如何模式匹配typeclass值？ [英] How to pattern match against a typeclass value?

问题描述

假设我在Haskell中定义了一个类型类，

Suppose that I define a typeclass in Haskell,

class (Bool a) where
    false :: a
    true  :: a

为了定义一个通用的不是操作对于任何 Bool ，都需要对其潜在值进行模式匹配：



In order to define a generic not operation for any Bool, one would need to pattern match against its potential values:

not :: Bool a => a → a
not true  = false
not false = true

不过，编译。我怎样才能使它工作？

That doesn't compile, though. How can I get it to work?

推荐答案

您无法像这样匹配类型类型值。类型类抽象出实际的基础类型，而模式匹配会暴露它。

You can't match against a typeclass value like this. Typeclasses abstract over the actual underlying type while pattern matching would expose it.

然而，仍然可以通过 ViewPatterns 扩展名。这个想法是包含一个函数，它接受你的抽象值，并为你提供一个代表视图结构的具体类型的值：

However, you can still get the same behavior and some nice syntax with the ViewPatterns extension. The idea is to include a function which takes your abstract value and gives you a value of a concrete type that represents a view into the structure:

class (Bool a) where
  true   :: a
  false  :: a
  asBool :: a -> Bool

现在您可以使用 asBool 函数在模式匹配中作为视图模式：

Now you can use the asBool function in a pattern match as a view pattern:

not :: Bool a => a -> a
not (asBool -> True)  = false
not (asBool -> False) = true

然而，请注意，这可能会导致 asBool 被计算两次，如果它隐藏昂贵的计算可能会成为问题。

However, note that this could result in asBool being computed twice which could be an issue if it hides an expensive computation.

这篇关于如何模式匹配typeclass值？的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！