====== Key Differences between CongoCC and Legacy JavaCC ======

From the end user's point of view, the most important difference is that CongoCC has undergone quite a bit of re-design to make it much more usable "out of the box" than the legacy JavaCC. One of the most basic (and obvious) things that CongoCC provides is the [[INCLUDE]] statement. With legacy JavaCC, the only way to reuse commonly used constructs across different grammars was via the classic copy-paste //antipattern//.

There has been an effort to clean up the set of configuration options. In general, the philosophy of JavaCC 21 is to make configuration options largely unnecessary, at least for typical usage, since the defaults are set sensibly and, in the absence of configuration settings, the tool simply infers naming via conventions. See [[convention over configuration]] for more information.

It seems quite clear that building an AST [[Abstract Syntax Tree]] is the most typical use case for this sort of tool. So, there has been a heavy focus on making the whole thing much simpler. In legacy JavaCC, generating a parser that builds an AST is actually a rather baroque build process. You write a grammar with special "tree-buildling annotations" that you process with the legacy JJTree tool, which is really a //pre-processor// that in turn generates a JavaCC grammar. Then you run JavaCC on that to generate your Java source code.

With Congo, the JJTree pre-processor functionality is merged into the JavaCC tool and, even in the absence of special tree-building annotations (though they are still supported) the generated parser simply builds an AST by default, following some common-sense conventions. (N.B. You can still generate a parser that does not automatically build an AST, but you need to specify that via TREE_BUILDING_ENABLED=false in the settings.)

===== Tree Building Enhancements =====

Aside from being the out-of-the-box default, tree building has been enhanced considerably compared to what JJTree offers. In particular, the generated **Token** class now implements the **Node** interface. So, optionally, Tokens (both regular tokens and special tokens) may be added to the generated parse tree. (Regular tokens are added to the AST by default, while "Special tokens" (which usually represent comments in source code) are not included. They can be included via the **SPECIAL_TOKENS_ARE_NODES** setting, which is false by default. Regular tokens are added to the AST as nodes by default, but this can be turned off by setting the **TOKENS_ARE_NODES** setting to false. See [[https://github.com/revusky/freecc/wiki/Tree-Building-Enhancements|Tree Building Enhancements]] for more info.

===== Code Injection =====

CongoCC introduces a new statement called **INJECT** that allows you to "inject" code into the files that the tool generates. This can help you to avoid the error-prone anti-pattern of generating code and editing it afterwards. See [[Code Injection in JavaCC 21]] for more information.

===== Streamlined Syntax =====

CongoCC incorporates an [[new syntax summary|alternative streamlined syntax]] that should be quite a bit more pleasant to write and easier to read.

The difference is frequently dramatic. Where the legacy tool required you to write things like:

<code>
    LOOKAHEAD (Foo() Bar()) Foo() Bar() Baz()
</code>

in CongoCC you could express the above as:

<code>
     Foo Bar =>|| Baz
</code>         

===== More powerful lookahead =====

Perhaps most importantly, the longstanding bug of nested syntactic lookahead not working correctly has [[https://javacc.com/2020/07/15/nested-syntactic-lookahead-works/|finally been squashed]]!

The ''SCAN'' construct (designed to supersede the legacy ''LOOKAHEAD'') offers a superset of the legacy ''LOOKAHEAD'' functionality. [[contextual_predicates]] predicates allow you to define conditions at [[choice points]] based on scanning backwards in the parse/lookahead stack. [[contextual_predicates]] also works in arbitrarily nested scanahead.

The new [[up to here]] construct should eliminate the need to write more verbose and error-prone numerical and syntactic lookahead constructs. 




===== CongoCC is being actively developed =====

CongoCC now supports the full Java language up through Java 19. Since the [[https://github.com/congo-cc/congo-parser-generator/blob/main/examples/java/Java.ccc|Java grammar]] is embedded in [[https://github.com/congo-cc/congo-parser-generator/blob/main/src/grammars/CongoCC.ccc#L445|CongoCC grammar]] using the [[INCLUDE]] mechanism, that Java grammar is usable on its own. Note that this grammar successfully parses all the Java source code in the OpenJDK 20, as well as all the Java source code in JRuby, Jython, and FreeMarker. So, if anybody needs a Java code parser for use in their own projects, this is quite usable!