“let 未完成。需要一个表达式”错误。但我不知道在哪里

open System

let highLowGame () = 
    let rng = new Random();
    let secretNumber = rng.Next() % 100 + 1

    let rec highLowGameStep () = 
        printfn "Guess a number: "
        let guessStr = Console.ReadLine()
        let guess = Int32.Parse(guessStr)
        match guess with
        | _ when guess > secretNumber -> printfn "Too high!" highLowGameStep ()
        | _ when guess = secretNumber -> printfn "You got it!" ()
        | _ when guess < secretNumber -> printfn "Too low!" highLowGameStep ()


[<EntryPoint>]
let main argv = 
    highLowGame ()
    0 // return an integer exit code

我知道有很多这样的问题，而且我知道 F# 中的函数必须有一个返回变量。我的在这里| _ when guess = secretNumber -> printfn "You got it!" ()所以我不明白为什么它一直告诉我我的区块尚未完成

此示例直接来自 F# 3.0 书籍。

/stdin(14,13): error FS0010: Unexpected identifier in expression. Expected incomplete structured construct at or before this point or other token.

是完整的错误。

你必须在结束时返回一些东西let陈述。否则，您的函数只是定义了一些值，但表达式不完整 - 您缺少返回值。有关更多详细信息，请参阅此 MSDN 链接。 https://msdn.microsoft.com/en-us/library/dd233238.aspx#Anchor_2

在这种情况下，您可以添加highLowGameStep ()最后调用函数并获取其返回值：

open System

let highLowGame () = 
    let rng = new Random();
    let secretNumber = rng.Next() % 100 + 1

    let rec highLowGameStep () = 
        printfn "Guess a number: "
        let guessStr = Console.ReadLine()
        let guess = Int32.Parse(guessStr)
        match guess with
        | _ when guess > secretNumber -> printfn "Too high!" highLowGameStep ()
        | _ when guess = secretNumber -> printfn "You got it!" ()
        | _ when guess < secretNumber -> printfn "Too low!" highLowGameStep ()

    highLowGameStep ()


[<EntryPoint>]
let main argv = 
    highLowGame ()
    0 // return an integer exit code