Module pattern 

The pattern compiler: a code snippet with metavariable holes → a tree-sitter query.

This is the atomic-tier pattern form. The idea (from ast-grep) is to let rule authors write a snippet of real code with $VAR holes instead of hand-authoring a tree-sitter S-expression query:

  $SRC?.$KEY ?? $DEFAULT

compiles to

  ((binary_expression
     left: (member_expression object: (_) @SRC (optional_chain) property: (_) @KEY)
     "??"
     right: (_) @DEFAULT) @__match)

How it works

1. Each $VAR is replaced with a unique placeholder identifier so the snippet parses as ordinary code in the target grammar.
2. We parse the substituted snippet — bare, then in a per-language wrapper context (e.g. a function body) when the fragment is not a valid compilation unit — and locate the snippet’s own subtree by its byte range in the parsed source.
3. We walk that subtree and mirror it into a query: every named child is emitted with its field name, every anonymous token (operators, keywords, punctuation) is emitted as a quoted literal so the structure is matched precisely, and every placeholder becomes a (_) @VAR wildcard capture.
4. Repeated metavariables unify: the second and later occurrences get helper captures plus an (#eq? …) predicate so $X … $X only matches when both holes carry identical text.

Variadic $$$ holes are not yet supported (tracked for the relational tier, #2833); they compile to a clear error.

Structs

CompiledPattern

A snippet pattern compiled to a tree-sitter query string.

Constants

ROOT_CAPTURE

The capture name bound to the whole matched pattern, used for range extraction. Chosen to not collide with a user metavar (which are uppercase by convention and never start with __).

Functions

compile_pattern

Compile a pattern snippet for language into a tree-sitter query.