没有从字面上的'1'产生的Num字符串的实例 [英] No instance for Num String arising from the literal `1'

问题描述

  main = do 
 putStrLn $ myLast [1,2,3,4] 
 
 myLast :: [a]  - > a 
 myLast [x] = x 
 myLast（_：xs）= myLast xs 
  

当我尝试运行此代码时，我收到以下消息：

没有针对从字面值'1'产生的（Num字符串）的实例
可能修复：为（Num字符串）添加一个实例声明

当我运行列表[1，2，3，4 ]。我没有指定类型，但它不适用于int。

解决方案

。

你遇到的问题就是这个

Prelude>：t putStrLn

putStrLn :: String - > IO（）

即该函数可以只处理字符串，而不处理数字。一个常见的解决方案是首先将要显示的内容转换为字符串： putStrLn（show x），但实际上这个组合存在一个更好的标准函数：

  main = do 
 print $ myLast [1,2,3,4] 
  

main = do  
    putStrLn $myLast [1,2,3,4]

myLast :: [a] -> a
myLast [x] = x
myLast (_:xs) = myLast xs

When i try to run this code i get this message:

"No instance for (Num String) arising from the literal `1' Possible fix: add an instance declaration for (Num String)"

It runs well when I run with the list ["1","2","3,"4"]. I didn't specify the type but it doesn't work with ints.

解决方案

"No instance for..." error messages are usually misleading.

The problem you have is simply this

Prelude> :t putStrLn
putStrLn :: String -> IO ()

i.e. that function can only deal with strings, not with numbers. An often-seen solution is to first translate the thing you want to show into a string: putStrLn (show x), but actually the combination exists as a much nicer standard function:

main = do  
    print $ myLast [1,2,3,4]

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