Mention the result in the error message for JoinKinds

Fixes #453.
well-typed · May 20, 2022 · ab4c027 · ab4c027
1 parent 337165b
commit ab4c027
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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"
diff --git a/optics/src/Optics.hs b/optics/src/Optics.hs
@@ -539,7 +539,7 @@ import Data.Either.Optics                    as P
 --
 -- >>> :t mapped % folded
 -- ...
--- ...A_Setter cannot be composed with A_Fold
+-- ...A_Setter cannot be composed with A_Fold...
 -- ...
 --
 -- The ('Control.Category..') operator from "Control.Category" cannot be used to