Question

请考虑以下事项：

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

如果我将其加载到ghci中，那么我会得到result类型的以下内容：

*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，但我遇到了困难，因为我无法找到一种方法将typeOf与类型* -> *的类型构造函数结合使用

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

Typeable现在是poly-kinded，制作Typeable1，Typeable2等，已过时，已弃用并已降级为Data.OldTypeable。此外，现在不允许用户编写的Typeable实例：使用deriving或新的扩展名-XAutoDeriveTypeable，它将创建Typeable 模块中声明的每种数据类型的实例。

Answer 1

一种可能性是自己创建一个Typeable实例。我为TyCon创建Container时遇到了一些困难，也许有更好的方法可以做到这一点：

{-# 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

带上一粒盐，我对Typeable没有经验。

typeOf类型构造函数类型* - ＆gt; * /打印程序内的值类型

1 个答案: