Question

我希望能够将Action应用于Thing，其中只有一些事情可以接受某些操作。

data Thing  = BoxVal Box | BallVal Ball   deriving (Show)
data Box    = Box  {name :: String}       deriving (Show)
data Ball   = Ball {size :: Int}          deriving (Show)
data Action = SetName String | Resize Int | ToFoo deriving (Show)

applyAction :: Thing -> Action -> Maybe String
applyAction (BoxVal box) a   = actionToBox box a
applyAction (BallVal ball) a = undefined
applyAction _ _ = Nothing

actionToBox :: Box -> Action -> Maybe String
actionToBox b (SetName s) = Just $ s
actionToBox b (ToFoo)     = Just "foo"
actionToBox _ _           = Nothing

上面的代码有效（并不是很优雅），但GHCI在Pattern match(es) are overlapped中抱怨applyAction。如何在不收到警告的情况下获得相同的功能？

Answer 1

冗余模式匹配是：

applyAction _ _ = Nothing

Thing只定义了两个构造函数BoxVal和BallVal，并且在前两个模式匹配中都明确处理了这些构造函数：

applyAction (BoxVal box) a   = actionToBox box a
applyAction (BallVal ball) a = undefined

这使得applyAction _ _变得不必要了。它永远不会匹配，因为前两个模式匹配所有可能性。

Answer 2

为applyAction定义一个类，然后定义适当的实例。

data Thing  = BoxVal Box | BallVal Ball   deriving (Show)
data Box    = Box  {name :: String}       deriving (Show)
data Ball   = Ball {size :: Int}          deriving (Show)
data Action = SetName String | Resize Int | ToFoo deriving (Show)

class Actionable x where
    applyAction :: x -> Action -> Maybe String

instance Actionable Box where
    applyAction :: Box -> Action -> Maybe String
    applyAction b (SetName s) = Just s
    applyAction b ToFoo = Just "foo"
    applyAction _ _ = Nothing

instance Actionable Thing where
    applyAction :: Thing -> Action -> Maybe String
    applyAction (BoxVal b) a = applyAction b a
    applyAction _ _ = undefined

（该类不直接解决重叠模式问题，但确实可以避免您需要提出新的函数名称。applyAction根据其第一个参数的类型做正确的事。）

根据值类型执行不同的功能

2 个答案: