Grammar: Preliminary Formatron KBNF support

backends/exllamav2/grammar.py

@@ -1,4 +1,5 @@
 import traceback
+import typing
 from exllamav2 import ExLlamaV2, ExLlamaV2Tokenizer
 from exllamav2.generator.filters import ExLlamaV2Filter
 from loguru import logger
@@ -7,6 +8,7 @@
 from formatron.formatter import FormatterBuilder
 from formatron.schemas import json_schema
 from formatron.integrations.exllamav2 import create_formatter_filter
+from formatron.extractor import NonterminalExtractor
 
 
 def clear_grammar_func_cache():
@@ -98,7 +100,11 @@ def add_kbnf_filter(
         try:
             # Validate KBNF and create formatter
             f = FormatterBuilder()
-            # TODO: Implement this
+            f.append_line(
+                f"{f.extractor(
+                    lambda nonterminal: CustomExtractor(nonterminal, kbnf_string)
+                    )}"
+            )
         except Exception:
             logger.error(
                 "Skipping because the KBNF string couldn't be parsed. "
@@ -111,3 +117,19 @@ def add_kbnf_filter(
 
         # Append the filters
         self.filters.append(lmfilter)
+
+
+class CustomExtractor(NonterminalExtractor):
+    def __init__(self, nonterminal: str, kbnf_string: str):
+        super().__init__(nonterminal)
+        self.kbnf_string = kbnf_string
+
+    # Fails without an extract function defined
+    # No idea what it does or why it's needed, but this seems to work
+    # TODO: Figure out how to do this properly
+    def extract(self, input_str: str) -> typing.Optional[tuple[str, typing.Any]]:
+        return input_str[len(input_str) :], input_str[: len(input_str)]
+
+    @property
+    def kbnf_definition(self) -> str:
+        return self.kbnf_string.replace("start", self.nonterminal)