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.NET 4 中的模式匹配

  • 2009-05-30
  • 本文字数:3531 字

    阅读完需:约 12 分钟

case 语句可以看作是 if/else 语法的特别版。他们的功能和作用是一样的,但有时 case 语句会令代码看起来更加清爽。考虑下面的 C#和 VB 示例。

复制代码
<span>double </span>CaclRateByDate(<span>DayOfWeek </span>day)
{
<span>if </span>(day == <span>DayOfWeek</span>.Monday)
{
<span>return </span>.42;
}
<span>else if </span>(day == <span>DayOfWeek</span>.Tuesday)
{
<span>return </span>.67;
}
<span>else if </span>(day == <span>DayOfWeek</span>.Wednesday)
{
<span>return </span>.56;
}
<span>else if </span>(day == <span>DayOfWeek</span>.Thursday)
{
<span>return </span>.34;
}
<span>else if </span>(day == <span>DayOfWeek</span>.Friday)
{
<span>return </span>.78;
}
<span>else if </span>(day == <span>DayOfWeek</span>.Saturday)
{
<span>return </span>.92;
}
<span>else if </span>(day == <span>DayOfWeek</span>.Sunday)
{
<span>return </span>.18;
}
<span>throw new </span><span>ArgumentOutOfRangeException</span>(<span>"Unexpected enum value"</span>);
}
复制代码
<span>Function </span>CaclRateByDate(<span>ByVal </span>day <span>As </span>DayOfWeek) <span>As Double
If </span>day = Monday <span>Then
Return </span>0.42
<span>ElseIf </span>day = Tuesday <span>Then
Return </span>0.67
<span>ElseIf </span>day = Wednesday <span>Then
Return </span>0.56
<span>ElseIf </span>day = Thursday <span>Then
Return </span>0.34
<span>ElseIf </span>day = Friday <span>Then
Return </span>0.78
<span>ElseIf </span>day = Saturday <span>Then
Return </span>0.92
<span>ElseIf </span>day = Sunday <span>Then
Return </span>0.18
<span>Else
Throw New </span>ArgumentOutOfRangeException(<span>"Unexpected enum value"</span>)
<span>End If
End Function</span>

开发者需要一遍又一遍地编写“ElseIf day =”或“else if (day ==”这种语句,但却并没有增加任何信息。这种语句简直就是一种折磨,不停地分散开发者的注意力,我指的是 DayOfWeek 和返回值。

在 VB 和 C#中,我们可以通过 case 语句进行简化。

复制代码
<span>double </span>CaclRateByDate2(<span>DayOfWeek </span>day)
{
<span>switch </span>(day)
{
<span>case </span><span>DayOfWeek</span>.Monday:
<span>return </span>.42;
<span>case </span><span>DayOfWeek</span>.Tuesday:
<span>return </span>.67;
<span>case </span><span>DayOfWeek</span>.Wednesday:
<span>return </span>.56;
<span>case </span><span>DayOfWeek</span>.Thursday:
<span>return </span>.34;
<span>case </span><span>DayOfWeek</span>.Friday:
<span>return </span>.78;
<span>case </span><span>DayOfWeek</span>.Saturday:
<span>return </span>.92;
<span>case </span><span>DayOfWeek</span>.Sunday:
<span>return </span>.18;
<span>default</span>:
<span>throw new </span><span>ArgumentOutOfRangeException</span>(<span>"Unexpected enum value"</span>);
}
}
复制代码
<span>Function </span>CalcRateByDate2(<span>ByVal </span>day <span>As </span>DayOfWeek) <span>As Double
Select Case </span>day
<span>Case </span>Monday
<span>Return </span>0.42
<span>Case </span>Tuesday
<span>Return </span>0.67
<span>Case </span>Wednesday
<span>Return </span>0.56
<span>Case </span>Thursday
<span>Return </span>0.34
<span>Case </span>Friday
<span>Return </span>0.78
<span>Case </span>Saturday
<span>Return </span>0.92
<span>Case </span>Sunday
<span>Return </span>0.18
<span>Case Else
Throw New </span>ArgumentOutOfRangeException(<span>"Unexpected enum value"</span>)
<span>End Select
End Function</span>

即便如此还是有不少的重复代码。为什么总是不断地说需要一个返回值呢?像下面这样写岂不更好?

