Some "GHC-internal" types leak to users via TH, and their constructors occasionally pick up new fields, causing breakage downstream. The extra field often has a sensible default (Nothing, [], ...) and it should be best practice to rename the constructor when adding the new field, while replacing the original constructor with a pattern synonym with the "old" signature. data Foo = ... | NewImprovedMkFoo X Y Z -- was MkFoo Y Z pattern MkFoo :: Foo pattern MkFoo Y Z = NewImprovedMkFoo Nothing Y Z When pattern synonyms are used to maintain a backwards-compatible API, there should of course be no special signalling to differentiate them from "real" constructors. The boundary between "GHC-internal" and external may not always be obvious, some care is required to reduce leaking breakage via TH. -- Viktor.