Hi devs, I would like to add the support for the following automatic instance-deriving extension: module M where class G a where doG :: a -> Int class P a where doP :: a -> Int doP _ = 10 deriving instance G a where -- automatically derived instance doG = doP data X = X instance P X -- derive G X automatically print (doG X) -- print 10 See the forwarded mail below for the real context. This extension has been proposed by someone before as InstanceTemplates: https://ghc.haskell.org/trac/ghc/wiki/InstanceTemplates I have modified the parser and the renamer accordingly to get: class M.G a_awb where M.doG :: a_awb -> Int class M.P a_ap6 where M.doP :: a_ap6 -> Int M.doP _ = 10 instance M.G a_ap6 where M.doG = M.doP I am new to the compiler part of GHC, so I have a few questions before I continue: 1a) does it make sense to store the renamed class instance declaration in an interface file? (supposing it only uses exported methods/types; we could check that) 1b) will it be possible to create a new instance declaration in another module by just doing the substitution [a_ap6 -> X] in it? (i.e. when we parse "instance P X", do we know that it means [a_ap6 -> X] in class P (and not [a -> X])?) 2) maybe I should go a different way and store only the derived instance methods as we store class default methods? Any insight appreciated! Thanks, Sylvain ---------- Forwarded message ---------- From: Sylvain Henry <hsyl20@gmail.com> Date: 2016-03-05 14:56 GMT+01:00 Subject: Reducing boilerplate To: Haskell Cafe <haskell-cafe@haskell.org> Hi, To write FFI bindings, I use c-storable-deriving [1] to automatically derive CStorable instances for many data types (the only difference between Storable and CStorable is that CStorable provides default methods for types that have Generic instances): {-# LANGUAGE DeriveGeneric #-} {-# LANGUAGE DeriveAnyClass #-} ... data X = X { fieldStatus :: Vector 24 Word8 , fieldPadding :: Word8 } deriving (Generic, CStorable) However I also need a Storable instance, hence I have to write (the "c*" methods are provided by CStorable): instance Storable X where peek = cPeek poke = cPoke alignment = cAlignment sizeOf = cSizeOf Is there a way to automatically generate this instance for every data that has an instance of CStorable? Ideally, I would like to say once and for all: instance CStorable a => Storable a where peek = cPeek poke = cPoke alignment = cAlignment sizeOf = cSizeOf As I don't think it is currently possible, would it be sensible to add the support for automatically derived instances attached to classes (as a GHC extension)? Regards, Sylvain [1] https://hackage.haskell.org/package/c-storable-deriving