F 的教學課程#Tour of F#

若要了解 F# 的最佳方式是讀取和寫入 F# 程式碼。The best way to learn about F# is to read and write F# code. 這篇文章會做為某些 F# 語言的重要功能的逐步教學課程,並提供您一些您可以在您的電腦執行的程式碼片段。This article will act as a tour through some of the key features of the F# language and give you some code snippets that you can execute on your machine. 若要瞭解如何設定開發環境, 請參閱消費者入門To learn about setting up a development environment, check out Getting Started.

F# 中有兩個主要概念: 函式和類型。There are two primary concepts in F#: functions and types. 本教學課程將強調屬於這兩個概念的語言功能。This tour will emphasize features of the language which fall into these two concepts.

線上執行程式碼Executing the code online

如果您的電腦F#上未安裝, 您可以使用WebAssembly 上的 Try F# 來執行瀏覽器中的所有範例。If you don't have F# installed on your machine, you can execute all of the samples in your browser with Try F# on WebAssembly. Fable 是直接在您F#的瀏覽器中執行的方言。Fable is a dialect of F# that executes directly in your browser. 若要查看複寫中接下來的範例, 請參閱範例 > 瞭解F# Fable 複寫左側功能表列中的 > 導覽。To view the samples that follow in the REPL, check out Samples > Learn > Tour of F# in the left-hand menu bar of the Fable REPL.

函數和模組Functions and Modules

任何的 F# 程式的最基本的部分是函式分成模組The most fundamental pieces of any F# program are functions organized into modules. 函式來產生輸出的輸入上執行的工作和其下組織模組,這是主要的方式分組在 F# 中的項目。Functions perform work on inputs to produce outputs, and they are organized under Modules, which are the primary way you group things in F#. 它們是使用 let 系結來定義, 這會提供函式名稱並定義其引數。They are defined using the let binding, which give the function a name and define its arguments.

module BasicFunctions = 

    /// You use 'let' to define a function. This one accepts an integer argument and returns an integer. 
    /// Parentheses are optional for function arguments, except for when you use an explicit type annotation.
    let sampleFunction1 x = x*x + 3

    /// Apply the function, naming the function return result using 'let'. 
    /// The variable type is inferred from the function return type.
    let result1 = sampleFunction1 4573

    // This line uses '%d' to print the result as an integer. This is type-safe.
    // If 'result1' were not of type 'int', then the line would fail to compile.
    printfn "The result of squaring the integer 4573 and adding 3 is %d" result1

    /// When needed, annotate the type of a parameter name using '(argument:type)'.  Parentheses are required.
    let sampleFunction2 (x:int) = 2*x*x - x/5 + 3

    let result2 = sampleFunction2 (7 + 4)
    printfn "The result of applying the 2nd sample function to (7 + 4) is %d" result2

    /// Conditionals use if/then/elif/else.
    ///
    /// Note that F# uses white space indentation-aware syntax, similar to languages like Python.
    let sampleFunction3 x = 
        if x < 100.0 then 
            2.0*x*x - x/5.0 + 3.0
        else 
            2.0*x*x + x/5.0 - 37.0

    let result3 = sampleFunction3 (6.5 + 4.5)

    // This line uses '%f' to print the result as a float.  As with '%d' above, this is type-safe.
    printfn "The result of applying the 3rd sample function to (6.5 + 4.5) is %f" result3

let系結也是您將值系結至名稱的方式, 類似于其他語言中的變數。let bindings are also how you bind a value to a name, similar to a variable in other languages. let系結預設為不可變, 這表示一旦值或函式系結至名稱, 就無法就地變更。let bindings are immutable by default, which means that once a value or function is bound to a name, it cannot be changed in-place. 這與其他語言中的變數相反, 這是可變動的, 這表示它們的值可以在任何時間點變更。This is in contrast to variables in other languages, which are mutable, meaning their values can be changed at any point in time. 如果您需要可變的系結, 您可以let mutable ...使用語法。If you require a mutable binding, you can use let mutable ... syntax.

module Immutability =

    /// Binding a value to a name via 'let' makes it immutable.
    ///
    /// The second line of code fails to compile because 'number' is immutable and bound.
    /// Re-defining 'number' to be a different value is not allowed in F#.
    let number = 2
    // let number = 3

    /// A mutable binding.  This is required to be able to mutate the value of 'otherNumber'.
    let mutable otherNumber = 2

    printfn "'otherNumber' is %d" otherNumber

    // When mutating a value, use '<-' to assign a new value.
    //
    // Note that '=' is not the same as this.  '=' is used to test equality.
    otherNumber <- otherNumber + 1

    printfn "'otherNumber' changed to be %d" otherNumber

數位、布林值和字串Numbers, Booleans, and Strings

為.NET 語言,F# 支援相同的基礎基本型別存在於.NET。As a .NET language, F# supports the same underlying primitive types that exist in .NET.

以下是如何各種數值類型表示 F# 中:Here is how various numeric types are represented in F#:

module IntegersAndNumbers = 

    /// This is a sample integer.
    let sampleInteger = 176

    /// This is a sample floating point number.
    let sampleDouble = 4.1

    /// This computed a new number by some arithmetic.  Numeric types are converted using
    /// functions 'int', 'double' and so on.
    let sampleInteger2 = (sampleInteger/4 + 5 - 7) * 4 + int sampleDouble

    /// This is a list of the numbers from 0 to 99.
    let sampleNumbers = [ 0 .. 99 ]

    /// This is a list of all tuples containing all the numbers from 0 to 99 and their squares.
    let sampleTableOfSquares = [ for i in 0 .. 99 -> (i, i*i) ]

    // The next line prints a list that includes tuples, using '%A' for generic printing.
    printfn "The table of squares from 0 to 99 is:\n%A" sampleTableOfSquares

