xtext:表达式/因子/术语语法 [英] xtext: expression/factor/term grammar

问题描述

这一定是互联网上某个知名示例之一，但我似乎找不到它.

This has got to be one of those well-known examples that's somewhere on the internet, but I can't seem to find it.

我正在尝试学习XText，我认为计算器表达式解析器将是一个好的开始.但是我的语法出现语法错误:

I'm trying to learn XText and I figured a calculator expression parser would be a good start. But I'm getting syntax errors in my grammar:

Expression:
    Term (('+'|'-') Term)*;

Term:
    Factor (('*'|'/') Factor)*;

Factor:
    number=Number | variable=ID | ('(' expression=Expression ')');

在表达式"和术语"行中出现此错误:

I get this error in the Expression and Term lines:

Multiple markers at this line
 - Cannot change type twice within a rule
 - An unassigned rule call is not allowed, when the 'current'
   was already created.

有什么作用?我怎样才能解决这个问题?以及什么时候在语法中有instanceName=Rule与Rule条目?

What gives? How can I fix this? And when do I have instanceName=Rule vs. Rule entries in a grammar?

推荐答案

我下载了与eclipse集成的xtext，它带有一个计算器示例，该示例可以完成您希望的算术运算.根据我的收集，您将需要为令牌分配关联性.这种语法对我来说很好:

I downloaded xtext integrated with eclipse and it comes with a calculator example which does approximately what you wish called arithmetics. From what I can gather you will need to assign an associativity to your tokens. This grammar runs fine for me:

Expression:
    Term (({Plus.left=current}'+'|{Minus.left=current}'-') right=Term)*;

Term:
    Factor (({Multiply.left=current} '*'| {Division.left=current}'/') right=Factor)*;

Factor:
    number=NUMBER | variable=ID | ('(' expression=Expression ')');

可以查看

The example grammar they have for arithmetics can be viewed here. It includes a bit more than your, like function calls, but the basics are the same.

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