f# - "let is unfinished. expect an expression"错误。我不知道在哪里

Question

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 | _ 当guess = secretNumber -> printfn “你明白了!” () 所以我不明白为什么它一直告诉我我的 block 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。

Answer 1

您必须在 let 语句的末尾返回一些内容。否则，您的函数只定义了一些值，但表达式不完整 - 您缺少返回值。 See this MSDN link for more details.

这种情况下，可以在最后加上highLowGameStep()来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