Main.hs

{-# LANGUAGE CPP #-}
{-# LANGUAGE FlexibleContexts #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE DataKinds #-}
{-# LANGUAGE KindSignatures #-}
{-# LANGUAGE TypeFamilies #-}
import GHC.TypeLits
import Data.Proxy

type family UpdateType (l :: Symbol) s b
type family FieldType (l :: Symbol) s

type instance UpdateType "fst" (x,y) a = (a,y)
type instance FieldType "fst" (x,y) = x

class 
#ifdef CXT
    FieldType l (UpdateType l s b) ~ b => 
#endif
    SetField l s b where
    setField :: Proxy l -> s -> b -> UpdateType l s b

instance SetField "fst" (a,b) x where
    setField _ (a,b) x = (x,b)

test1 xs x = setField (Proxy :: Proxy "fst") xs x :: ( Int , ())
{- with

ghci-8.0.0.20160204 -DCXT 
ghci-7.8.4
-- just not 7.10

test1
  :: (SetField "fst" s Int, UpdateType "fst" s Int ~ (Int, ())) =>
     s -> Int -> (Int, ())

without -DCXT

test1
  :: (SetField "fst" s b, UpdateType "fst" s b ~ (Int, ())) =>
     s -> b -> (Int, ())
-}