[Haskell-cafe] Need help with scrap-your-boilerplate

[Roman Cheplyaka]

On 06/10/14 21:24, Michael Sperber wrote:
> Would using a different generic-programming framework help me do this
> without the unsafe stuff?

Yes. For example, using my traverse-with-class library:

  {-# LANGUAGE TemplateHaskell, MultiParamTypeClasses,
      FlexibleInstances, ConstraintKinds, UndecidableInstances,
      ImplicitParams, OverlappingInstances #-}
  import Data.Generics.Traversable
  import Data.Generics.Traversable.TH
  import Data.Proxy

  data Foo a = Foo a

  deriveGTraversable ''Foo

  -- define a class around our generic operation
  class Bar b where
    bar :: b -> b

  -- default case
  instance Bar a where
    bar x = x

  -- Foo-specific case
  instance Bar (Foo a) where
    bar x = x

  -- deep traversal
  recursiveBar :: (Bar a, GTraversable (Rec Bar) a) => a -> a
  recursiveBar x =
    let ?c = Proxy :: Proxy Bar -- traversal with the Bar dictionary
    in everywhere bar x

Roman