我想定义一个函数来计算列表中满足给定谓词的元素数量:
number_of_elements :: (a -> Bool) -> [a] -> Int
number_of_elements f xs = length (filter f xs)
例如:
number_of_elements (==2) [2,1,54,1,2]
应该返回2。
我们可以写得更短:
number_of_elements f = length . filter f
没有f参数可以写吗?