[Haskell-beginners] Data.Foldable/foldMap question

Hi, I'm reading the LYAH book and on page 263 it says that if you define Data.Foldable.foldMap you get Data.Foldable.foldl and Data.Foldable.foldl for "free". The default implementation code of Data.Foldable.foldr is: foldr :: (a -> b -> b) -> b -> t a -> b foldr f z t = appEndo (foldMap (Endo . f) t) z I don't understand this code, but more specifically I don't get how there can a Monoid constraint on foldMap's return type and not on foldr/foldl. Can any one explain what Endo is and how it works? Thanks a lot, Patrick -- ===================== Patrick LeBoutillier Rosemère, Québec, Canada