以下是布林值和執行基本條件式邏輯的樣子:Here's what Boolean values and performing basic conditional logic looks like:

module Booleans =

    /// Booleans values are 'true' and 'false'.
    let boolean1 = true
    let boolean2 = false

    /// Operators on booleans are 'not', '&&' and '||'.
    let boolean3 = not boolean1 && (boolean2 || false)

    // This line uses '%b'to print a boolean value.  This is type-safe.
    printfn "The expression 'not boolean1 && (boolean2 || false)' is %b" boolean3

以下是基本字串操作的樣子:And here's what basic string manipulation looks like:

module StringManipulation = 

    /// Strings use double quotes.
    let string1 = "Hello"
    let string2  = "world"

    /// Strings can also use @ to create a verbatim string literal.
    /// This will ignore escape characters such as '\', '\n', '\t', etc.
    let string3 = @"C:\Program Files\"

    /// String literals can also use triple-quotes.
    let string4 = """The computer said "hello world" when I told it to!"""

    /// String concatenation is normally done with the '+' operator.
    let helloWorld = string1 + " " + string2 

    // This line uses '%s' to print a string value.  This is type-safe.
    printfn "%s" helloWorld

    /// Substrings use the indexer notation.  This line extracts the first 7 characters as a substring.
    /// Note that like many languages, Strings are zero-indexed in F#.
    let substring = helloWorld.[0..6]
    printfn "%s" substring

TupleTuples

Tuple是 F# 中是什麼大問題。Tuples are a big deal in F#. 它們是未命名但已排序值的群組, 可視為值本身。They are a grouping of unnamed, but ordered values, that can be treated as values themselves. 請將它們想成是從其他值匯總而來的值。Think of them as values which are aggregated from other values. 它們有許多用途, 例如方便地從函式傳回多個值, 或將值分組以進行特定的便利性。They have many uses, such as conveniently returning multiple values from a function, or grouping values for some ad-hoc convenience.

module Tuples =

    /// A simple tuple of integers.
    let tuple1 = (1, 2, 3)

    /// A function that swaps the order of two values in a tuple. 
    ///
    /// F# Type Inference will automatically generalize the function to have a generic type,
    /// meaning that it will work with any type.
    let swapElems (a, b) = (b, a)

    printfn "The result of swapping (1, 2) is %A" (swapElems (1,2))

    /// A tuple consisting of an integer, a string,
    /// and a double-precision floating point number.
    let tuple2 = (1, "fred", 3.1415)

    printfn "tuple1: %A\ttuple2: %A" tuple1 tuple2

自 F# 4.1,您也可以建立structtuple。As of F# 4.1, you can also create struct tuples. 這些也會與 c # 7/Visual Basic 15 元組 (也struct就是元組) 完全交互操作:These also interoperate fully with C#7/Visual Basic 15 tuples, which are also struct tuples:

/// Tuples are normally objects, but they can also be represented as structs.
///
/// These interoperate completely with structs in C# and Visual Basic.NET; however,
/// struct tuples are not implicitly convertible with object tuples (often called reference tuples).
///
/// The second line below will fail to compile because of this.  Uncomment it to see what happens.
let sampleStructTuple = struct (1, 2)
//let thisWillNotCompile: (int*int) = struct (1, 2)

// Although you can
let convertFromStructTuple (struct(a, b)) = (a, b)
let convertToStructTuple (a, b) = struct(a, b)

printfn "Struct Tuple: %A\nReference tuple made from the Struct Tuple: %A" sampleStructTuple (sampleStructTuple |> convertFromStructTuple)

請務必注意, 因為struct元組是實值型別, 所以不能隱含地轉換成參考元組, 反之亦然。It's important to note that because struct tuples are value types, they cannot be implicitly converted to reference tuples, or vice versa. 您必須在參考和結構元組之間明確轉換。You must explicitly convert between a reference and struct tuple.

管線和組合Pipelines and Composition

透過管道傳送這類運算子|>廣泛處理 F# 中的資料時。Pipe operators such as |> are used extensively when processing data in F#. 這些運算子是函數, 可讓您以彈性的方式建立函式的「管線」。These operators are functions that allow you to establish "pipelines" of functions in a flexible manner. 下列範例會逐步解說如何利用這些運算子來建立簡單的功能管線:The following example walks through how you can take advantage of these operators to build a simple functional pipeline:

module PipelinesAndComposition =

    /// Squares a value.
    let square x = x * x

    /// Adds 1 to a value.
    let addOne x = x + 1

    /// Tests if an integer value is odd via modulo.
    let isOdd x = x % 2 <> 0

    /// A list of 5 numbers.  More on lists later.
    let numbers = [ 1; 2; 3; 4; 5 ]

    /// Given a list of integers, it filters out the even numbers,
    /// squares the resulting odds, and adds 1 to the squared odds.
    let squareOddValuesAndAddOne values = 
        let odds = List.filter isOdd values
        let squares = List.map square odds
        let result = List.map addOne squares
        result

    printfn "processing %A through 'squareOddValuesAndAddOne' produces: %A" numbers (squareOddValuesAndAddOne numbers)
    
    /// A shorter way to write 'squareOddValuesAndAddOne' is to nest each
    /// sub-result into the function calls themselves.
    ///
    /// This makes the function much shorter, but it's difficult to see the
    /// order in which the data is processed.
    let squareOddValuesAndAddOneNested values = 
        List.map addOne (List.map square (List.filter isOdd values))

    printfn "processing %A through 'squareOddValuesAndAddOneNested' produces: %A" numbers (squareOddValuesAndAddOneNested numbers)

    /// A preferred way to write 'squareOddValuesAndAddOne' is to use F# pipe operators.
    /// This allows you to avoid creating intermediate results, but is much more readable
    /// than nesting function calls like 'squareOddValuesAndAddOneNested'
    let squareOddValuesAndAddOnePipeline values =
        values
        |> List.filter isOdd
        |> List.map square
        |> List.map addOne

    printfn "processing %A through 'squareOddValuesAndAddOnePipeline' produces: %A" numbers (squareOddValuesAndAddOnePipeline numbers)

