haskell IO : Couldn't match expected type `IO a0' with actual type

Question

我是 Haskell 的新手，我尝试了解如何正确执行 IO。

以下工作正常:

main = do
  action <- cmdParser
  putStrLn "Username to add to the password manager:"
  username <- getLine
  case action of
    Add -> persist entry
      where
        entry = Entry username "somepassword"

而以下导致编译错误:

main = do
  action <- cmdParser
  case action of
    Add -> persist entry
      where
        entry = Entry promptUsername "somepassword"

promptUsername = do
  putStrLn "Username to add to the password manager:"
  username <- getLine

错误在这里:

Couldn't match expected type `IO b0' with actual type `[Char]'
Expected type: IO b0
  Actual type: String
In the expression: username
[...]

这里发生了什么？为什么第一个版本有效，而第二个版本无效？

我知道在 Stack Overflow 中有一些类似的问题，但似乎没有一个可以向我解释这个问题。

Answer 1

username是 String , 但是 promptUsername是 IO String .您需要执行以下操作:

username <- promptUsername
let entry = Entry username "somepassword"
persist entry