joins :: String -> String -> Bool
joins left right = last left == head right

example :: [String]
example =
  [ unwords [w1, w2, w3, w4]
  | w1 <- ["the", "that", "a"]
  , w2 <- ["frog", "elephant", "thing"]
  , w3 <- ["walked", "treaded", "grows"]
  , w4 <- ["slowly", "quickly"]
  , joins w1 w2
  , joins w2 w3
  , joins w3 w4 ]

main :: IO ()
main = print example