haskell - putStrLn 函数不接受 [Char] 参数

Question

下面的代码:

Prelude> :t putStrLn
putStrLn :: String -> IO ()
Prelude> putStrLn "test"
test
it :: ()
Prelude> putStrLn "test" ++ "test"

<interactive>:25:1:
    Couldn't match expected type ‘[Char]’ with actual type ‘IO ()’
    In the first argument of ‘(++)’, namely ‘putStrLn "test"’
    In the expression: putStrLn "test" ++ "test"
Prelude> :t "test"
"test" :: [Char]
Prelude> :t "test" ++ "test" 
"test" ++ "test" :: [Char]
Prelude> 

我不明白错误:

" Couldn't match expected type ‘[Char]’ with actual type ‘IO ()’
        In the first argument of ‘(++)’, namely ‘putStrLn "test"’
        In the expression: putStrLn "test" ++ "test""

如 putStrLn接受 [Char] 类型作为第一个参数(我假设它隐式转换为 String ？)。但这不会检查 "test" ++ "test"作为参数传递，即使 "test" ++ "test"是 [Char] 类型还 ？

Answer 1

您的代码未通过 "test" ++ "test"作为参数。函数应用程序比任何中缀运算符绑定(bind)得更紧密。您的代码被解析为

(putStrLn "test") ++ "test"

您得到的错误是指 putStrLn不返回字符串。

为了解决这个问题，写

putStrLn ("test" ++ "test")

注: String定义为 type String = [Char] ，即它只是同一类型的不同名称。