Hi all I am experimenting with Trees that Grow [1] in the context of the GHC HsSyn AST, and wanting to express that a given extension point needs to have certain properties. The specific case is to be able to contain a SourceText, in the context of HsLit So I have (stripped down) data GHCX type family XHsString x type instance XHsString GHCX = SourceText class HasSourceText a where -- Provide setters to mimic existing constructors noSourceText :: a sourceText :: String -> a getSourceText :: a -> SourceText instance HasSourceText (XHsString GHCX) where noSourceText = NoSourceText sourceText s = SourceText s getSourceText a = a But this gives an error compiler/hsSyn/HsExtension.hs:80:10: error: • Illegal type synonym family application in instance: XHsString GHCX • In the instance declaration for ‘HasSourceText (XHsString GHCX)’ Is there some way to achieve this, or is it simply impossible? The full work-in-progress source is here[2] Regards Alan [1] https://arxiv.org/abs/1610.04799 [2] https://github.com/alanz/ghc/blob/df1c3b3d42284dd121086e6c571793f19e758977/c...