BNFC规则中的不良胁迫

Question

我正试图用BNFC编写一个编译器。我将使用BNFC来生成抽象语法树。但是，我得到了错误，我似乎不知道为什么。它似乎没有太多的文档。

以下是我所犯的错误：

Bad coercion in rule _. Prog ::= Block
Bad coercion in rule _. Declarations ::= Declaration ";" Declarations
Bad coercion in rule _. Declarations ::=
Bad coercion in rule _. Declaration ::= Var_declaration
Bad coercion in rule _. Declaration ::= Fun_declaration
Bad coercion in rule _. Type ::= "int"
Bad coercion in rule _. Type ::= "real"
Bad coercion in rule _. Type ::= "bool"
Bad coercion in rule _. Array_dimensions ::= "[" Expr "]" Array_dimensions
Bad coercion in rule _. Array_dimensions ::=
Bad coercion in rule _. Fun_block ::= Declarations Fun_body
Bad coercion in rule _. Param_list ::= "(" Parameters ")"
Bad coercion in rule _. Parameters ::= Basic_declaration More_parameters
Bad coercion in rule _. Parameters ::=
Bad coercion in rule _. More_parameters ::= "," Basic_declaration More_parameters
Bad coercion in rule _. More_parameters ::=
Bad coercion in rule _. Basic_declaration ::= Ident Basic_array_dimensions ":" Type
Bad coercion in rule _. Basic_array_dimensions ::=
Bad coercion in rule _. Program_body ::= "begin" Prog_stmts "end"
Bad coercion in rule _. Fun_body ::= "begin" Prog_stmts "return" Expr ";" "end"
Bad coercion in rule _. Prog_stmts ::= Prog_stmt ";" Prog_stmts
Bad coercion in rule _. Prog_stmts ::=
Bad coercion in rule _. Identifier ::= Ident Array_dimensions
Bad coercion in rule _. Expr ::= Bint_term
Bad coercion in rule _. Bint_term ::= Bint_factor
Bad coercion in rule _. Bint_factor ::= Int_expr Compare_op Int_expr
Bad coercion in rule _. Bint_factor ::= Int_expr
Bad coercion in rule _. Int_expr ::= Int_expr Addop Int_term
Bad coercion in rule _. Int_expr ::= Int_term
Bad coercion in rule _. Int_term ::= Int_term Mulop Int_factor
Bad coercion in rule _. Int_term ::= Int_factor
Bad coercion in rule _. Int_factor ::= "(" Expr ")"
Bad coercion in rule _. Modifier_list ::= "(" Arguments ")"
Bad coercion in rule _. Modifier_list ::= Array_dimensions
Bad coercion in rule _. Arguments ::= Expr More_arguments
Bad coercion in rule _. Arguments ::=
Bad coercion in rule _. More_arguments ::= "," Expr More_arguments
Bad coercion in rule _. More_arguments ::=

下面是BNFC文件的示例：

_.Prog ::= Block;

M_Prog. Block ::= Declarations Program_body;

_.Declarations ::= Declaration ";" Declarations;
_. Declarations ::= ;

_. Declaration ::= Var_declaration;
_. Declaration ::= Fun_declaration;

M_Var. Var_declaration ::= "var" Ident Array_dimensions ":" Type;

_. Type ::= "int";
_. Type ::= "real";
_. Type ::= "bool"; 

_. Array_dimensions ::= "[" Expr "]" Array_dimensions;
_. Array_dimensions ::=;

M_Fun. Fun_declaration ::= "fun" Ident Param_list ":" Type "{" Fun_block "}";

_. Fun_block ::= Declarations Fun_body; 

_. Param_list ::= "(" Parameters ")";

_. Parameters ::= Basic_declaration More_parameters;
_. Parameters ::= ;

_. More_parameters ::= ","  Basic_declaration More_parameters;
_. More_parameters ::= ;

_. Basic_declaration ::= Ident Basic_array_dimensions ":" Type;

_. Basic_array_dimensions ::= "[" "]" Basic_array_dimensions;
_. Basic_array_dimensions ::=;

我似乎不正确地使用了_.标签。但是手册中只有一两行描述它的用法。我在这里做错什么了？

Answer 1

来自文档

当然，在值类型与参数类型相同的情况下，下划线仅作为一个参数构造函数的替换有意义。

这意味着，只有在规则的右侧有一个非终端，并且非终端与左侧站点相同的情况下，您才能使用_。所以您可以做一些类似_. A ::= "(" A ")" ;的事情，但不能做_. A ::= "(" B ")" ;或_. A ::= "(" A A ")" ;。

在您的示例中，我建议您为每条规则指定一个标签，在某些情况下，_主要用于简化AST。

顺便说一句，物品清单还有一个语法快捷方式。