haskell - typeOf 带有类型构造函数 *->*/从程序中打印值的类型

Question

考虑以下几点:

module Main where

data Tree a = EmptyTree | Node a (Tree a) (Tree a) deriving (Show, Read, Eq) 

data Container a b = Container{contField :: b a} deriving (Show)

result = Container {contField = Node 'a' EmptyTree EmptyTree}

main = do 
    print result

如果我将它加载到 ghci 中，那么我将得到以下类型的 result :

*Main> :t result
result :: Container Char Tree

如何从程序中打印 Container Char Tree 类型？我试图调整 Haskell — get TypeRep from concrete type instance 给出的解决方案但是我卡住了，因为我找不到将 typeOf 与 * -> *

类型的构造函数结合使用的方法

[编辑] : 本文中的一些方法已在 ghc 7.8.1 中弃用 Release notes for version 7.8.1 :

Typeable is now poly-kinded, making Typeable1, Typeable2, etc., obsolete, deprecated, and relegated to Data.OldTypeable. Furthermore, user-written instances of Typeable are now disallowed: use deriving or the new extension -XAutoDeriveTypeable, which will create Typeable instances for every datatype declared in the module.

Answer 1

一种可能性是自己创建一个Typeable 实例。我在为 Container 创建 TyCon 时遇到了一些困难，也许有更好的方法:

{-# LANGUAGE ExplicitForAll #-}
{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE DeriveDataTypeable #-}
module Main where

import Data.Dynamic
import Data.Typeable

data Tree a = EmptyTree | Node a (Tree a) (Tree a)
    deriving (Show, Read, Eq, Typeable)

-- copy a representation of a type constructor from 
-- an existing representation
copyTyCon :: Typeable a => a -> String -> TyCon
copyTyCon x = mkTyCon3 (tyConPackage tc) (tyConModule tc)
  where tc = typeRepTyCon (typeOf x)

data Dummy = Dummy -- just to get package/module names for Container
    deriving (Typeable)

data Container a b = Container { contField :: b a }
    deriving (Show)
instance (Typeable a, Typeable1 f) => Typeable (Container a f) where
    typeOf (Container x) = mkTyConApp (copyTyCon Dummy "Container")
                                      [typeOf (undefined :: a), typeOf1 x]


result = Container { contField = Node 'a' EmptyTree EmptyTree }

main = do 
    print $ typeOf result
    print result

对它持保留态度，我对 Typeable 不是很有经验。