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Question

当我尝试创建类型为instance的{​​{1}}时，我得到Show使用的地方，并显示以下消息：

show

Company.hs

Ambiguous occurrence `show'
    It could refer to either `Prelude.show',
                             imported from `Prelude' at [..]\Company.hs   (and originally defined in `GHC.Show')
                          or `Company.show',
                             defined at [..]\Company.hs:29:5

PS 添加了我刚刚在Prelude中加载的所有模块。错误是module Company where import Data.List data Worker=Worker{ age::Int, name::String, title::Title, income::Int } names=["age","name","title","income"] accesors=[show . age,show . name, show . title,show .income] data Title=Manager | Dev | Tester deriving (Show) data Company=Company{ cname::String, people::[Man], yf::Int }deriving (Show) instance Show Man where show w = intercalate "," (zipWith (\name acc->name++":"++acc w) names accesors) 应用于所有show。

Answer 1

编辑后：您的问题是由于缩进造成的。您需要在show w = ...之前添加至少一个空格，否则它将成为Company模块内的顶级定义，从而导致有两个单独的show定义，其中一个没有重载，而不是instance Show Man定义的一部分。

在编辑之前：由于缺少部分，代码不会执行，但是添加这些缺少的部分也不会产生警告：

module Main where

import Data.List

data Title = Manager | Dev | Tester deriving (Show)

data Man = Man { _age    :: Int
               , _name   :: String
               , _title  :: Title
               , _income :: Int
               }

instance Show Man where
  show m = intercalate "," (zipWith (\name acc -> name ++ ":" ++ acc m) names accessors)

data Company = Company { _cname  :: String
                       , _people :: [Man]
                       , _yf     :: Int
                       }

instance Show Company where
  show (Company cname people yf) =
    intercalate "\n  " $
      ("Company (" ++ cname ++ ", " ++ show yf ++ "):") : map show people

names :: [String]
names = ["age", "name", "title", "income"]

accessors :: [Man -> String]
accessors = [show . _age, show . _name, show . _title, show . _income]

main :: IO ()
main = do
  print alice
  print bob
  print (Company "Alice & Bob's" [alice, bob] 1)

alice, bob :: Man
alice = Man 41 "Alice" Manager 41000
bob = Man 42 "Bob" Manager 42000

礼物：

$ ./show
age:41,name:"Alice",title:Manager,income:41000
age:42,name:"Bob",title:Manager,income:42000
Company (Alice & Bob's, 1):
  age:41,name:"Alice",title:Manager,income:41000
  age:42,name:"Bob",title:Manager,income:42000

尽管您可能会对deriving (Show)感到满意。