Sorry to be negative and this comment probably doesn't belong in a discussion about a specific parsing toolkit but I've become unconvinced that parser generators are useful. My experience is limited to Yacc/lex back in the old days (quickly jumped to Bison/flex), more recently Antlr and a couple of functional parser combinator libraries. In nearly all case it was to deal with "real world" (i.e. not toy) programming languages.

The last time I needed a parser (in Java), I started studying the Antlr docs (it's changed quite a bit since I used it last) but became disillusioned quickly with the amount of reading and studying I would have to do to get something working.

So I quickly wrote a "hand crafted" tokenizer and recursive descent parser. I found this so satisfying that it made me wonder why I had bothered learning relatively complex tools in the past particularly since I had been exposed to recursive descent parsing as an undergrad.

Advantages that pop into my head:

- The code was clean, readable and very concise. For debugging, the stacktraces were helpful and I could use my regular debugger/IDE to step through the parsing process. The method names in my Parser class mostly matched the names of corresponding grammar rules.

- You can code around the theoretical limitations of recursive descent parsing in a very intuitive manner (e.g. "if (tokens.peekAhead(1).getType() == Token.LEFT_BRACE) { parseX(); } else { parseY(); }"). In theory it might seem this would lead to a mess but it actually allows very flexible and natural abstractions.

- You have complete control over the building of the AST - the parseX(...) methods can take arguments or the calling parse method can manipulate the returned AST - doing stuff like flattening (normalising) node trees or re-ordering child nodes, etc. The shape of the AST can be independent of the structure of the grammar rules.

- It's easy to provide helpful error messages and even error recovery without fighting with the toolkit. Better still, you can start with a fairly lazy generic error handler and later, in a natural style, add special cases to make the messages more and more helpful for specific common user mistakes. I sneakily logged all parse failures by users to constantly improve error reporting. After a while the parser seemed almost like an AI when reporting errors.

- For parsing expressions, there is a relatively well-known way to deal with operators with different arities and associativity rules (by adding a numeric "context binding strength" parameter to your parseExpr() method) - a quick google provided the template.

- The entire parser was self contained in a small number of reasonably compact classes: a Lexer/Tokenizer class, a Parser class and a SymbolTable class (and of course a TokenType enum and an ASTNode class). Other developers could grok the code because it was compact and self contained without having to learn a parsing toolkit.

- You feel in control; i.e. you can add features to the language and the parser incrementally without fearing that sinking feeling you get when you think you're 99% of the way there only to realize that the tool you're using makes the last 1% impossible forcing you to rethink/rewrite already "banked" functionality.

- Zero dependencies and trivial to integrate into the build and test process.

edit: paragraphs