F# 匹配可变对象（字符串）

Question

这是目前为止的完整代码：

module clean
#light
open System
open System.IO
let pause() = Console.ReadLine()
let drive = System.IO.Directory.GetDirectoryRoot(System.IO.Directory.GetCurrentDirectory())
printfn "You're using the %s drive.\n\n" drive

let game1 = "Assassin's Creed"
let game2 = "Crysis"
let game3 = "Mass Effect"

let local1 = "\%APPDATA\%\\Ubisoft\\Assassin's Creed\\Saved Games\\"
let local2 = "\%USERPROFILE\%\\Documents\\My Games\\Crysis\\SaveGames\\"
let local3 = "\%USERPROFILE\%\\Documents\\BioWare\\Mass Effect\\Save\\"

let roam1 = drive + "Saves\\Abraxas\\" + game1 + "\\"
let roam2 = drive + "Saves\\Abraxas\\" + game2 + "\\"
let roam3 = drive + "Saves\\Abraxas\\" + game3 + "\\"




let rec getGame() =
  printfn "Which Game?\n\n   1.%s\n   2.%s\n   3.%s\n\n" game1 game2 game3
  match Int32.TryParse(stdin.ReadLine()) with
  | true,1 -> game1
  | true,2 -> game2
  | true,3 -> game3
  | _ ->
     printfn "You did not enter a valid choice."
     let _ = pause()
     Console.Clear()
     getGame()

let mutable gameprint = getGame()
printf "You have chosen %s\n\n" gameprint

let roaming =
  match gameprint with
  | game1 -> roam1
  | game2 -> roam2
  | game3 -> roam3
  | _ -> "test"

printf "Roaming set to %s\n\n" roaming

let local =
  match gameprint with
  | game1 -> local1
  | game2 -> local2
  | game3 -> local3
  | _ -> "test"

printf "Local set to %s\n\n" local

printf "Check gameprint  %s" gameprint

在设置漫游和本地对象的部分中，它告诉我它永远不会与“game1”以外的任何东西匹配。

我在匹配本地和漫游对象之前和之后执行了“printf”检查...游戏打印在两个 printf 命令中都正确显示，但与 game1 以外的任何内容都不匹配...我'我不确定我在哪里犯了错误。

Answer 1

两件事。

1. 在 F# 中，绑定可以被隐藏。特别是，在您的match 中，当您在模式中使用game1、game2 和game3 时，您实际上是在用这些名称声明new 绑定。因此，第一个模式将始终匹配，并且只会在评估右侧之前将您尝试与之匹配的任何值分配给新绑定 game1。

   解决此问题的一种方法是使用 [<Literal>] 属性声明您的 gameN 绑定（但请注意，它们也必须以大写字母开头才能用作常量）：

   [<Literal>] let Game1 = "Assassin's Creed"

   现在您可以在模式匹配中使用Game1，它会按您的预期工作。

1234563 .

Answer 2

请参阅F# matching with two values 了解说明。

当与一些非文字值进行比较时，我只会使用 if-then-else

if gameprint = game1 then ...
elif gameprint = game2 then ...
...

Answer 3

也许更像这样的东西可以让您使其更具可扩展性（如果您在运行时填充游戏列表）......对不起，代码有点匆忙我正在努力准备工作：

open System 
type Game = {Title:string; local:string; roam:string}

let game1 = {
    Title= "Assassin's Creed"; 
    local = "\%APPDATA\%\\Ubisoft\\Assassin's Creed\\Saved Games\\"; 
    roam = "Saves\\Abraxas\\\Assassin's Creed\\"
}
let game2 = {
    Title= "Crysis"; 
    local = "\%USERPROFILE\%\\Documents\\My Games\\Crysis\\SaveGames\\"; 
    roam = "Saves\\Abraxas\\\Crysis\\"
}
let games = [game1; game2]
let printGamelListItem i g = printfn "%i: %s" i g.Title

let printChoice() = 
    printfn "Which Game?\n"
    games |> List.fold (fun acc g -> 
                            printGamelListItem acc g 
                            acc+1) 1
    |> ignore

let rec getGame() = 
    printChoice()
    match Int32.TryParse(Console.ReadLine()) with
    |true, x  when x <= games.Length -> games.[x-1]
    | _ ->
        printfn "You did not enter a valid choice."
        let _ = Console.ReadLine()
        Console.Clear()
        getGame()

let selection = getGame()
printfn "Roaming set to: %s" (selection.roam)
printfn "Local set to: %s" (selection.local)