Haskell尾函数用于空列表

Question

我有一个函数的问题,该函数应该只返回列表的尾部.函数是myTail,即使输入是空列表,也应该给出一个可用的结果.

我想了解所有3种方式:模式匹配,保护方程和条件表达式

这工作:

> myTail_pat :: [a] -> [a]

> myTail_pat (x:xs) = xs
> myTail_pat [] = []

但是这个:

> myTail_guard (x:xs)   | null xs = []
>               | otherwise = xs

给我错误:程序错误:模式匹配失败:myTail_guard []如何声明没有模式的函数？

谢谢.

Answer 1

该模式x:xs与空列表不匹配.你需要这样做:

myTail_guard xs
  | null xs   = []
  | otherwise = tail xs

Answer 2

1滴是安全的

drop 1 []
-- result: []