Mention the result in the error message for JoinKinds

Fixes #453.
well-typed · May 20, 2022 · 5fcc03f · 5fcc03f
1 parent 337165b
commit 5fcc03f
Showing 1 changed file with 3 additions and 1 deletion.
diff --git a/optics-core/src/Optics/Internal/Optic/Subtyping.hs b/optics-core/src/Optics/Internal/Optic/Subtyping.hs
@@ -434,6 +434,8 @@ instance k ~ A_Setter           => JoinKinds A_Setter           A_Traversal
 
 instance {-# OVERLAPPABLE #-}
   ( JoinKinds k l m
-  , TypeError ('ShowType k ':<>: 'Text " cannot be composed with " ':<>: 'ShowType l)
+  , TypeError ('ShowType k ':<>: 'Text " cannot be composed with " ':<>:
+               'ShowType l ':<>: 'Text " into " ':<>:
+               'ShowType m)
   ) => JoinKinds k l m where
   joinKinds _ = error "unreachable"