diff --git a/src/Data/Abstract/Type.hs b/src/Data/Abstract/Type.hs
index 9c8b2a80c..9585097c9 100644
--- a/src/Data/Abstract/Type.hs
+++ b/src/Data/Abstract/Type.hs
@@ -4,6 +4,7 @@ import Control.Monad.Fail
 import Data.Traversable
 import Prelude hiding (fail)
 
+-- | The type of type variable names.
 type TName = Int
 
 data Type