从概念上讲，它就像一系列if ... then语句。
在高级方面，模式匹配遵循F#中的此语法-
match expr with | pat1 - result1 | pat2 -> result2 | pat3 when expr2 -> result3 | _ -> defaultResult
- 每个|符号定义条件。
- -> 符号表示“如果条件为真，则返回该值...”。
- _ 符号提供默认模式，这意味着它与所有其他内容(例如通配符)匹配。
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)
编译并执行程序时，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
您还可以将多个条件链接在一起，这些条件返回相同的值。如-
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"
编译并执行程序时，将产生以下输出-
Winter Spring Autumn Rainy
匹配函数
F#允许您使用 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")
编译并执行程序时，将产生以下输出-
10 20.5 21 8.75 15
Filters&Guards
您可以使用 when 关键字添加过滤(Filters)或防护(Guards)。
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)
编译并执行程序时，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)
编译并执行程序时，将产生以下输出-
11 is less than 25 72 is greater than 10 0 equals 0
元组匹配
以下示例演示了与元组的模式匹配-
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"))
编译并执行程序时，将产生以下输出-
Hello, Zara Hello, Raman from the department of Computer Sc. You are not registered into the system
记录匹配
以下示例演示了与记录Record的模式匹配-
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 }
编译并执行程序时，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).
