updated documentation strings to include the case of empty tag in a u…

…nion

dhall/src/Dhall/Marshal/Encode.hs

@@ -943,6 +943,7 @@ encodeFieldWith name encodeType = RecordEncoder $ Dhall.Map.singleton name encod
 data Status = Queued Natural
             | Result Text
             | Errored Text
+            | Unreachable ()
 :}
 
     And assume that we have the following Dhall union that we would like to
@@ -951,6 +952,7 @@ data Status = Queued Natural
 > < Result : Text
 > | Queued : Natural
 > | Errored : Text
+> | Unreachable 
 > >.Result "Finish successfully"
 
     Our encoder has type 'Encoder' @Status@, but we can't build that out of any
@@ -963,11 +965,13 @@ injectStatus = adapt >$< unionEncoder
   (   encodeConstructorWith "Queued"  inject
   >|< encodeConstructorWith "Result"  inject
   >|< encodeConstructorWith "Errored" inject
+  >|< encodeConstructorWith "Unreachable" inject
   )
   where
-    adapt (Queued  n) = Left n
-    adapt (Result  t) = Right (Left t)
-    adapt (Errored e) = Right (Right e)
+    adapt (Queued  n) = Just (Left n)
+    adapt (Result  t) = Just (Right (Left t))
+    adapt (Errored e) = Just (Right (Right e))
+    adapt Unreachable = Nothing
 :}
 
     Or, since we are simply using the `ToDhall` instance to inject each branch, we could write
@@ -978,11 +982,13 @@ injectStatus = adapt >$< unionEncoder
   (   encodeConstructor "Queued"
   >|< encodeConstructor "Result"
   >|< encodeConstructor "Errored"
+  >|< encodeConstructor "Unreachable"
   )
   where
-    adapt (Queued  n) = Left n
-    adapt (Result  t) = Right (Left t)
-    adapt (Errored e) = Right (Right e)
+    adapt (Queued  n) = Just (Left n)
+    adapt (Result  t) = Just (Right (Left t))
+    adapt (Errored e) = Just (Right (Right e))
+    adapt Unreachable = Nothing
 :}
 
 -}