Confusing behavior of Implicit Params

implicit_param_invalid_substitution.hs

{-# LANGUAGE ImplicitParams #-}

example1 :: (?a :: Int) => [Int]
example1 = let ?a = 4 in [foo,bar + bar,(?a + 3) + (?a + 3)]
  where
  foo = bar + bar
  bar = ?a + 3

example2 :: (?a :: Int) => [Int]
example2 = let ?a = 4 in [foo,bar + bar,(?a + 3) + (?a + 3)]
  where
  foo :: (?a :: Int) => Int
  foo = bar + bar
  bar :: (?a :: Int) => Int
  bar = ?a + 3

main :: IO ()
main = do
  print (let ?a = 11 in example1)
  print (let ?a = 11 in example2)

-- $ ./implicit_badness 
-- [28,28,14]
-- [14,14,14]