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ghci> data Foo = Foo { name :: String, age :: Int } deriving Show ghci> let x = Foo { name = "hello", age = undefined } ghci> let y = x { age = 42 } ghci> y Foo {name = "hello", age = 42} ghci> x Foo {name = "hello", age = *** Exception: Prelude.undefined CallStack (from HasCallStack): undefined, called at <interactive>:17:37 in interactive:Ghci13 ghci>