As mentioned by Sam (280Z28), ANTLR 4 does not have rewrite operators. When generating the parser, ANTLR 4 creates some listener classes that you can use to listen for “enter” and “exit” events of all parser rules. Also, ANTLR 4 supports “direct left recursive rules”, so your expression rules can be defined in a single … Read more
Since I’ve had a little bit of a struggle with the two existing answers, I’d like to share the solution I ended up with. First of all I created my own version of an ErrorListener like Sam Harwell suggested: public class ThrowingErrorListener extends BaseErrorListener { public static final ThrowingErrorListener INSTANCE = new ThrowingErrorListener(); @Override public … Read more
There was 2 problems. One was the file has to be named “Hello.g4” not “hello.g4” because the grammar is specified as Hello. The second was the classpath, it requires the path and name of the jar file, as well as the current directory. The following command worked; javac -classpath .;C:\JavaLib\antlr-4.5-complete.jar *.java
ANTLR 4 produces parse trees based on the grammar instead of ASTs based on arbitrary AST operators and/or rewrite rules. This allows ANTLR 4 to automatically produce listener and visitor interfaces that you can implement in the code using your grammar. The change can be dramatic for users upgrading existing applications from version 3, but … Read more
Negating can occur inside lexer and parser rules. Inside lexer rules you can negate characters, and inside parser rules you can negate tokens (lexer rules). But both lexer- and parser rules can only negate either single characters, or single tokens, respectively. A couple of examples: lexer rules To match one or more characters except lowercase … Read more
The error message means that the expected token type containing the value ‘void’ does not match the actual token type produced by consuming the string ‘void’ from the input. Looking at your lexer rules suggests that the input string ‘void’ is being consumed by the IDENTIFIER rule, producing a token of type IDENTIFIER, not VOID. … Read more
This seems to be a common misunderstanding of ANTLR: Language Processing in ANTLR: The Language Processing is done in two strictly separated phases: Lexing, i.e. partitioning the text into tokens Parsing, i.e. building a parse tree from the tokens Since lexing must preceed parsing there is a consequence: The lexer is independent of the parser, … Read more
In ANTLR, the lexer is isolated from the parser, which means it will split the text into typed tokens according to the lexer grammar rules, and the parser has no influence on this process (it cannot say “give me an INTEGER now” for instance). It produces a token stream by itself. Furthermore, the parser doesn’t … Read more
By default, ANTLR 4 generates listeners. But if you give org.antlr.v4.Tool the command line parameter -visitor, ANTLR generates visitor classes for you. These work much like listeners, but give you more control over which (sub) trees are walked/visited. This is particularly useful if you want to exclude certain (sub) trees (like else/if blocks, as in … Read more
Ok, let’s build a simple math example. Building an AST is totally overkill for such a task but it’s a nice way to show the principle. I’ll do it in C# but the Java version would be very similar. The grammar First, let’s write a very basic math grammar to work with: grammar Math; compileUnit … Read more