    /// You can shorten 'squareOddValuesAndAddOnePipeline' by moving the second `List.map` call
    /// into the first, using a Lambda Function.
    ///
    /// Note that pipelines are also being used inside the lambda function.  F# pipe operators
    /// can be used for single values as well.  This makes them very powerful for processing data.
    let squareOddValuesAndAddOneShorterPipeline values =
        values
        |> List.filter isOdd
        |> List.map(fun x -> x |> square |> addOne)

    printfn "processing %A through 'squareOddValuesAndAddOneShorterPipeline' produces: %A" numbers (squareOddValuesAndAddOneShorterPipeline numbers)

先前所做的範例使用的許多功能的 F#,包括清單處理函式,第一級函式,並部分的應用程式The previous sample made use of many features of F#, including list processing functions, first-class functions, and partial application. 雖然深入瞭解每個概念可能會變得很重要, 但請清楚瞭解在建立管線時, 函數可以如何用來處理資料。Although a deep understanding of each of those concepts can become somewhat advanced, it should be clear how easily functions can be used to process data when building pipelines.

清單、陣列和順序Lists, Arrays, and Sequences

清單、 陣列和順序是 F# 核心程式庫中的三種主要集合類型。Lists, Arrays, and Sequences are three primary collection types in the F# core library.

清單是相同類型之元素的已排序、不可變集合。Lists are ordered, immutable collections of elements of the same type. 它們是單一連結的清單, 這表示它們是用來進行列舉, 但如果很大, 則不適合用于隨機存取和串連。They are singly-linked lists, which means they are meant for enumeration, but a poor choice for random access and concatenation if they're large. 相較于其他熱門語言的清單, 通常不會使用單一連結的清單來表示清單。This in contrast to Lists in other popular languages, which typically do not use a singly-linked list to represent Lists.

module Lists =

    /// Lists are defined using [ ... ].  This is an empty list.
    let list1 = [ ]  

    /// This is a list with 3 elements.  ';' is used to separate elements on the same line.
    let list2 = [ 1; 2; 3 ]

    /// You can also separate elements by placing them on their own lines.
    let list3 = [
        1
        2
        3
    ]

    /// This is a list of integers from 1 to 1000
    let numberList = [ 1 .. 1000 ]  

    /// Lists can also be generated by computations. This is a list containing 
    /// all the days of the year.
    let daysList = 
        [ for month in 1 .. 12 do
              for day in 1 .. System.DateTime.DaysInMonth(2017, month) do 
                  yield System.DateTime(2017, month, day) ]

    // Print the first 5 elements of 'daysList' using 'List.take'.
    printfn "The first 5 days of 2017 are: %A" (daysList |> List.take 5)

    /// Computations can include conditionals.  This is a list containing the tuples
    /// which are the coordinates of the black squares on a chess board.
    let blackSquares = 
        [ for i in 0 .. 7 do
              for j in 0 .. 7 do 
                  if (i+j) % 2 = 1 then 
                      yield (i, j) ]

    /// Lists can be transformed using 'List.map' and other functional programming combinators.
    /// This definition produces a new list by squaring the numbers in numberList, using the pipeline 
    /// operator to pass an argument to List.map.
    let squares = 
        numberList 
        |> List.map (fun x -> x*x) 

    /// There are many other list combinations. The following computes the sum of the squares of the 
    /// numbers divisible by 3.
    let sumOfSquares = 
        numberList
        |> List.filter (fun x -> x % 3 = 0)
        |> List.sumBy (fun x -> x * x)

    printfn "The sum of the squares of numbers up to 1000 that are divisible by 3 is: %d" sumOfSquares

陣列是一種固定大小 、可變動的元素集合, 屬於相同類型的專案。Arrays are fixed-size, mutable collections of elements of the same type. 它們支援的項目,快速隨機存取,且速度比 F# 清單因為它們只是連續記憶體區塊。They support fast random access of elements, and are faster than F# lists because they are just contiguous blocks of memory.

module Arrays =

    /// This is The empty array.  Note that the syntax is similar to that of Lists, but uses `[| ... |]` instead.
    let array1 = [| |]

    /// Arrays are specified using the same range of constructs as lists.
    let array2 = [| "hello"; "world"; "and"; "hello"; "world"; "again" |]

    /// This is an array of numbers from 1 to 1000.
    let array3 = [| 1 .. 1000 |]

    /// This is an array containing only the words "hello" and "world".
    let array4 = 
        [| for word in array2 do
               if word.Contains("l") then 
                   yield word |]

    /// This is an array initialized by index and containing the even numbers from 0 to 2000.
    let evenNumbers = Array.init 1001 (fun n -> n * 2) 

    /// Sub-arrays are extracted using slicing notation.
    let evenNumbersSlice = evenNumbers.[0..500]

    /// You can loop over arrays and lists using 'for' loops.
    for word in array4 do 
        printfn "word: %s" word

    // You can modify the contents of an array element by using the left arrow assignment operator.
    //
    // To learn more about this operator, see: https://docs.microsoft.com/dotnet/fsharp/language-reference/values/index#mutable-variables
    array2.[1] <- "WORLD!"

