Haskell 无法匹配由签名给出的类型

Question

Okay hi guys, I am having a weird problem.

hist :: [Int] -> [Int]
hist x = (foldr (+) 0 x)

The code above does not work, because trying to compile I am getting the error, that expected type '[Int]' could not be matched with actual type 'Int'. I don't get that.

Fun Fact: when I delete the signature, the function works fine! Does anybody know the fault?

Thank you!

Answer 1

The output of the foldr (+) 0 x will be an Int , not an [Int] . If you want a list, you can use scanl , but I guess, you still want foldr :

hist :: [Int] -> 
hist = foldr (+) 0

I however strongly advise to make use of sum :: (Foldable f, Num a) => fa -> a which also calculates the sum, but on all types that are instances of Num , and for all Foldable s (and thus not only a list).