<Vec<String>>;
   %type stmt_list Vec<Vec<String>>;
   %type func_list Vec<Vec<String>>;
   %type arg_list Vec<String>;
   %type type_list Vec<String>;
   %type package String;
   %type imports Vec<String>;
   %type f_decl Vec<String>;
   %type type_t Vec<Vec<String>>;
   %type type_s Vec<Vec<String>>;
   %type decl Vec<Vec<String>>;
   %type assignment String;
   %type CheckMark String;
   %type Todo String;
   %left Else;
   %right Eq;
   %left Or;
   %left And;
   %nonassoc Equal Neq;
   %nonassoc Less LessEq Greater GreaterEq;
   %left Add Sub;
   %left Mul Div;
   %nonassoc Not;
   input ::= type_s?(v) {v /*assigning worthless values to make the compiler happy*/};
   type_t ::= stmt(a) {vec![a]}
   type_s ::= type_t(t) {t}
   type_s ::= type_s(mut a) type_t(b) {a.extend(b);a}
   stmt ::= KeyFunction Ident(name) LParen arg_list?(args) RParen block(code) {vec![name]}
   assignment ::= Ident(a) Eq expr(b) {a}
   arg_list ::= Ident(n)  [Semicolon] { vec![n] }
   arg_list ::= assignment(n) {vec![n]}
   arg_list ::= arg_list(mut args) Comma Ident(n) { args.push(n); args }
   block ::= LBrace stmt_list?(ss) RBrace { ss.unwrap_or(Vec::new()) }
   stmt_list ::= stmt(s) { vec![s] }
   stmt_list ::= stmt_list(mut ss) stmt(s) { ss.push(s); ss }
   stmt ::= block(ss) { ss.get(0).unwrap_or(&Vec::new()).clone() }
   stmt ::= expr(e) Semicolon {e}
   stmt ::= Ident(a) Eq expr(b) Semicolon { vec![a] }
   stmt ::= Var Ident(a) Eq expr(b) Semicolon { vec![a]}    
   stmt ::= If LParen expr(e) RParen stmt(s1) Else stmt(s2) {println!("should execute this"); s1  }
   stmt ::= If LParen expr(e) RParen stmt(s1) [Else] {println!("should not execute this"); s1 }
   stmt ::= While LParen expr(e) RParen stmt(s) { s }
   stmt ::= Return expr(e) Semicolon { e }
   stmt ::= Return Semicolon { vec!["".to_string()] }
   stmt ::= Break Semicolon { vec!["".to_string()] }
   stmt ::= Continue Semicolon {vec!["".to_string()] }
   stmt ::= Todo(a) expr(b) Semicolon {vec![a]}
   stmt ::= CheckMark(a) {vec![a]}
   expr ::= Num(n) { vec!["".to_string()] }
   expr ::= String(n) {vec!["".to_string()]}
   expr ::= Ident(n) {vec!["".to_string()] }
   expr ::= Ident(n) LParen expr_list?(es) RParen {vec![n]}
   expr ::= LParen expr(e) RParen { e }
   expr ::= expr(a) Add expr(b) { a}
   expr ::= expr(a) Sub expr(b) { a }
   expr ::= expr(a) Mul expr(b) {a}
   expr ::= expr(a) Div expr(b) {a}
   expr ::= Sub expr(a) [Not]     { a }
   expr ::= expr(a) Equal expr(b) { a }
   expr ::= expr(a) Neq expr(b) { a }
   expr ::= expr(a) And expr(b) { a }
   expr ::= expr(a) Or expr(b) {a }
   expr ::= Not expr(a) { a }
   expr ::= expr(a) Less expr(b) { a }
   expr ::= expr(a) Greater expr(b) { a }
   expr ::= expr(a) LessEq expr(b) { a }
   expr ::= expr(a) GreaterEq expr(b) { a }
   expr_list ::= expr(e) { vec![e] }
   expr_list ::= expr_list(mut es) Comma expr(e) { es.push(e); es }
}
fn main() {
    let mut parse = parser::Parser::new();
    let code = vec![
        parser::Token::If,
        parser::Token::LParen,
        parser::Token::Num(1),
        parser::Token::RParen,
        parser::Token::LBrace,
        parser::Token::Return,
        parser::Token::Semicolon,
        parser::Token::RBrace,
        parser::Token::Else,
        parser::Token::LBrace,
        parser::Token::Return,
        parser::Token::Semicolon,
        parser::Token::RBrace   
    ];
    for i in code {
        println!("gettin {:?}",i);
        parse.parse(i).unwrap();
    }
    parse.end_of_input().unwrap();
}

Answer 1

对于遇到类似问题的任何人来说，问题在于else标记的关联性，将else标记从左关联更改为右关联起到了作用