[post] ANTLR parser generator and visitor pattern

[at15]

Type

• request post from @at15

Related

• https://github.com/at15/gegecece

Description

While working on gegecece and writing SQL parser (ANTLR rule). The main problem is converting parser tree to AST.
AST in SQL is (unresolved) logical plan. There are many ways to define the rule and to traverse the parser tree.
A common pattern is called visitor pattern. However, when looking at code in some dbs e.g. tidb, es. they didn't build the plan in a very visitor way. This post tries to address the following questions

• what is visitor pattern, how to implement one (in different languages)
• use visitor pattern in ANTLR
• not using visitor pattern in ANTLR (when it comes to SQL)
• alternative label in ANTLR & visitor

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