[Haskell-beginners] Data.Foldable/foldMap question

[Patrick LeBoutillier]

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





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Patrick LeBoutillier
Rosemère, Québec, Canada