diff --git a/outlines/generate/generator.py b/outlines/generate/generator.py
index 1a6dfde48..f76e449a1 100644
--- a/outlines/generate/generator.py
+++ b/outlines/generate/generator.py
@@ -6,6 +6,7 @@
 
 if TYPE_CHECKING:
     from outlines.generate.samplers import Sampler
+    from outlines.index.index import Index
 
 
 @dataclass
@@ -15,7 +16,7 @@ class GenerationState:
     kv_cache: Optional[torch.Tensor] = None
 
 
-def process(generator: Generator, index, state: GenerationState):
+def process(generator: Generator, index: "Index", state: GenerationState):
     """This generator drives the text generation process by
     walking through the FSM."""
     next(generator)
diff --git a/outlines/index/index.py b/outlines/index/index.py
new file mode 100644
index 000000000..89e49393f
--- /dev/null
+++ b/outlines/index/index.py
@@ -0,0 +1,11 @@
+from typing import Callable, NamedTuple, NewType
+
+import torch
+
+State = NewType("State", int)
+
+
+class Index(NamedTuple):
+    next_instruction: Callable[[State], torch.Tensor]
+    next_state: Callable[[State, torch.Tensor], State]
+    is_final: Callable[[State], bool]