Haskell语言学习笔记（81）Data.Typeable

Data.Typeable

利用 Data.Typeable，可以打印动态类型信息。

class Typeable (a :: k) where

  typeRep# :: TypeRep a

typeRep :: Typeable a => TypeRep a

typeRep = typeRep#

typeOf :: Typeable a => a -> TypeRep a

typeOf _ = typeRep

typeOf 函数可以返回某个值的类型信息。

{-# LANGUAGE DeriveDataTypeable, GADTs #-}

import Data.Typeable

import Data.Data

data Person = Person { name :: String, age :: Int } deriving (Typeable, Data)

{-

*Main> typeOf (undefined :: Person)

Person

*Main> tyConName $ typeRepTyCon $ typeOf (undefined :: Person)

"Person"

*Main> tyConModule $ typeRepTyCon $ typeOf (undefined :: Person)

"Main"

*Main> dataTypeOf (undefined :: Person)

DataType {tycon = "Person", datarep = AlgRep [Person]}

*Main> dataTypeConstrs $ dataTypeOf (undefined :: Person)

[Person]

*Main> constrFields $ head $ dataTypeConstrs $ dataTypeOf (undefined :: Person)

["name","age"]

-}

f :: Typeable a => a -> String

f x = case (cast x :: Maybe Int) of

           Just i -> "I can treat i as an int in this branch " ++ show (i * i)

           Nothing -> case (cast x :: Maybe Bool) of

                           Just b -> "I can treat b as a bool in this branch " ++ if b then "yes" else "no"

                           Nothing -> "x was of some type other than Int or Bool"

{-

*Main> f True

"I can treat b as a bool in this branch yes"

*Main> f (3 :: Int)

"I can treat i as an int in this branch 9"

-}

data Admissible a where

    AdInt :: Admissible Int

    AdBool :: Admissible Bool

f2 :: Admissible a -> a -> String

f2 AdInt i = "I can treat i as an int in this branch " ++ show (i * i)

f2 AdBool b = "I can treat b as a bool in this branch " ++ if b then "yes" else "no"

{-

*Main> f2 AdInt 3

"I can treat i as an int in this branch 9"

*Main> f2 AdBool True

"I can treat b as a bool in this branch yes"

-}

How can I read the metadata of a type at runtime?

巴特西

Haskell语言学习笔记（81）Data.Typeable

Data.Typeable

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