{-# LANGUAGE TemplateHaskell #-}
{-# LANGUAGE GADTs #-}
module Widget (
  widget
) where

import Control.Lens.TH

data Widget = Widget -- the second Widget is value constructor you missed
  { _widgetX :: Int  -- _ is a convention, because _widgetX is useless
  , _widgetY :: Int  -- because of bad design of records
  , _widgetZ :: Int  -- the prefixes are also a convention, mere _x is useless t                                                                                                                oo
  } deriving (Show, Ord, Eq)

makeLenses ''Widget

class WidgetClass a where
  moveX :: Int -> a -> a -- sort of virtual method declaration

instance WidgetClass Widget where
  moveX = undefined -- sort of virtual method definition

widget :: Int -> Int -> Int -> Widget
widget x y z | x > 0 && y > 0 && z > 0 = Widget x y z -- smart constructor
widget _ _ _ = error "parameters out of range"

data IWidget where
  IWidget :: (Show a, WidgetClass a) => a -> IWidget

instance Show IWidget where
  show (IWidget x) = show x

data Dummy = Dummy Int deriving Show
instance WidgetClass Dummy where
   moveX = undefined

main = print $  [IWidget $ widget 1 1 2, IWidget $ Dummy 42]