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langcc: A Next-Generation Compiler Compiler

langcc can serve as a replacement for lex+yacc, but is much more powerful. langcc enables a completely new paradigm for developing programming languages based on iteratively improving the intuitive declarative specification of your language instead of laboriously updating your hand-written compiler frontend for every grammar change.

• langcc generates efficient, linear-time parsers for an extremely broad class of grammars (a strict superset of canonical LR). In particular, the project includes complete specifications of Python 3.9.12 (grammars/py.lang) and Golang 1.17.8 (grammars/go.lang), and generates parsers for each language that are faster than their respective standard parsers (resp., 1.2x and 4.3x faster).
• langcc requires only a single input file in its declarative .lang format to generate a full compiler frontend, including AST struct definitions and associated traversals, hashing, pretty-printing, and automatic integration with the generated parser (no need to sprinkle C++ code into your grammar as in lex+yacc).
• langcc provides a built-in "conflict tracing" algorithm which traces LR conflicts back to "confusing input pairs", which provide explicit example strings instead of opaque shift/reduce conflicts.
• langcc extends the existing LR paradigm to include both recursive-descent parsing actions (which result in much smaller and more intuitive automata) and also per-symbol attribute constraints (which provide support for many industrial language constructs).
• langcc implements a novel transformation for LR grammars we call "continuation-passing style", which significantly expands the class of grammars the tool supports.
• Finally, langcc ships with a standalone datatype compiler called datacc, which generates full C++ implementations of algebraic datatypes (including sum types) from a simple declarative language whose spec is provided in data.lang.

langcc is self-hosting. The file grammars/meta.lang contains the specification for the "language of languages" in which langcc expects .lang files to be written. In this canonical implementation, the file bootstrap.sh uses langcc to generate its own compiler front-end.

langcc is stable and feature-complete. It is ready to be used as a research prototype to facilitate rapid exploration of new compilers and programming languages. It is free and open-source and available under the Apache 2.0 License.

langcc is also described in the following companion technical reports, which provide the theoretical basis for its functionality.

• Zimmerman, Joe. Practical LR Parser Generation. arXiv, 2022.
• Zimmerman, Joe. langcc: A Next-Generation Compiler Compiler. arXiv, 2022.

Build

This build has been verified to work on Ubuntu 22.04 and MacOS 12.5.

For Ubuntu 22.04:

./deps_ubuntu.sh
make -j8
sudo make install

For macOS 12.5 (requires Homebrew):

./deps_macos.sh
make -j8
sudo make install

And, in order to bootstrap the langcc front-end itself, subsequently run:

./bootstrap.sh

Examples

Once langcc (and its companion datacc) have been built, you can run the various provided examples:

• The most fully featured example is in examples/lox, which includes a complete interpreter for a full scripting language called Lox designed by Robert Nystrom at Crafting Interpreters. The Lox example is explored in detail in the user manual and is the recommended way to learn about langcc's feature set.
• examples/basic illustrates the basic syntax of a declarative language spec, while examples/calc implements a calculator for arithmetic expressions, including user-friendly error reporting.
• grammars/go.lang and grammars/py.lang provide complete specifications for the syntax of Golang 1.17.8 and Python 3.9.12, respectively. They showcase the full feature set of langcc and are compiled during the main build process itself, which produces tests build/go_standalone_test and build/py_standalone_test. (Note: These binaries require repositories for Golang 1.17.8 and Python 3.9.12 located in the directories ../go and ../cpython.)
• The language of datatypes, grammars/data.lang, describes the input format expected by the standalone datatype compiler datacc for specifying algebraic datatypes.
• Finally, the language of languages itself, grammars/meta.lang, describes the input format expected by langcc. This language also serves as basic documentation, as it enumerates all of its own features.

Documentation

For full documentation, please see the user manual.