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Rationale

竜 TatSu was created to address some recurring problems encountered over decades of working with parser generation tools:

Some programming languages allow the use of keywords as identifiers, or have different meanings for symbols depending on context (Ruby). A parser needs control of lexical analysis to be able to handle those languages.
LL and LR grammars become contaminated with myriads of lookahead statements to deal with ambiguous constructs in the source language. PEG parsers address ambiguity from the onset.
Separating the grammar from the code that implements the semantics, and using a variation of a well-known grammar syntax (EBNF) allows for full declarative power in language descriptions. General-purpose programming languages are not up to the task.
Semantic actions do not belong in a grammar. They create yet another programming language to deal with when doing parsing and translation: the source language, the grammar language, the semantics language, the generated parser’s language, and the translation’s target language. Most grammar parsers do not check the syntax of embedded semantic actions, so errors get reported at awkward moments, and against the generated code, not against the grammar.
Preprocessing (like dealing with includes, fixed column formats, or structure-through-indentation) belongs in well-designed program code; not in the grammar.
It is easy to recruit help with knowledge about a mainstream programming language like Python, but help is hard to find for working with complex grammar-description languages. 竜 TatSu grammars are in the spirit of a Translators and Interpreters 101 course (if something is hard to explain to a college student, it’s probably too complicated, or not well understood).
Generated parsers should be easy to read and debug by humans. Looking at the generated source code is sometimes the only way to find problems in a grammar, the semantic actions, or in the parser generator itself. It’s inconvenient to trust generated code that one cannot understand.
Python is a great language for working with language parsing and translation.