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Implementing Domain-Specific Languages with Xtext and Xtend

You're reading from  Implementing Domain-Specific Languages with Xtext and Xtend

Product type Book
Published in Aug 2013
Publisher Packt
ISBN-13 9781782160304
Pages 342 pages
Edition 1st Edition
Languages
Author (1):
Lorenzo Bettini Lorenzo Bettini
Profile icon Lorenzo Bettini

Table of Contents (21) Chapters

Implementing Domain-Specific Languages with Xtext and Xtend
Credits
About the Author
Acknowledgement
About the Reviewers
www.PacktPub.com
Preface
1. Implementing a DSL 2. Creating Your First Xtext Language 3. The Xtend Programming Language 4. Validation 5. Code Generation 6. Customizations 7. Testing 8. An Expression Language 9. Type Checking 10. Scoping 11. Building and Releasing 12. Xbase 13. Bibliography
Index

Left recursive grammars


When moving on to a more complex expression, such as addition, we need to write a recursive rule since the left and right parts of an addition are expressions themselves. It would be natural to express such a rule as follows:

Expression:
  ... as above
  {Plus} left=Expression '+' right=Expression;

However, this results in an error from the Xtext editor as shown in the following screenshot:

Xtext uses a parser algorithm that is suitable for use for interactive editing due to its better handling of error recovery. Unfortunately this parser algorithm does not deal with left recursive rules. A rule is left recursive when the first symbol of the rule is non-terminal and refers to the rule itself. The preceding rule for addition is indeed left recursive and is rejected by Xtext.

Note

Xtext generates an ANTLR parser (Parr 2007), which relies on an LL(*) algorithm; we will not go into details about parsing algorithms; we refer the interested reader to Aho et al. 2007. Such parsers...

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