无法与实际类型IO（）匹配的预期类型[a0] [英] couldn't match expected type [a0] with actual type IO ()

问题描述

我的代码有什么问题：

  insertValue file x = 
 if x ==10then ok
 else putStrLnError;文件
  

解决方案

您需要使用 return :: a - > IO a 将您的字符串提升为 IO字符串：

  insertValue file x = 
 if x ==10
然后返回ok
 else putStrLnError
返回文件
  

但是您确定不想调用 putStrLnok（而不是 returnok）并返回一个Maybe值？否则，您将返回文件或ok，调用者无法确定调用<$时是否有错误c $ c> insertValue 放在名为ok的文件上。

What is wrong in my code:

insertValue file x = 
    if x == "10" then "ok"
    else do putStrLn "Error"; file

解决方案

You need to use return :: a -> IO a to "lift" your strings into IO String:

insertValue file x = 
    if x == "10"
      then return "ok"
      else do putStrLn "Error"
              return file

But are you sure you don't want to call putStrLn "ok" (instead of return "ok") and return a Maybe value? Otherwise you are returning file or "ok" and your caller could never determine if there was an error when calling insertValue on a file named "ok".

这篇关于无法与实际类型IO（）匹配的预期类型[a0]的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！