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英文: Folding.

foldl :: Foldable t => (b -> a -> b) -> b -> t a -> b
foldr :: Foldable t => (a -> b -> b) -> b -> t a -> b

foldl1 :: Foldable t => (a -> a -> a) -> t a -> a
foldr1 :: Foldable t => (a -> a -> a) -> t a -> a

类似 C++ 的 std::ranges::fold_left 和 std::ranges::fold_right.

sum' = foldl (+) 0
and' = foldl1 (&&) True
or' = foldl1 (||) False

参考

• https://wiki.haskell.org/Fold.

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