    /// You can transform arrays using 'Array.map' and other functional programming operations.
    /// The following calculates the sum of the lengths of the words that start with 'h'.
    let sumOfLengthsOfWords = 
        array2
        |> Array.filter (fun x -> x.StartsWith "h")
        |> Array.sumBy (fun x -> x.Length)

    printfn "The sum of the lengths of the words in Array 2 is: %d" sumOfLengthsOfWords

序列是專案的邏輯系列, 全都是相同的類型。Sequences are a logical series of elements, all of the same type. 這些是比清單和陣列更一般的類型, 可以將您的「視圖」放入任何邏輯專案序列中。These are a more general type than Lists and Arrays, capable of being your "view" into any logical series of elements. 它們也會脫穎而出, 因為它們可能是延遲的, 也就是說, 只有在需要時才會計算元素。They also stand out because they can be lazy, which means that elements can be computed only when they are needed.

module Sequences = 

    /// This is the empty sequence.
    let seq1 = Seq.empty

    /// This a sequence of values.
    let seq2 = seq { yield "hello"; yield "world"; yield "and"; yield "hello"; yield "world"; yield "again" }

    /// This is an on-demand sequence from 1 to 1000.
    let numbersSeq = seq { 1 .. 1000 }

    /// This is a sequence producing the words "hello" and "world"
    let seq3 = 
        seq { for word in seq2 do
                  if word.Contains("l") then 
                      yield word }

    /// This sequence producing the even numbers up to 2000.
    let evenNumbers = Seq.init 1001 (fun n -> n * 2) 

    let rnd = System.Random()

    /// This is an infinite sequence which is a random walk.
    /// This example uses yield! to return each element of a subsequence.
    let rec randomWalk x =
        seq { yield x
              yield! randomWalk (x + rnd.NextDouble() - 0.5) }

    /// This example shows the first 100 elements of the random walk.
    let first100ValuesOfRandomWalk = 
        randomWalk 5.0 
        |> Seq.truncate 100
        |> Seq.toList

    printfn "First 100 elements of a random walk: %A" first100ValuesOfRandomWalk

遞迴函式Recursive Functions

處理集合或序列的項目通常是使用遞迴F# 中。Processing collections or sequences of elements is typically done with recursion in F#. 雖然 F# 提供 for 迴圈和命令式程式設計的支援,但遞迴建議,因為很容易就能保證正確性。Although F# has support for loops and imperative programming, recursion is preferred because it is easier to guarantee correctness.

注意

下列範例會使用透過match運算式的模式比對。The following example makes use of the pattern matching via the match expression. 本文稍後將討論此基本結構。This fundamental construct is covered later in this article.

module RecursiveFunctions = 
              
    /// This example shows a recursive function that computes the factorial of an 
    /// integer. It uses 'let rec' to define a recursive function.
    let rec factorial n = 
        if n = 0 then 1 else n * factorial (n-1)

    printfn "Factorial of 6 is: %d" (factorial 6)

    /// Computes the greatest common factor of two integers.
    ///
    /// Since all of the recursive calls are tail calls,
    /// the compiler will turn the function into a loop,
    /// which improves performance and reduces memory consumption.
    let rec greatestCommonFactor a b =
        if a = 0 then b
        elif a < b then greatestCommonFactor a (b - a)
        else greatestCommonFactor (a - b) b

    printfn "The Greatest Common Factor of 300 and 620 is %d" (greatestCommonFactor 300 620)

    /// This example computes the sum of a list of integers using recursion.
    let rec sumList xs =
        match xs with
        | []    -> 0
        | y::ys -> y + sumList ys

    /// This makes 'sumList' tail recursive, using a helper function with a result accumulator.
    let rec private sumListTailRecHelper accumulator xs =
        match xs with
        | []    -> accumulator
        | y::ys -> sumListTailRecHelper (accumulator+y) ys
    
    /// This invokes the tail recursive helper function, providing '0' as a seed accumulator.
    /// An approach like this is common in F#.
    let sumListTailRecursive xs = sumListTailRecHelper 0 xs

    let oneThroughTen = [1; 2; 3; 4; 5; 6; 7; 8; 9; 10]

    printfn "The sum 1-10 is %d" (sumListTailRecursive oneThroughTen)

F# 也有完整支援 Tail 呼叫最佳化,這是一種最佳化,使其只是最快的速度迴圈建構的遞迴呼叫。F# also has full support for Tail Call Optimization, which is a way to optimize recursive calls so that they are just as fast as a loop construct.

記錄和區分聯集類型Record and Discriminated Union Types

記錄和等位型別中 F# 程式碼,使用兩種基本資料類型,通常的最佳方式來表示 F# 程式中的資料。Record and Union types are two fundamental data types used in F# code, and are generally the best way to represent data in an F# program. 雖然這會使它們類似于其他語言中的類別, 但其中一個主要差異在於它們有結構相等的語義。Although this makes them similar to classes in other languages, one of their primary differences is that they have structural equality semantics. 這表示它們是「原生」的可比較和相等, 只需檢查其中一個是否等於另一個。This means that they are "natively" comparable and equality is straightforward - just check if one is equal to the other.

記錄是已命名值的匯總, 具有選擇性成員 (例如方法)。Records are an aggregate of named values, with optional members (such as methods). 如果您熟悉C#或 JAVA, 則這些內容應該類似于 Poco 或 pojo, 只是結構化的相等和較少的儀式。If you're familiar with C# or Java, then these should feel similar to POCOs or POJOs - just with structural equality and less ceremony.

module RecordTypes = 

    /// This example shows how to define a new record type.  
    type ContactCard = 
        { Name     : string
          Phone    : string
          Verified : bool }
              
    /// This example shows how to instantiate a record type.
    let contact1 = 
        { Name = "Alf" 
          Phone = "(206) 555-0157" 
          Verified = false }

    /// You can also do this on the same line with ';' separators.
    let contactOnSameLine = { Name = "Alf"; Phone = "(206) 555-0157"; Verified = false }

    /// This example shows how to use "copy-and-update" on record values. It creates 
    /// a new record value that is a copy of contact1, but has different values for 
    /// the 'Phone' and 'Verified' fields.
    ///
    /// To learn more, see: https://docs.microsoft.com/dotnet/fsharp/language-reference/copy-and-update-record-expressions
    let contact2 = 
        { contact1 with 
            Phone = "(206) 555-0112"
            Verified = true }

