Скачать или смотреть Resolving Syntax Errors in Flex-Bison Lexer and Parser for Comma-Separated Values

Learn how to effectively ignore whitespace in your flex-bison parser and avoid syntax errors while parsing comma-separated sequences.
---
This video is based on the question https://stackoverflow.com/q/62859818/ asked by the user 'Maifee Ul Asad' ( https://stackoverflow.com/u/10305444/ ) and on the answer https://stackoverflow.com/a/62861443/ provided by the user 'rici' ( https://stackoverflow.com/u/1566221/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

Visit these links for original content and any more details, such as alternate solutions, latest updates/developments on topic, comments, revision history etc. For example, the original title of the Question was: flex-bison ignore whitespace

Also, Content (except music) licensed under CC BY-SA https://meta.stackexchange.com/help/l...
The original Question post is licensed under the 'CC BY-SA 4.0' ( https://creativecommons.org/licenses/... ) license, and the original Answer post is licensed under the 'CC BY-SA 4.0' ( https://creativecommons.org/licenses/... ) license.

If anything seems off to you, please feel free to write me at vlogize [AT] gmail [DOT] com.
---
Resolving Syntax Errors in Flex-Bison Lexer and Parser for Comma-Separated Values

When working with flex and bison, many newcomers encounter issues with syntax errors, particularly when parsing sequences that include whitespace. This post will delve into a common issue encountered when building a simple lexer and parser for comma-separated values and provide a detailed explanation on how to effectively ignore whitespace and fix the problem.

The Problem: Syntax Errors with Comma-Separated Input

Imagine you are developing a lexer and parser to handle a simple grammar that accepts sequences of values separated by commas, such as:

a

a,b

a ,b

a, b

a,b,c

In your current setup, you experience syntax errors when passing input that barely includes spaces. This frustration often stems from not properly configuring the lexer to ignore whitespace characters or misunderstanding how to declare tokens and patterns.

The Solution: A Step-by-Step Approach

Let's break down the proper configuration for both the lexer and parser.

Step 1: Configure the Lexer

The lexer, built using flex, is tasked with tokenizing input. Here is how you can configure it correctly:

Define Tokens: You should ensure that key tokens like COMMA and KEY are correctly declared. The KEY pattern must support multiple characters:

[[See Video to Reveal this Text or Code Snippet]]

Ignore Whitespace: You need a simple rule that recognizes whitespace characters and does nothing, making it easy to ignore them:

[[See Video to Reveal this Text or Code Snippet]]

Handle Other Inputs: Any unrecognized characters can be sent to the parser using:

[[See Video to Reveal this Text or Code Snippet]]

By implementing these changes, the lexer can effectively ignore whitespace, avoiding unnecessary syntax errors.

Here's how your lexer should look:

[[See Video to Reveal this Text or Code Snippet]]

Step 2: Configure the Parser

Now that the lexer is well-defined, let's address the parser built with bison. Consider the following adjustments:

Remove Unused Tokens: Since whitespace is ignored, the IGNORE token isn't needed in your parser. Bison often warns about such unused declarations.

Simplify the Production Rules: Your grammar can be simplified for readability and function without compromising on correctness. Improve the productions to simply handle the key sets and commas:

[[See Video to Reveal this Text or Code Snippet]]

Loop through Input: Modify the main function to read input continuously and parse each line accordingly. This prevents unnecessary parsing of empty lines.

Here's an updated version of your parser:

[[See Video to Reveal this Text or Code Snippet]]

Step 3: Compilation Instructions

When compiling your project, avoid using the legacy -y flag with bison. It may lead to compatibility issues. Instead, use:

[[See Video to Reveal this Text or Code Snippet]]

Conclusion: Error-Free Parsing

By following these steps to configure both your flex lexer and bison parser, you should be able to handle comma-separated input without experiencing syntax errors.

This solution guides you through a common issue encountered in constructing parsers and aids in creating a robust lexer that appropriately handles whitespace, simplifies your parsing logic, and sets you on a path towards easily parsing your desired input format.

Now, get coding and see your lexer and parser function efficiently!