In the Trees that Grow paper, it is explained that GHC has a single data type HsSyn that crosses several compiler phases; a second data type TH.Syntax for Template Haskell and that other Haskell libraries e.g. haskell-src-exts defnining yet others. Ideally, HsSyn would be reused in Template Haskell and these third-party libraries and motivates the flexibilities offered by the TTG (Trees That Grow) techniques.