复制代码
<span>double </span>CaclRateByDate2(<span>DayOfWeek </span>day)
{
<span>return switch </span>(day)
{
<span>DayOfWeek</span>.Monday: .42;
<span>DayOfWeek</span>.Tuesday: .67;
<span>DayOfWeek</span>.Wednesday: .56;
<span>DayOfWeek</span>.Thursday: .34;
<span>DayOfWeek</span>.Friday: .78;
<span>DayOfWeek</span>.Saturday: .92;
<span>DayOfWeek</span>.Sunday: .18;
<span>default</span>:
<span>throw new </span><span>ArgumentOutOfRangeException</span>(<span>"Unexpected enum value"</span>);
}
}
复制代码
<span>Function </span>CalcRateByDate2(<span>ByVal </span>day <span>As </span>DayOfWeek) <span>As Double
Return Select Case </span>day
Monday: 0.42
Tuesday: 0.67
Wednesday: 0.56
Thursday: 0.34
Friday: 0.78
Saturday: 0.92
Sunday: 0.18
<span>Case Else
Throw New </span>ArgumentOutOfRangeException(<span>"Unexpected enum value"</span>)
<span>End Select
End Function</span>

在消除了那些不必要的重复后,你会发现 C#和 VB 代码看起来是如此的接近。剩下的代码就是寻找的模式以及与模式所匹配的结果了。这就是众所周知的模式匹配

遗憾的是,在C# 4 和VB 10 中并没有提供该特性,但却有一门新语言提供了对模式匹配的支持。看看下面这个由 Mathew Podwysocki 编写的 F#示例(需要说明的是,在下面这些示例中都创建了相应的函数)。

复制代码
<span>let</span> calcRateByDay2 (day:System.DayOfWeek) =
<span>match</span> day <span>with </span>
| System.DayOfWeek.Monday -> 0.42
| System.DayOfWeek.Tuesday -> 0.67
| System.DayOfWeek.Wednesday -> 0.56
| System.DayOfWeek.Thursday -> 0.34
| System.DayOfWeek.Friday -> 0.78
| System.DayOfWeek.Saturday -> 0.92
| System.DayOfWeek.Sunday -> 0.18
| _ -> failwith "Unexpected enum value"

接下来 Mathew 又介绍了同时检查多个参数的方式。下面这个示例将下划线当作通配符。

复制代码
<span>let</span> allowUrl url port =
<span>match</span> (url, port) <span>with</span>
| "http://www.microsoft.com/", 80 -> true
| "http://example.com/", 8888 -> true
| _, 80 -> true
| _ -> false

遗憾的是,F#的语法并不简洁。如果想要操纵某个值,那就不得不通过名称或占位符来指定了。

复制代码
<span>let</span> calcRateByDay3 (day:System.DayOfWeek) =
<span>match</span> day <span>with </span>
| x when x >= System.DayOfWeek.Monday && x <= System.DayOfWeek.Friday -> 0.42
| System.DayOfWeek.Saturday -> 0.92
| System.DayOfWeek.Sunday -> 0.18
| _ -> failwith "Unexpected enum value"
<span>let</span> calcRateByDay3 (day:System.DayOfWeek) =
<span>match</span> day <span>with </span>
| _ when day >= System.DayOfWeek.Monday && day <= System.DayOfWeek.Friday -> 0.42
| System.DayOfWeek.Saturday -> 0.92
| System.DayOfWeek.Sunday -> 0.18
| _ -> failwith "Unexpected enum value"

下面的代码用 VB 的 case 语句实现同样的功能。

复制代码
<span>Function </span>CaclRateByDate3(<span>ByVal </span>day <span>As </span>DayOfWeek) <span>As Double
Select Case </span>day
<span>Case </span>Monday <span>To </span>Friday : <span>Return </span>0.42
<span>Case </span>Saturday : <span>Return </span>0.92
<span>Case </span>Sunday : <span>Return </span>0.18
<span>Case Else
Throw New </span>ArgumentOutOfRangeException(<span>"Unexpected enum value"</span>)
<span>End Select
End Function</span>

如你所见,.NET 平台上的每种语言都有自己的一些语法优势,可以将他们应用到其他语言上而无需改变语言的核心。

查看英文原文: Pattern Matching in .NET 4

2009-05-30 22:311920
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