Haskell IO:无法将预期类型"IO a0"与实际类型匹配

Question

我是Haskell的新手,我试着理解如何正确地执行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"

以下结果导致编译错误:

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,而第二个版本没有？

我知道在Stack Overflow中有一些类似的问题,但是他们似乎都没有向我解释这个问题.

Answer 1

username是一个String,但是promptUsername是IO String.你需要做一些事情:

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