Question

如何从字符串创建some string？

我正在浏览F# Koans tutorials，我是否坚持这一点：

[<Koan>]
let ProjectingValuesFromOptionTypes() =
    let chronoTrigger = { Name = "Chrono Trigger"; Platform = "SNES"; Score = Some 5 }
    let halo = { Name = "Halo"; Platform = "Xbox"; Score = None }

    let decideOn game =

        game.Score
        |> Option.map (fun score -> if score > 3 then "play it" else "don't play")

    //HINT: look at the return type of the decide on function
    AssertEquality (decideOn chronoTrigger) (Some "play it")
    AssertEquality (decideOn halo) (Some "don't play")

我得到的例外是：

You have not yet reached enlightenment ...
  Expected: null
  But was:  <Some(don't play)>

如何将字符串向上转换为option string类型？

Answer 1

如何将字符串向上转换为选项字符串类型？

施法具有非常特殊的含义。您要做的是用string包裹Option，而不是投射它。为此，请使用Some构造函数：

let x = Some myString //x: string option

但是，我认为这不会解决你得到的断言错误（至少不是自己）。我不想在这里给你完整的答案（特别是因为那不是你所要求的，找到答案就是做Koan的全部意义）但我会留下这个线索作为为什么你在断言中看到null：

None |> printfn "Value: %A" // Value: <null>

有关该行为的更多信息，请参阅Why is None represented as null?。

如何向选项类型转发？

1 个答案: