Antlr - Parse Tree (AST)

About

The tree parser is an AST that is created by the parser from a text input.

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Management

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Grun

with grun, see the -tree of -gui option of

Example:

grun grammar startRule -tree
grun grammar startRule -gui

Type your text and end with a End of File character (Ctrl+Z or Ctrl+D)

Idea

With the idea plugin:

• right click on the line of the start rule
• start to type a text to see the tree

Java

• api/Java/org/antlr/v4/runtime/tree/Trees.html - Static method
• api/Java/org/antlr/v4/runtime/tree/ParseTree.html - Parse tree

Build

Example from the getting started with an Hello.g4 grammar file that was generated in:

• a lexer file called HelloLexer.java
• a parser file called HelloParser.java

The grammar below will parse every hello [a-z]+ text.

grammar Hello;
r  : 'hello' ID ;   // Top parser rule of the tree
ID : [a-z]+ ;      
WS : [ \t\r\n]+ -> skip ;

To build a tree, you need to:

• Create a ANTLRInputStream stream of characters that will be used by the lexer

ANTLRInputStream input = new ANTLRInputStream("hello nico");

• Build the lexer from the input

HelloLexer lexer = new HelloLexer(input);

• Wrap the lexer into a stream of tokens CommonTokenStream (The parser can then read this stream back and forth)

CommonTokenStream tokens = new CommonTokenStream(lexer);

• Create a parser that will build the tree

HelloParser parser = new HelloParser(tokens);

• Finally, you can build the tree (a ParseTree) from the parser rule called r (present in the grammar file and generated in the parser as a function)

ParseTree tree = parser.r();    
System.out.println(tree.toStringTree(parser)); // print LISP-style tree