gistfile1.hs

data Foo = Foo { foo :: Int }

x0 = Foo { foo = 3 }          -- create
x1 = x0 { foo = 4 }           -- replace
x2 = foo x1                   -- access
x3 = x1 { foo = foo x1 + 1 }  -- apply

gistfile2.hs

upd :: Name -> Q Exp

x3 = $(upd 'foo) (+ 1) x1

gistfile3.hs

old :: State Foo ()
old = modify (\x -> x { foo = foo x + 1 })

new :: State Foo ()
new = modify $ $(upd 'foo) (+ 1)