Haskell——从具体类型实例获取 TypeRep

发布于 2024-11-05 20:42:23 字数 407 浏览 8 评论 0原文

我想编写一个具有此类型签名的函数：

getTypeRep :: Typeable a => t a -> TypeRep

其中 TypeRep 将是 a 的类型表示，而不是 t a 的类型表示。也就是说，编译器应该在任何调用站点自动返回正确的类型表示形式 [to getTypeRep]，这将具有 a 的具体类型。

为了添加一些上下文，我想创建一个“动态类型”数据类型，其特点是它将记住顶级类型，但不记住其参数。例如，我想将MyClass a转换为Dynamic MyClass，上面的函数将用于创建存储a的Dynamic MyClass实例类型参数a的表示。

原文

I want to write a function with this type signature:

getTypeRep :: Typeable a => t a -> TypeRep

where the TypeRep will be the type representation for a, not for t a. That is, the compiler should automatically return the correct type representation at any call sites [to getTypeRep], which will have concrete types for a.

To add some context, I want to create a "Dynamic type" data type, with the twist that it will remember the top-level type, but not its parameter. For example, I want to turn MyClass a into Dynamic MyClass, and the above function will be used to create instances of Dynamic MyClass that store a representation of the type parameter a.

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用心笑 2024-11-12 20:42:23

好吧，如何使用作用域类型变量来选择内部组件：

{-# LANGUAGE ExplicitForAll #-}
{-# LANGUAGE ScopedTypeVariables #-}

import Data.Dynamic
import Data.Typeable

getTypeRep :: forall t a . Typeable a => t a -> TypeRep
getTypeRep _ = typeOf (undefined :: a)

对我有用：

*Main> getTypeRep (Just ())
()
*Main> getTypeRep (Just 7)
Integer
*Main> getTypeRep ([True])
Bool

有趣的设计。

Well, how about using scoped type variables to select the inner component:

{-# LANGUAGE ExplicitForAll #-}
{-# LANGUAGE ScopedTypeVariables #-}

import Data.Dynamic
import Data.Typeable

getTypeRep :: forall t a . Typeable a => t a -> TypeRep
getTypeRep _ = typeOf (undefined :: a)

Works for me:

*Main> getTypeRep (Just ())
()
*Main> getTypeRep (Just 7)
Integer
*Main> getTypeRep ([True])
Bool

Interesting design.

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私藏温柔 2024-11-12 20:42:23

与 Don 的解决方案无关的是，请注意代码很少需要 ScopedTypeVariables。它只是使解决方案更清洁（但便携性较差）。没有作用域类型的解决方案是：

{-# LANGUAGE ExplicitForAll #-}
import Data.Typeable

helper :: t a -> a
helper _ = undefined

getTypeRep :: forall t a. Typeable a => t a -> TypeRep
getTypeRep = typeOf . helper

On a tangential note to Don's solution, notice that code rarely require ScopedTypeVariables. It just makes the solution cleaner (but less portable). The solution without scoped types is:

{-# LANGUAGE ExplicitForAll #-}
import Data.Typeable

helper :: t a -> a
helper _ = undefined

getTypeRep :: forall t a. Typeable a => t a -> TypeRep
getTypeRep = typeOf . helper

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