看起来还是很简单，但是深入到内部怎么理解？为什么要叫这个名字？
把
sum' :: (Num a) => [a] -> a
sum' xs = foldl (+) 0 xs
柯里化
sum' :: (Num a) => [a] -> a
sum' = foldl (+) 0
看起来还是很简单，但是深入到内部怎么理解？为什么要叫这个名字？
把
sum' :: (Num a) => [a] -> a
sum' xs = foldl (+) 0 xs
柯里化
sum' :: (Num a) => [a] -> a
sum' = foldl (+) 0