How to understand “(Num b) => length :: [a] -> b” in Haskell?

Question

I am reading Learn You a Haskell for Great Good! , and come across the following word:

For instance, the length function has a type declaration of length :: [a] -> Int instead of having a more general type of (Num b) => length :: [a] -> b.

How to understand " (Num b) => length :: [a] -> b " here? Shouldn't it be " length :: (Num b) => [a] -> b "?

Answer 1

你是对的：这本书中的错误应该按照你的建议阅读。