kosmikus

module FoldMutRec where

-- Note: DeriveFoldable could be used, but I am providing
-- hand-written instances to better illustrate the mechanics.

import Data.Monoid

data Forest a = Forest [Tree a]
data Tree a = Node a (Forest a)

kosmikus

{-# LANGUAGE DataKinds, TypeOperators, GADTs, TypeFamilies #-}
{-# LANGUAGE MultiParamTypeClasses, FlexibleInstances #-}
{-# LANGUAGE ScopedTypeVariables #-}
module FirstField where

import Generics.SOP

class HasHead (y :: *) (xs :: [*]) where
  hhead :: NP I xs -> K y xs
instance (x ~ y) => HasHead y (x ': xs) where