小编rea*_*ado的帖子

f1 [] = 1
f1 (x:xs) = x * f1 xs

f2 [] = 0
f2 (x:xs) = 1 + f2 xs

f3 [] = 0
f3 (x:xs) = x + f3 xs

f4 [] = []
f4 (x:xs) = x ++ f4 xs

这些都有一个共同的行为,我究竟如何识别模式并编写高阶函数来捕获它？