理解F#中的模式匹配与活动模式

F# Code - 对简单值和.NET类型进行匹配

// 对简单值进行匹配。

let rec fibonacci x =

match x with

| x when x <= 0 -> failwith "x必须是正整数。"

| 1 -> 1

| 2 -> 1

| x -> fibonacci(x - 1) + fibonacci(x - 2)



printfn "%i" (fibonacci 2) // -> 1

printfn "%i" (fibonacci 4) // -> 3



// 对.NET类型进行匹配。

open System

let typeToString x =

match box x with

| :? Int32 -> "Int32"

| :? Double -> "Double"

| :? String -> "String"

| _ -> "Other Type"

F# Code - 对列表应用模式匹配

// 对列表应用模式匹配。

let listOfList = [[2; 3; 5]; [7; 11; 13]; [17; 19; 23; 29]]



let rec concatenateList list =

match list with

| head :: tail -> head @ (concatenateList tail)

| [] -> []



let rec concatenateList2 list =

if List.nonempty list then

let head = List.hd list in

let tail = List.tl list in

head @ (concatenateList2 tail)

else

[]



let primes = concatenateList listOfList

print_any primes // [2; 3; 5; 7; 11; 13; 17; 19; 23; 29]

F# Code - 对Union类型应用模式匹配

type BinaryTree<'a> =

    | Leaf of 'a

    | Node of BinaryTree<'a> * BinaryTree<'a> 



let rec printBinaryTreeValues t =

match t with

| Leaf x -> printfn "%i" x

| Node (l, r) ->

printBinaryTreeValues l

printBinaryTreeValues r



printBinaryTreeValues (Node ((Node (Leaf 1, Leaf 2)), (Node (Leaf 3, Leaf 4))))

F# Code - Single-Case活动模式

let (|UpperCase|) (x:string) = x.ToUpper()

let result = match "foo" with

| UpperCase "FOO" -> true

| _ -> false



printfn "%b" result // -> true

F# Code - Multi-Case活动模式

let (|Odd|Even|) x = if x % 2 = 0 then Even else Odd

let isDivisibleByTwo x = match x with Even -> true | Odd -> false

print_any (isDivisibleByTwo 2) // -> true

print_any (isDivisibleByTwo 3) // -> false

F# Code - Partial活动模式

// Partial Active Patterns

open System

let (|DivisibleBySeven|_|) input = if input % 7 = 0 then Some() else None



let (|IsPerfectSquare|_|) (input : int) =

let sqrt = int (Math.Sqrt(float input))

if sqrt * sqrt = input then

Some()

else

None



let describeNumber x =

match x with

| DivisibleBySeven & IsPerfectSquare -> 

printfn "x is divisible by 7 and is a perfect square."

| DivisibleBySeven                   -> printfn "x is divisible by seven."

| IsPerfectSquare                    -> printfn "x is a perfect square."

| _                                  -> printfn "x looks normal."



describeNumber 49 // x is divisible by 7 and is a perfect square.

describeNumber 35 // x is divisible by seven.

describeNumber 25 // x is a perfect square.

describeNumber 20 // x looks normal.

F# Code - 应用活动模式操作XML文档

// 定义针对XML节点的模式

let (|Node|Leaf|) (node : #System.Xml.XmlNode) =

if node.HasChildNodes then

Node (node.Name, { for x in node.ChildNodes -> x })

else

Leaf (node.InnerText)



// 打印XML节点的函数 

let printXml node =

let rec printXml indent node =

match node with

| Leaf (text) -> printfn "%s%s" indent text

| Node (name, nodes) ->

printfn "%s%s:" indent name

nodes |> Seq.iter (printXml (indent + "    "))

printXml "" node



// 定义XML节点

let doc =

let temp = new System.Xml.XmlDocument()

let text =  "<fruit>

                 <apples>

<gannySmiths>1</gannySmiths>

<coxsOrangePippin>3</coxsOrangePippin>

</apples>

<organges>2</organges>

<bananas>4</bananas>

</fruit>"

    temp.LoadXml(text)

temp



printXml (doc.DocumentElement :> System.Xml.XmlNode)