haskell - 模式与守卫 : otherwise does not match?

Question

当给定一个空字符串时，以下两个函数的行为有所不同:

guardMatch l@(x:xs) 
    | x == '-'        = "negative " ++ xs
    | otherwise       = l

patternMatch ('-':xs) = "negative " ++ xs
patternMatch l        = l

这是我的输出:

*Main> guardMatch ""
"*** Exception: matching.hs:(1,1)-(3,20): Non-exhaustive patterns in function guardMatch

*Main> patternMatch ""
""

问题:为什么“否则”关闭不捕获空字符串？

Answer 1

otherwise在模式l@(x:xs)的范围内, 只能匹配一个非空字符串。看看这(有效地)在内部转化为什么可能会有所帮助:

guardMatch   l = case l of
                   (x  :xs) -> if x == '-' then "negative " ++ xs else l
patternMatch l = case l of
                   ('-':xs) ->                  "negative " ++ xs
                   _        ->                                         l

(实际上，我认为 if 被翻译成 case + 守卫而不是相反。)