图案重叠,为什么会发生？

Question

我有一个文件OptionList.hs,有一个名为OptionList的新数据类型.我想在显示OptionList时隐藏EmptyOpt:

module OptionList (
    OptionList,
    voidOption,
    (+:)
) where


data OptionList a b = EmptyOpt | OptionList { optListHead :: a, optListTail :: b } deriving (Read)

instance (Show a, Show b) => Show (OptionList a b) where
    show (OptionList voidOption a) = "{" ++ (show a) ++"}"
    show (OptionList a voidOption) = "{" ++ (show a) ++"}"
    show (OptionList a b) = "{"++ (show a) ++ ", " ++ (show b) ++"}"
    show voidOption = ""




voidOption::(OptionList Int Int)
voidOption = EmptyOpt



(+:) :: a -> b -> (OptionList a b)
infixr 5 +:
t1 +: t2 = OptionList t1 t2

然后我有主文件 todo.hs

import OptionList


main = do
    print ( 0 +: "test" +: voidOption )

但编译器告诉OptionList.hs中的模式匹配是重叠的:

OptionList.hs:12:9: Warning:
    Pattern match(es) are overlapped
    In an equation for ‘show’:
        show (OptionList a voidOption) = ...
        show (OptionList a b) = ...

当我执行它时,它确实是重叠的.它产生以下输出:

{{}}

(我希望它是{0, {"test"}})

但为什么这些模式重叠？

Answer 1

在线

show (OptionList voidOption a) = "{" ++ (show a) ++"}"

voidOption是一个新的局部变量,就像a是.它与voidOption下面定义的变量没有关系.基本上,上面的行相当于

show (OptionList b a) = "{" ++ (show a) ++"}"

因此,它总是与以下行匹配并重叠.

如果我们将模式中的所有变量视为模式定义的变量,则很容易记住这一点.从某种意义上说,它们是价值"走出"的模式,而不是"在".

打开警告(-Wall)应该警告你这个错误,因为voidOption全局阴影的新本地绑定.

在模式中,您应该使用EmptyOpt构造函数.例如

show (OptionList EmptyOpt a) = "{" ++ (show a) ++"}"