    /// This example shows how to write a function that processes a record value.
    /// It converts a 'ContactCard' object to a string.
    let showContactCard (c: ContactCard) = 
        c.Name + " Phone: " + c.Phone + (if not c.Verified then " (unverified)" else "")

    printfn "Alf's Contact Card: %s" (showContactCard contact1)

    /// This is an example of a Record with a member.
    type ContactCardAlternate =
        { Name     : string
          Phone    : string
          Address  : string
          Verified : bool }

        /// Members can implement object-oriented members.
        member this.PrintedContactCard =
            this.Name + " Phone: " + this.Phone + (if not this.Verified then " (unverified)" else "") + this.Address

    let contactAlternate = 
        { Name = "Alf" 
          Phone = "(206) 555-0157" 
          Verified = false 
          Address = "111 Alf Street" }
   
    // Members are accessed via the '.' operator on an instantiated type.
    printfn "Alf's alternate contact card is %s" contactAlternate.PrintedContactCard

自 F# 4.1,您也可以代表記錄當做structs。As of F# 4.1, you can also represent Records as structs. 這會透過[<Struct>]屬性來完成:This is done with the [<Struct>] attribute:

/// Records can also be represented as structs via the 'Struct' attribute.
/// This is helpful in situations where the performance of structs outweighs
/// the flexibility of reference types.
[<Struct>]
type ContactCardStruct = 
    { Name     : string
      Phone    : string
      Verified : bool }

區分等位 (DUs)是可能是一些命名表單或案例的值。Discriminated Unions (DUs) are values which could be a number of named forms or cases. 儲存在類型中的資料可以是數個相異值的其中一個。Data stored in the type can be one of several distinct values.

module DiscriminatedUnions = 

    /// The following represents the suit of a playing card.
    type Suit = 
        | Hearts 
        | Clubs 
        | Diamonds 
        | Spades

    /// A Discriminated Union can also be used to represent the rank of a playing card.
    type Rank = 
        /// Represents the rank of cards 2 .. 10
        | Value of int
        | Ace
        | King
        | Queen
        | Jack

        /// Discriminated Unions can also implement object-oriented members.
        static member GetAllRanks() = 
            [ yield Ace
              for i in 2 .. 10 do yield Value i
              yield Jack
              yield Queen
              yield King ]
                                   
    /// This is a record type that combines a Suit and a Rank.
    /// It's common to use both Records and Discriminated Unions when representing data.
    type Card = { Suit: Suit; Rank: Rank }
              
    /// This computes a list representing all the cards in the deck.
    let fullDeck = 
        [ for suit in [ Hearts; Diamonds; Clubs; Spades] do
              for rank in Rank.GetAllRanks() do 
                  yield { Suit=suit; Rank=rank } ]

    /// This example converts a 'Card' object to a string.
    let showPlayingCard (c: Card) = 
        let rankString = 
            match c.Rank with 
            | Ace -> "Ace"
            | King -> "King"
            | Queen -> "Queen"
            | Jack -> "Jack"
            | Value n -> string n
        let suitString = 
            match c.Suit with 
            | Clubs -> "clubs"
            | Diamonds -> "diamonds"
            | Spades -> "spades"
            | Hearts -> "hearts"
        rankString  + " of " + suitString

    /// This example prints all the cards in a playing deck.
    let printAllCards() = 
        for card in fullDeck do 
            printfn "%s" (showPlayingCard card)

您也可以使用 DUs 作為單一案例的區分等位, 以協助透過基本型別進行領域模型化。You can also use DUs as Single-Case Discriminated Unions, to help with domain modeling over primitive types. 通常會使用字串和其他基本型別來代表某個專案, 因此會提供特定意義。Often times, strings and other primitive types are used to represent something, and are thus given a particular meaning. 不過, 只使用資料的基本標記法可能會造成錯誤地指派不正確的值!However, using only the primitive representation of the data can result in mistakenly assigning an incorrect value! 將每一種類型的資訊表示為不同的單一案例聯集, 可以在此案例中強制執行正確性。Representing each type of information as a distinct single-case union can enforce correctness in this scenario.

// Single-case DUs are often used for domain modeling.  This can buy you extra type safety
// over primitive types such as strings and ints.
//
// Single-case DUs cannot be implicitly converted to or from the type they wrap.
// For example, a function which takes in an Address cannot accept a string as that input,
// or vice versa.
type Address = Address of string
type Name = Name of string
type SSN = SSN of int

// You can easily instantiate a single-case DU as follows.
let address = Address "111 Alf Way"
let name = Name "Alf"
let ssn = SSN 1234567890

/// When you need the value, you can unwrap the underlying value with a simple function.
let unwrapAddress (Address a) = a
let unwrapName (Name n) = n
let unwrapSSN (SSN s) = s

// Printing single-case DUs is simple with unwrapping functions.
printfn "Address: %s, Name: %s, and SSN: %d" (address |> unwrapAddress) (name |> unwrapName) (ssn |> unwrapSSN)

如上述範例所示, 若要取得單一案例的區分等位中的基礎值, 您必須明確地將它解除包裝。As the above sample demonstrates, to get the underlying value in a single-case Discriminated Union, you must explicitly unwrap it.

此外, DUs 也支援遞迴定義, 可讓您輕鬆地表示樹狀結構和原本就遞迴的資料。Additionally, DUs also support recursive definitions, allowing you to easily represent trees and inherently recursive data. 例如, 以下說明如何使用existsinsert函式來表示二進位搜尋樹狀目錄。For example, here's how you can represent a Binary Search Tree with exists and insert functions.

