如何理解这种类型类错误，告诉我我使用了错误的类型？

这是我计算数字列表的平均值的功能

arithMean :: (Fractional a) => [a] -> a
arithMean list = (foldr (+) 0 list)/ (length list)

但是，它在下面引发了一个错误

HigherOrder.hs:10:39: error:
? Couldn't match expected type ‘a’ with actual type ‘Int’
  ‘a’ is a rigid type variable bound by
    the type signature for:
      arithMean :: forall a. Fractional a => [a] -> a
    at HigherOrder.hs:9:1-39
? In the second argument of ‘(/)’, namely ‘(length list)’
  In the expression: (foldr (+) 0 list) / (length list)
  In an equation for ‘arithMean’:
      arithMean list = (foldr (+) 0 list) / (length list)
? Relevant bindings include
    list :: [a] (bound at HigherOrder.hs:10:11)
    arithMean :: [a] -> a (bound at HigherOrder.hs:10:1)

关于此错误，我对为什么迫使我将“列表”更改为[Int]感到非常困惑，我认为无论它是哪种类型，它都应该起作用

此外，我认为当我尝试使用“长度”函数时可能发生了某些事情，因为当我在下面编写此代码时，它可以工作

arithMean :: (Fractional a) => [a] -> a
arithMean list = (foldr (+) 0 list)