F# - 模式匹配

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模式匹配允许你“将数据与逻辑结构或结构进行比较，将数据分解成组成部分，或以各种方式从数据中提取信息”。

换句话说，它提供了一种更灵活、更强大的方法来测试数据是否符合一系列条件，并根据满足的条件执行一些计算。

从概念上讲，它就像一系列 if… then 语句。

语法

从高级层面来看，F# 中的模式匹配遵循以下语法：

match expr with
| pat1 - result1
| pat2 -> result2
| pat3 when expr2 -> result3
| _ -> defaultResult

其中：

• 每个 | 符号定义一个条件。
• -> 符号表示“如果条件为真，则返回此值…”。
• _ 符号提供默认模式，这意味着它像通配符一样匹配所有其他内容。

示例 1

以下示例使用模式匹配语法计算斐波那契数：

let rec fib n =
   match n with
   | 0 -> 0
   | 1 -> 1
   | _ -> fib (n - 1) + fib (n - 2)
for i = 1 to 10 do
   printfn "Fibonacci %d: %d" i (fib i)

编译并执行程序后，将产生以下输出：

Fibonacci 1: 1
Fibonacci 2: 1
Fibonacci 3: 2
Fibonacci 4: 3
Fibonacci 5: 5
Fibonacci 6: 8
Fibonacci 7: 13
Fibonacci 8: 21
Fibonacci 9: 34
Fibonacci 10: 55

你还可以将多个返回相同值的条件链接在一起。例如：

示例 2

let printSeason month =
   match month with
   | "December" | "January" | "February" -> printfn "Winter"
   | "March" | "April" -> printfn "Spring"
   | "May" | "June" -> printfn "Summer"
   | "July" | "August" -> printfn "Rainy"
   | "September" | "October" | "November" -> printfn "Autumn"
   | _ -> printfn "Season depends on month!"

printSeason "February"
printSeason "April"
printSeason "November"
printSeason "July"

编译并执行程序后，将产生以下输出：

Winter
Spring
Autumn
Rainy

模式匹配函数

F# 允许你使用 function 关键字编写模式匹配函数：

let getRate = function
   | "potato" -> 10.00
   | "brinjal" -> 20.50
   | "cauliflower" -> 21.00
   | "cabbage" -> 8.75
   | "carrot" -> 15.00
   | _ -> nan (* nan is a special value meaning "not a number" *)

printfn "%g"(getRate "potato")
printfn "%g"(getRate "brinjal")
printfn "%g"(getRate "cauliflower")
printfn "%g"(getRate "cabbage")
printfn "%g"(getRate "carrot")

编译并执行程序后，将产生以下输出：

10
20.5
21
8.75
15

向模式添加过滤器或保护

你可以使用 when 关键字向模式添加过滤器或保护。

示例 1

let sign = function
   | 0 -> 0
   | x when x < 0 -> -1
   | x when x > 0 -> 1

printfn "%d" (sign -20)
printfn "%d" (sign 20)
printfn "%d" (sign 0)

编译并执行程序后，将产生以下输出：

-1
1
0

示例 2

let compareInt x =
   match x with
   | (var1, var2) when var1 > var2 -> printfn "%d is greater than %d" var1 var2
   | (var1, var2) when var1 < var2 -> printfn "%d is less than %d" var1 var2
   | (var1, var2) -> printfn "%d equals %d" var1 var2

compareInt (11,25)
compareInt (72, 10)
compareInt (0, 0)

编译并执行程序后，将产生以下输出：

11 is less than 25
72 is greater than 10
0 equals 0

使用元组进行模式匹配

以下示例演示了使用元组进行模式匹配：

let greeting (name, subject) =
   match (name, subject) with
   | ("Zara", _) -> "Hello, Zara"
   | (name, "English") -> "Hello, " + name + " from the department of English"
   | (name, _) when subject.StartsWith("Comp") -> "Hello, " + name + " from the department of Computer Sc."
   | (_, "Accounts and Finance") -> "Welcome to the department of Accounts and Finance!"
   | _ -> "You are not registered into the system"

printfn "%s" (greeting ("Zara", "English"))
printfn "%s" (greeting ("Raman", "Computer Science"))
printfn "%s" (greeting ("Ravi", "Mathematics"))

编译并执行程序后，将产生以下输出：

Hello, Zara
Hello, Raman from the department of Computer Sc.
You are not registered into the system

使用记录进行模式匹配

以下示例演示了使用记录进行模式匹配：

type Point = { x: float; y: float }
let evaluatePoint (point: Point) =
   match point with
   | { x = 0.0; y = 0.0 } -> printfn "Point is at the origin."
   | { x = xVal; y = 0.0 } -> printfn "Point is on the x-axis. Value is %f." xVal
   | { x = 0.0; y = yVal } -> printfn "Point is on the y-axis. Value is %f." yVal
   | { x = xVal; y = yVal } -> printfn "Point is at (%f, %f)." xVal yVal

evaluatePoint { x = 0.0; y = 0.0 }
evaluatePoint { x = 10.0; y = 0.0 }
evaluatePoint { x = 0.0; y = 10.0 }
evaluatePoint { x = 10.0; y = 10.0 }

编译并执行程序后，将产生以下输出：

Point is at the origin.
Point is on the x-axis. Value is 10.000000.
Point is on the y-axis. Value is 10.000000.
Point is at (10.000000, 10.000000).