Hello, I just used the new GHC generics together with the DefaultSignatures extension to provide a default generic implementation for toJSON and parseJSON in the aeson package: https://github.com/mailrank/aeson/pull/26 It appears that the generic representation of a sum type has a tree shape as in: (a :+: b) :+: (c :+: d) In my case this tree-shaped representation is problematic when parsing a JSON value to this type. My overloaded parsing function is parameterized with a key which specifies which of the a, b, c or d constructors to parse. When it encounters a constructor it checks if it matches the key, if so it is parsed, if not parsing will fail. Because of the tree-shaped representation of sum types I have to recursively parse the left and right branch and join them using <|>: https://github.com/basvandijk/aeson/blob/d5535817ceb192aa9d7d0d0b291e1901f3f... I don't know for sure but I suspect that this can cause memory leaks since the <|> has to keep the right value in memory when it is parsing the left. Ideally the generic representation of sum types is right associative as in: a :+: (b :+: (c :+: d)) This way I only have to check if 'a' matches, if it does the right branch can be forgotten. Is there a good reason for not having it right associative? Regards, Bas