diff --git a/src/Data/Syntax/Assignment.hs b/src/Data/Syntax/Assignment.hs
index b957cd3d1..9e06a0c91 100644
--- a/src/Data/Syntax/Assignment.hs
+++ b/src/Data/Syntax/Assignment.hs
@@ -147,6 +147,7 @@ advanceState state@AssignmentState{..}
   | Rose (_ :. range :. span :. _) _ : rest <- stateNodes = AssignmentState (Info.end range) (Info.spanEnd span) (Source.drop (Info.end range - stateOffset) stateSource) rest
   | otherwise = state
 
+-- | State kept while running 'Assignment's.
 data AssignmentState grammar = AssignmentState
   { stateOffset :: Int -- ^ The offset into the Source thus far reached, measured in bytes.
   , statePos :: Info.SourcePos -- ^ The (1-indexed) line/column position in the Source thus far reached.