/// Discriminated Unions also support recursive definitions.
///
/// This represents a Binary Search Tree, with one case being the Empty tree,
/// and the other being a Node with a value and two subtrees.
type BST<'T> =
    | Empty
    | Node of value:'T * left: BST<'T> * right: BST<'T>

/// Check if an item exists in the binary search tree.
/// Searches recursively using Pattern Matching.  Returns true if it exists; otherwise, false.
let rec exists item bst =
    match bst with
    | Empty -> false
    | Node (x, left, right) ->
        if item = x then true
        elif item < x then (exists item left) // Check the left subtree.
        else (exists item right) // Check the right subtree.

/// Inserts an item in the Binary Search Tree.
/// Finds the place to insert recursively using Pattern Matching, then inserts a new node.
/// If the item is already present, it does not insert anything.
let rec insert item bst =
    match bst with
    | Empty -> Node(item, Empty, Empty)
    | Node(x, left, right) as node ->
        if item = x then node // No need to insert, it already exists; return the node.
        elif item < x then Node(x, insert item left, right) // Call into left subtree.
        else Node(x, left, insert item right) // Call into right subtree.

由於 DUs 可讓您以資料類型表示樹狀結構的遞迴結構, 因此在此遞迴結構上操作十分簡單, 並保證正確性。Because DUs allow you to represent the recursive structure of the tree in the data type, operating on this recursive structure is straightforward and guarantees correctness. 模式比對也支援, 如下所示。It is also supported in pattern matching, as shown below.

此外, 您可以struct [<Struct>]使用屬性來將 DUs 表示為:Additionally, you can represent DUs as structs with the [<Struct>] attribute:

/// Discriminated Unions can also be represented as structs via the 'Struct' attribute.
/// This is helpful in situations where the performance of structs outweighs
/// the flexibility of reference types.
///
/// However, there are two important things to know when doing this:
///     1. A struct DU cannot be recursively-defined.
///     2. A struct DU must have unique names for each of its cases.
[<Struct>]
type Shape =
    | Circle of radius: float
    | Square of side: float
    | Triangle of height: float * width: float

不過, 在這麼做時, 有兩個重要的事項要牢記在心:However, there are two key things to keep in mind when doing so:

  1. 結構 DU 無法以遞迴方式定義。A struct DU cannot be recursively-defined.
  2. 結構 DU 的每一個案例都必須有唯一的名稱。A struct DU must have unique names for each of its cases.

如果無法遵循上述動作, 將會導致編譯錯誤。Failure to follow the above will result in a compilation error.

模式比對Pattern Matching

模式比對是 F# 語言功能,可讓 F# 類型上操作的正確性。Pattern Matching is the F# language feature which enables correctness for operating on F# types. 在上述範例中, 您可能已注意到相當多match x with ...的語法。In the above samples, you probably noticed quite a bit of match x with ... syntax. 此結構可讓編譯器 (可以瞭解資料類型的「圖形」) 在使用資料類型時, 透過所謂的完整模式比對來強制您考慮所有可能的情況。This construct allows the compiler, which can understand the "shape" of data types, to force you to account for all possible cases when using a data type through what is known as Exhaustive Pattern Matching. 這在正確性方面非常強大, 而且可以應變用來「增益」通常會在編譯時期發生的執行時間問題。This is incredibly powerful for correctness, and can be cleverly used to "lift" what would normally be a runtime concern into compile-time.

module PatternMatching =

    /// A record for a person's first and last name
    type Person = {
        First : string
        Last  : string
    }

    /// A Discriminated Union of 3 different kinds of employees
    type Employee =
        | Engineer of engineer: Person
        | Manager of manager: Person * reports: List<Employee>
        | Executive of executive: Person * reports: List<Employee> * assistant: Employee

    /// Count everyone underneath the employee in the management hierarchy,
    /// including the employee. The matches bind names to the properties 
    /// of the cases so that those names can be used inside the match branches.
    /// Note that the names used for binding do not need to be the same as the 
    /// names given in the DU definition above.
    let rec countReports(emp : Employee) =
        1 + match emp with
            | Engineer(person) ->
                0
            | Manager(person, reports) ->
                reports |> List.sumBy countReports
            | Executive(person, reports, assistant) ->
                (reports |> List.sumBy countReports) + countReports assistant


    /// Find all managers/executives named "Dave" who do not have any reports.
    /// This uses the 'function' shorthand to as a lambda expression.
    let rec findDaveWithOpenPosition(emps : List<Employee>) =
        emps
        |> List.filter(function
                       | Manager({First = "Dave"}, []) -> true // [] matches an empty list.
                       | Executive({First = "Dave"}, [], _) -> true
                       | _ -> false) // '_' is a wildcard pattern that matches anything.
                                     // This handles the "or else" case.

您可能已經注意到, 使用_模式。Something you may have noticed is the use of the _ pattern. 這就是所謂的萬用字元模式, 這是一個指出「我不在意什麼東西」的方法。This is known as the Wildcard Pattern, which is a way of saying "I don't care what something is". 雖然方便, 但如果您不小心使用_, 可以不小心略過完整的模式比對, 而不再受益于編譯時期實行原則。Although convenient, you can accidentally bypass Exhaustive Pattern Matching and no longer benefit from compile-time enforcements if you aren't careful in using _. 在模式比對時, 如果您不在意分解類型的特定片段, 或當您在模式比對運算式中列舉了所有有意義的案例時, 最好使用此方式。It is best used when you don't care about certain pieces of a decomposed type when pattern matching, or the final clause when you have enumerated all meaningful cases in a pattern matching expression.

