📝 the Eval instance for Union.

src/Data/Abstract/Eval.hs

@@ -29,6 +29,7 @@ class Monad m => Eval term v m constr where
   default eval :: (MonadFail m, Show1 constr) => ((v -> m v) -> term -> m v) -> ((v -> m v) -> constr term -> m v)
   eval _ _ expr = fail $ "Eval unspecialized for " ++ liftShowsPrec (const (const id)) (const id) 0 expr ""
 
+-- | If we can evaluate any syntax which can occur in a 'Union', we can evaluate the 'Union'.
 instance (Monad m, Apply (Eval t v m) fs) => Eval t v m (Union fs) where
   eval ev yield = apply (Proxy :: Proxy (Eval t v m)) (eval ev yield)