BAML grammar companion for treesitter-chunker

This optional package provides a pinned Tree-sitter BAML grammar through a Python language() capsule. It is separate from the universal treesitter-chunker wheel so projects that do not use BAML do not install a native extension.

The grammar comes from BoundaryML's read-only baml-treesitter mirror at the commit recorded in pin.json. overlay.patch makes one source change: prompt_field accepts the grammar's existing backtick_string production as well as raw_string. This preserves the original source bytes, including interpolation and line endings. The overlay is temporary until the upstream BAML grammar accepts this prompt form. The package still needs a separate distribution until treesitter-chunker deliberately adopts an upstream BAML grammar in its pinned language pack.

from tree_sitter import Language, Parser
from treesitter_chunker_baml_grammar import language

parser = Parser(Language(language()))
tree = parser.parse(b"function greet() -> string { prompt: `Hello` }")
assert not tree.root_node.has_error

The source distribution needs a C compiler at installation time. The wheel workflow builds cp311-abi3 wheels for Linux glibc x86_64/aarch64, macOS x86_64/arm64, and Windows x86_64 and tests them on Python 3.11–3.13. Other platforms use the source distribution until a wheel is tested there. Run python scripts/check_baml_grammar.py to verify vendored hashes offline; explicit regeneration requires the pinned tree-sitter-cli executable.

Feasibility evidence

At the pinned mirror commit, the unmodified grammar rejected the BAML 0.20.1 backtick prompt: form. The one-rule overlay generated an ABI-15 parser with Tree-sitter CLI 0.25.10 without a conflict or warning. All 65 upstream corpus cases passed. The focused fixtures cover the release example, old raw prompts, multiline and block interpolation, adjacent declarations, bigints, generics, Unicode, CRLF, and every initial declaration kind. They parse without ERROR or MISSING nodes. Tests check original byte spans for prompt delimiters, interpolations, and CRLF declarations. pin.json records the upstream source, overlay, generator executable, and generated output SHA-256 values.

implements_for_declaration exposes interface and target fields but no name field. The chunker integration must derive its stable symbol from those fields; the grammar package preserves both spans.