現用模式是另一個搭配模式比對使用的強大結構。Active Patterns are another powerful construct to use with pattern matching. 它們可讓您將輸入資料分割成自訂表單, 並在模式比對呼叫位置分解它們。They allow you to partition input data into custom forms, decomposing them at the pattern match call site. 它們也可以參數化, 因此可讓將資料分割定義為函數。They can also be parameterized, thus allowing to define the partition as a function. 展開先前的範例以支援現用模式看起來像這樣:Expanding the previous example to support Active Patterns looks something like this:

// Active Patterns are another powerful construct to use with pattern matching.
// They allow you to partition input data into custom forms, decomposing them at the pattern match call site. 
//
// To learn more, see: https://docs.microsoft.com/dotnet/fsharp/language-reference/active-patterns
let (|Int|_|) = parseInt
let (|Double|_|) = parseDouble
let (|Date|_|) = parseDateTimeOffset
let (|TimeSpan|_|) = parseTimeSpan

/// Pattern Matching via 'function' keyword and Active Patterns often looks like this.
let printParseResult = function
    | Int x -> printfn "%d" x
    | Double x -> printfn "%f" x
    | Date d -> printfn "%s" (d.ToString())
    | TimeSpan t -> printfn "%s" (t.ToString())
    | _ -> printfn "Nothing was parse-able!"

// Call the printer with some different values to parse.
printParseResult "12"
printParseResult "12.045"
printParseResult "12/28/2016"
printParseResult "9:01PM"
printParseResult "banana!"

選擇性類型Optional Types

差別聯集類型的其中一個特殊案例是選項類型,這很有用,它是 F# 核心程式庫的一部分。One special case of Discriminated Union types is the Option Type, which is so useful that it's a part of the F# core library.

選項類型是代表兩個案例之一的類型: 值, 或完全沒有。The Option Type is a type which represents one of two cases: a value, or nothing at all. 在任何情況下, 如果值不一定是由特定作業所產生, 就會使用它。It is used in any scenario where a value may or may not result from a particular operation. 這會強制您將這兩種情況都列入考慮, 使其成為編譯時期的考慮, 而不是執行時間的顧慮。This then forces you to account for both cases, making it a compile-time concern rather than a runtime concern. 這些通常用於用來代表「 null無」的 api, 因此NullReferenceException在許多情況下都不需要擔心。These are often used in APIs where null is used to represent "nothing" instead, thus eliminating the need to worry about NullReferenceException in many circumstances.

/// Option values are any kind of value tagged with either 'Some' or 'None'.
/// They are used extensively in F# code to represent the cases where many other
/// languages would use null references.
///
/// To learn more, see: https://docs.microsoft.com/dotnet/fsharp/language-reference/options
module OptionValues = 

    /// First, define a zip code defined via Single-case Discriminated Union.
    type ZipCode = ZipCode of string

    /// Next, define a type where the ZipCode is optional.
    type Customer = { ZipCode: ZipCode option }

    /// Next, define an interface type that represents an object to compute the shipping zone for the customer's zip code, 
    /// given implementations for the 'getState' and 'getShippingZone' abstract methods.
    type IShippingCalculator =
        abstract GetState : ZipCode -> string option
        abstract GetShippingZone : string -> int

    /// Next, calculate a shipping zone for a customer using a calculator instance.
    /// This uses combinators in the Option module to allow a functional pipeline for
    /// transforming data with Optionals.
    let CustomerShippingZone (calculator: IShippingCalculator, customer: Customer) =
        customer.ZipCode 
        |> Option.bind calculator.GetState 
        |> Option.map calculator.GetShippingZone

測量單位Units of Measure

F# 型別系統的一項獨特功能是能夠透過單位的量值的數值常值的提供內容。One unique feature of F#'s type system is the ability to provide context for numeric literals through Units of Measure.

測量單位可讓您建立數數值型別與單位 (例如計量) 的關聯, 並讓函式在單位上執行工作, 而不是數值常值。Units of Measure allow you to associate a numeric type to a unit, such as Meters, and have functions perform work on units rather than numeric literals. 這可讓編譯器確認傳入的數值常數值型別在特定內容之下有意義, 因而排除與該類型工作相關聯的執行階段錯誤。This enables the compiler to verify that the types of numeric literals passed in make sense under a certain context, thus eliminating runtime errors associated with that kind of work.

/// Units of measure are a way to annotate primitive numeric types in a type-safe way.
/// You can then perform type-safe arithmetic on these values.
///
/// To learn more, see: https://docs.microsoft.com/dotnet/fsharp/language-reference/units-of-measure
module UnitsOfMeasure = 

    /// First, open a collection of common unit names
    open Microsoft.FSharp.Data.UnitSystems.SI.UnitNames

    /// Define a unitized constant
    let sampleValue1 = 1600.0<meter>          

    /// Next, define a new unit type
    [<Measure>]
    type mile =
        /// Conversion factor mile to meter.
        static member asMeter = 1609.34<meter/mile>

    /// Define a unitized constant
    let sampleValue2  = 500.0<mile>          

    /// Compute  metric-system constant
    let sampleValue3 = sampleValue2 * mile.asMeter   

    // Values using Units of Measure can be used just like the primitive numeric type for things like printing.
    printfn "After a %f race I would walk %f miles which would be %f meters" sampleValue1 sampleValue2 sampleValue3

F# 核心程式庫會定義許多的 SI 單位類型和單位轉換。The F# Core library defines many SI unit types and unit conversions. 若要深入瞭解, 請參閱Microsoft.FSharp.Data.UnitSystems.SI 命名空間To learn more, check out the Microsoft.FSharp.Data.UnitSystems.SI Namespace.

類別和介面Classes and Interfaces

F# 也有完整的支援,.NET 類別介面抽象類別繼承,依此類推。F# also has full support for .NET classes, Interfaces, Abstract Classes, Inheritance, and so on.

類別是代表 .net 物件的類型, 可以有屬性、方法和事件做為其成員Classes are types that represent .NET objects, which can have properties, methods, and events as its Members.

