--- A map of a Core type to the corresponding type name
-type TypeMap = Map.Map HType (AST.VHDLId, Either AST.TypeDef AST.SubtypeIn)
+-- A map of a Core type to the corresponding type name, or Nothing when the
+-- type would be empty.
+type TypeMap = Map.Map HType (Maybe (AST.VHDLId, Either AST.TypeDef AST.SubtypeIn))