获取Haskell记录的字段名称作为字符串列表？ [英] Get a Haskell record's field names as a list of strings?

问题描述

假设我有以下几项：

pre code data Rec = Rec {
alpha :: Int，
beta :: Double，
phi :: Float
}

sample = Rec 1 2.3 4.5

我理解Template Haskell& reify 函数可以获取记录的字段名称。即：

  print $（f sample） - > [alpha，beta，phi] 
  

没有模板Haskell。有人可以提供这样的示例实现可以完成？

解决方案

可以使用数据（大多数GHC版本）或Generic（7.2.x及以上）实例，GHC可以为您推导。下面是一个如何使用Data类型类转储记录字段的例子：

  { - ＃LANGUAGE DeriveDataTypeable＃ - } 
 
导入Data.Data 
 
数据Rec = Rec {
 alpha :: Int，
 beta :: Double，
 phi :: Float 
）派生（Data，Typeable）
 
 sample = Rec 1 2.3 4.5 
 
 main :: IO（）
 main = print。 constrFields。 toConstr $ sample 
  

Say I have the following:

data Rec = Rec {
    alpha :: Int,
    beta  :: Double,
    phi   :: Float 
} 

sample = Rec 1 2.3 4.5

I understand Template Haskell & the reify function can get me the record's field names. That is:

print $(f sample) --> ["alpha", "beta", "phi"]

There is also a claim that this can be done without Template Haskell. Can someone provide an example implementation for this can be accomplished?

解决方案

It can be done with a Data (most GHC versions) or Generic (7.2.x and up) instance, which GHC can derive for you. Here's an example of how to dump record fields with the Data typeclass:

{-# LANGUAGE DeriveDataTypeable #-}

import Data.Data

data Rec = Rec {
    alpha :: Int,
    beta  :: Double,
    phi   :: Float 
}  deriving (Data, Typeable)

sample = Rec 1 2.3 4.5

main :: IO ()
main = print . constrFields . toConstr $ sample 

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