/// Classes are a way of defining new object types in F#, and support standard Object-oriented constructs.
/// They can have a variety of members (methods, properties, events, etc.)
///
/// To learn more about Classes, see: https://docs.microsoft.com/dotnet/fsharp/language-reference/classes
///
/// To learn more about Members, see: https://docs.microsoft.com/dotnet/fsharp/language-reference/members
module DefiningClasses = 

    /// A simple two-dimensional Vector class.
    ///
    /// The class's constructor is on the first line,
    /// and takes two arguments: dx and dy, both of type 'double'.
    type Vector2D(dx : double, dy : double) =

        /// This internal field stores the length of the vector, computed when the 
        /// object is constructed
        let length = sqrt (dx*dx + dy*dy)

        // 'this' specifies a name for the object's self-identifier.
        // In instance methods, it must appear before the member name.
        member this.DX = dx

        member this.DY = dy

        member this.Length = length

        /// This member is a method.  The previous members were properties.
        member this.Scale(k) = Vector2D(k * this.DX, k * this.DY)
    
    /// This is how you instantiate the Vector2D class.
    let vector1 = Vector2D(3.0, 4.0)

    /// Get a new scaled vector object, without modifying the original object.
    let vector2 = vector1.Scale(10.0)

    printfn "Length of vector1: %f\nLength of vector2: %f" vector1.Length vector2.Length

定義泛型類別也非常簡單。Defining generic classes is also very straightforward.

/// Generic classes allow types to be defined with respect to a set of type parameters.
/// In the following, 'T is the type parameter for the class.
///
/// To learn more, see: https://docs.microsoft.com/dotnet/fsharp/language-reference/generics/
module DefiningGenericClasses = 

    type StateTracker<'T>(initialElement: 'T) = 

        /// This internal field store the states in a list.
        let mutable states = [ initialElement ]

        /// Add a new element to the list of states.
        member this.UpdateState newState = 
            states <- newState :: states  // use the '<-' operator to mutate the value.

        /// Get the entire list of historical states.
        member this.History = states

        /// Get the latest state.
        member this.Current = states.Head

    /// An 'int' instance of the state tracker class. Note that the type parameter is inferred.
    let tracker = StateTracker 10

    // Add a state
    tracker.UpdateState 17

若要執行介面, 您可以使用interface ... with語法或物件運算式To implement an Interface, you can use either interface ... with syntax or an Object Expression.

/// Interfaces are object types with only 'abstract' members.
/// Object types and object expressions can implement interfaces.
///
/// To learn more, see: https://docs.microsoft.com/dotnet/fsharp/language-reference/interfaces
module ImplementingInterfaces =

    /// This is a type that implements IDisposable.
    type ReadFile() =

        let file = new System.IO.StreamReader("readme.txt")

        member this.ReadLine() = file.ReadLine()

        // This is the implementation of IDisposable members.
        interface System.IDisposable with
            member this.Dispose() = file.Close()


    /// This is an object that implements IDisposable via an Object Expression
    /// Unlike other languages such as C# or Java, a new type definition is not needed 
    /// to implement an interface.
    let interfaceImplementation =
        { new System.IDisposable with
            member this.Dispose() = printfn "disposed" }

要使用的類型Which Types to Use

類別、記錄、區分等位和元組的存在會導致一個重要的問題: 您應該使用哪一項?The presence of Classes, Records, Discriminated Unions, and Tuples leads to an important question: which should you use? 就像生活中的大部分專案一樣, 答案取決於您的情況。Like most everything in life, the answer depends on your circumstances.

元組非常適合用來從函式傳回多個值, 並使用值的臨機操作匯總作為值本身。Tuples are great for returning multiple values from a function, and using an ad-hoc aggregate of values as a value itself.

記錄是來自元組的「逐步執行」, 具有選擇性成員的命名標籤和支援。Records are a "step up" from Tuples, having named labels and support for optional members. 它們非常適合用來透過您的程式進行傳輸中的資料的低人。They are great for a low-ceremony representation of data in-transit through your program. 因為它們的結構相等, 所以比較容易使用。Because they have structural equality, they are easy to use with comparison.

區分聯集有許多用途, 但核心優點是能夠將它們與模式比對搭配使用, 以考慮資料可擁有的所有可能「圖形」。Discriminated Unions have many uses, but the core benefit is to be able to utilize them in conjunction with Pattern Matching to account for all possible "shapes" that a data can have.

類別非常適用于許多因素, 例如當您需要表示資訊, 以及將該資訊系結至功能時。Classes are great for a huge number of reasons, such as when you need to represent information and also tie that information to functionality. 根據經驗法則, 當您具有概念上與某些資料系結的功能時, 使用類別和麵向物件程式設計的原則是一個很大的好處。As a rule of thumb, when you have functionality which is conceptually tied to some data, using Classes and the principles of Object-Oriented Programming is a big benefit. 當與C#和 Visual Basic 交互操作時, 類別也是慣用的資料類型, 因為這些語言幾乎都是使用類別。Classes are also the preferred data type when interoperating with C# and Visual Basic, as these languages use classes for nearly everything.

後續步驟Next Steps

既然您已了解的一些主要功能的語言,您應該準備好開始撰寫第一個 F# 程式 !Now that you've seen some of the primary features of the language, you should be ready to write your first F# programs! 查看消費者入門以瞭解如何設定您的開發環境, 並撰寫一些程式碼。Check out Getting Started to learn how to set up your development environment and write some code.

進一步瞭解的後續步驟可以是您喜歡的任何方式, 但我們建議您在中F#進行功能程式設計的簡介, 以熟悉核心功能程式設計概念。The next steps for learning more can be whatever you like, but we recommend Introduction to Functional Programming in F# to get comfortable with core Functional Programming concepts. 這些會在建置強固的程式,在 F# 不可或缺。These will be essential in building robust programs in F#.

此外,請參閱F# 語言參考在 F# 中看到完整的概念性內容。Also, check out the F# Language Reference to see a comprehensive collection of conceptual content on F#.