{-# LANGUAGE TemplateHaskell #-}

import Data.Foldable
import Language.Haskell.TH

data Point = Point { x :: Double
                   , y :: Double
                   }
  deriving Show
data Point3 = Point3 { x3, y3, z3 :: Double }
  deriving Show

let
  zipFields :: Q Exp -> Q Exp -> [Q Exp] -> Q Exp
  zipFields ctor op fields =
    foldl' (\prefix field -> [|$prefix ($op ($field a) ($field b))|]) ctor fields
  in [d|
    addPoint :: Point -> Point -> Point
    addPoint a b = $(zipFields [|Point|] [|(+)|] [[|x|], [|y|]])
    
    addPoint3 :: Point3 -> Point3 -> Point3
    addPoint3 a b = $(zipFields [|Point3|] [|(+)|] [[|x3|], [|y3|], [|z3|]])
  |]

main :: IO ()
main = do
  print $ addPoint (Point 1.0 2.0) (Point 0.3 0.4)
  print $ addPoint3 (Point3 1.0 2.0 3.0) (Point3 0.4 0.5 0.6)