如何为某些类型创建专用类型类，其余类型的默认实现(How to make specialized type classes for certain types, default implementat

如何为某些类型创建专用类型类，其余类型的默认实现(How to make specialized type classes for certain types, default implementation for the rest of types)

我希望有一个类型类型，可能在可能的情况下转换为其他类型。

class Castable a b where cast :: a -> Maybe b cast _ = Nothing -- default implementation

现在，该类将针对某些类型实现，对于所有其他类型，我希望具有默认实现。

怎么能这样做？

I would like to have a type class of types that can possibly casted to other types when possible.

class Castable a b where cast :: a -> Maybe b cast _ = Nothing -- default implementation

Now the class would be implemented for some types and for all the others I would like to have default implementation.

How one can do that?

最满意答案

它不一定是安全的或Haskell-y的事情，但它肯定是可能的，使用OverlappingInstances

首先，启用它们：

{-# LANGUAGE MultiParamTypeClasses #-} {-# LANGUAGE FlexibleInstances #-} {-# LANGUAGE OverlappingInstances #-}

写你的铸造类：

class Castable a b where cast :: a -> Maybe b cast _ = Nothing -- default implementation

“优化”实例：

instance Castable Int Bool where cast 0 = Just False cast _ = Just True

最后，所有类型的一般实例：

instance Castable a b where

使用示例：

main = do print $ (cast (7 :: Int) :: Maybe Bool) print $ (cast (7 :: Int) :: Maybe Integer)

运行此选项时，如果类型不是专用的，则选择默认值：

*Main> main Just True Nothing

It's not necessarily a safe or Haskell-y thing to do, but it is certainly possible, using OverlappingInstances

First, enable them:

{-# LANGUAGE MultiParamTypeClasses #-} {-# LANGUAGE FlexibleInstances #-} {-# LANGUAGE OverlappingInstances #-}

Write your casting class:

class Castable a b where cast :: a -> Maybe b cast _ = Nothing -- default implementation

An "optimized" instance:

instance Castable Int Bool where cast 0 = Just False cast _ = Just True

and finally, a general instance for all types:

instance Castable a b where

Example use:

main = do print $ (cast (7 :: Int) :: Maybe Bool) print $ (cast (7 :: Int) :: Maybe Integer)

Running this, the default is chosen when the types aren't specialized:

*Main> main Just True Nothing