Question

我的问题是在Rascal语法定义中处理保留关键字。采用以下Rascal模块，该模块源自在线文档：

module Test
// Taken from http://tutor.rascal-mpl.org/Rascal/Declarations/SyntaxDefinition/SyntaxDefinition.html

start syntax Top = Identifier | ReservedKeyword;

layout MyLayout = [\t\n\ \r\f]*;

// Is OK, parse(#Top,"if") succeeds, parse(#Identifier,"if") fails
lexical Identifier = [a-z] !<< [a-z]+ !>> [a-z] \ MyKeywords;

// Goes wrong, parse(#Top,"if") fails, parse(#Identifier,"if") succeeds, 
// so "if" is not exluded
//lexical Identifier = [a-z0-9] !<< [a-z][a-z0-9]* !>> [a-z0-9] \ MyKeywords;

keyword MyKeywords = "if" | "then" | "else" | "fi";

syntax ReservedKeyword = "if" | "then" | "else" | "fi";

关键是保留关键字构造\ MyKeywords仅在词法定义为[a-z] !<< [a-z]+ !>> [a-z]时才有效。如果词汇变得稍微复杂[a-z0-9] !<< [a-z][a-z0-9]* !>> [a-z0-9]，则不再排除关键字。

我在这里做错了什么？如果标识符为[a-z][a-z0-9]*？

，如何排除关键字？

Answer 1

问题的原因是[a-z]旁边的[a-z0-9]+等符号的并置不如\运算符和!>>运算符强。

所以我们只在标识符的尾MyKeywords中保留[a-z0-9]+：

lexical Identifier = [a-z] !<< [a-z][a-z0-9]+ \ MyKeywords;

要解决此问题，您可以添加括号以从整个序列中删除MyKeywords：

lexical Identifier = [a-z] !<< ([a-z][a-z0-9]+) \ MyKeywords;

然后您可以再次添加限制：

lexical Identifier = [a-z] !<< ([a-z][a-z0-9]+) \ MyKeywords !>> [a-z];

或者像这样，等同于：

lexical Identifier = [a-z] !<< ([a-z][a-z0-9]+) !>> [a-z] \ MyKeywords;

处理保留关键字取决于词法定义

1 个答案: