Record Update with RecordWildCards

recordupdate.hs

{-# LANGUAGE RecordWildCards #-}
{-# LANGUAGE DuplicateRecordFields #-}
{-# LANGUAGE NamedFieldPuns#-}

data C = C { c::[Int] } deriving Show
data B = B { b::Bool, c::C } deriving Show
data A = A { a::String, b::B } deriving Show

mkA = A "hello world" (B True (C [1,2,3]))

updateC A{b=B{c=C{..},..},..} =
  let c = [2,3,4] -- not confused by the 'c' in 'B'!
      b = False
  in A{b=B{c=C{c,..},..},..}

main = print (updateC mkA)
-- A {a = "hello world", b = B {b = False, c = C {c = [2,3,4]}}}