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F# 的教學課程

了解 F# 的最佳方式是讀取和寫入 F# 程式碼。 本文將導覽 F# 的一些重要功能,並提供一些程式碼片段供您在機器上執行。 若想了解如何設定開發環境,請參閱使用者入門

F# 有兩個主要概念:函式和型別。 此導覽強調屬於這兩個概念的語言功能。

線上執行程式碼

如果您未在電腦上安裝 F#,可參閱在 Fable 中嘗試使用 F#,以在瀏覽器中執行所有範例。 Fable 是直接在瀏覽器中執行的 F# 方言。 若想檢視 REPL 中的後續範例,請參閱 Fable REPL 左側功能表列中的範例 > 了解 > F# 導覽

函式和模組

任何 F# 程式最基本的部分都是組成模組函式函式會在輸入上執行工作以產生輸出,並在模組下形成組織,這會是您在 F# 中將項目組成群組的主要方式。 它們透過 let 繫結定義,其可命名函式並定義其引數。

F#
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 繫結也是您將值繫結至名稱的方式,類似其他語言中的變數。 let 繫結預設為不可變,這表示一旦值或函式繫結至名稱,便無法就地變更。 這與其他語言中的變數相反,那些變數是可變動的,也就是說,其值可在任何時間點進行變更。 如果您需要可變動的繫結,可以使用 let mutable ... 語法。

F#
module Immutability =

    /// Binding a value to a name via 'let' makes it immutable.
    ///
    /// The second line of code compiles, but 'number' from that point onward will shadow the previous definition.
    /// There is no way to access the previous definition of 'number' due to shadowing.
    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 {otherNumber}"

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

    printfn $"'otherNumber' changed to be {otherNumber}"

數字、布林值和字串

作為 .NET 語言,F# 支援存在於 .NET 中相同基礎的基本類型

以下是 F# 中各種數值型別的表示方式:

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 an interpolated string.
    printfn $"The table of squares from 0 to 99 is:\n{sampleTableOfSquares}"

以下示範布林值和執行基本條件式邏輯:

F#
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}"

以下示範基本字串操作:

F#
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 $"{substring}"

元組

元組是 F# 中的重要部分。 它們是未命名但已排序之值的群組,可視為值本身。 您應該要將其視為從其他值彙總而來的值。 它們具有許多功能,例如供您輕易從函式傳回多個值,或臨機操作對一些值進行分組。

F#
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 {(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: {tuple1}\ttuple2: {tuple2}"

您也可以建立 struct 元組。 這些也會與 C#7/Visual Basic 15 元組完全相互操作,其也是 struct 元組:

F#
/// 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: {sampleStructTuple}\nReference tuple made from the Struct Tuple: {(sampleStructTuple |> convertFromStructTuple)}"

請務必注意,因為 struct 元組是實值型別,所以無法以隱含方式轉換成參考元組,反之亦然。 您必須在參考和結構元組之間明確轉換。

Pipelines

在 F# 中處理資料時,會廣泛使用管道運算子 |>。 此運算子可讓您以彈性方式建立函式的「管道」。 下列範例將逐步解說如何使用這些運算子來組建簡單的函式管道:

F#
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.
    ///
    /// '<>' is a binary comparison operator that means "not equal to".
    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 {numbers} through 'squareOddValuesAndAddOne' produces: {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 {numbers} through 'squareOddValuesAndAddOneNested' produces: {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 {numbers} through 'squareOddValuesAndAddOnePipeline' produces: {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 {numbers} through 'squareOddValuesAndAddOneShorterPipeline' produces: {squareOddValuesAndAddOneShorterPipeline numbers}"

    /// Lastly, you can eliminate the need to explicitly take 'values' in as a parameter by using '>>'
    /// to compose the two core operations: filtering out even numbers, then squaring and adding one.
    /// Likewise, the 'fun x -> ...' bit of the lambda expression is also not needed, because 'x' is simply
    /// being defined in that scope so that it can be passed to a functional pipeline.  Thus, '>>' can be used
    /// there as well.
    ///
    /// The result of 'squareOddValuesAndAddOneComposition' is itself another function which takes a
    /// list of integers as its input.  If you execute 'squareOddValuesAndAddOneComposition' with a list
    /// of integers, you'll notice that it produces the same results as previous functions.
    ///
    /// This is using what is known as function composition.  This is possible because functions in F#
    /// use Partial Application and the input and output types of each data processing operation match
    /// the signatures of the functions we're using.
    let squareOddValuesAndAddOneComposition =
        List.filter isOdd >> List.map (square >> addOne)

    printfn $"processing {numbers} through 'squareOddValuesAndAddOneComposition' produces: {squareOddValuesAndAddOneComposition numbers}"

先前的範例運用了 F# 的許多功能,包括清單處理函式、第一級函式和部分應用程式。 雖然這些屬於進階概念,但在組建管道時,應該要清楚如何使用函式來處理資料。

清單、陣列和序列

清單、陣列和序列是 F# 核心程式庫中的三個主要集合型別。

清單是相同型別元素已排序且不可變的集合。 這些清單是單向連結清單,也就是說,其目的是列舉,但如果隨機存取和串連很大,就不是好的選擇。 這與其他語言中的清單相反,它們通常不會使用單向連結清單來表示清單。

F#
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.
    ///
    /// 'yield' is used for on-demand evaluation. More on this later in Sequences.
    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: {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}"

陣列是相同型別元素固定大小且可變的集合。 其支援快速隨機的元素存取,而且速度比 F# 清單更快,因為它們只是連續的記憶體區塊。

F#
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: {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://learn.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'.
    ///
    /// Note that in this case, similar to Lists, array2 is not mutated by Array.filter.
    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}"

序列是元素的一系列邏輯,全都屬於相同型別。 這些是比清單和陣列更通用的型別,能夠將您的「檢視」轉變成任何的邏輯元素系列。 它們也特別顯著,因為可能會延遲,這意味著元素在必要時才能進行計算。

F#
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 is a 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: {first100ValuesOfRandomWalk}"

遞迴函式

集合或元素序列的處理通常會透過 F# 中的遞迴來完成。 雖然 F# 支援迴圈和命令式程式設計,但還是建議您使用遞迴,因為它更容易確保正確性。

注意

下列範例透過 match 運算式使用模式比對。 本文稍後會介紹此基本建構。

F#
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.
    ///
    /// '::' is used to split a list into the head and tail of the list,
    /// the head being the first element and the tail being the rest of the list.
    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# 也完全支援 Tail Call Optimization,這是最佳化遞迴呼叫、使其與迴圈建構一樣快速的一種方式。

記錄和差異聯集型別

記錄和聯集型別是用於 F# 程式碼中的兩種基本資料型別,通常是在 F# 程式中表示資料的最佳方式。 雖然這使它們類似於其他語言中的類別,但其中的主要差異之一在於:它們具有結構相等語意。 這表示它們「原生」可比較,相等也直接明瞭 - 只需檢查一個是否等於另一個即可。

記錄是具名值的彙總,不一定具有成員 (例如方法)。 如果您熟悉 C# 或 JAVA,應該會覺得這些看起來與 POCOs 或 POJO 相似 - 只是結構相等,而且比較不繁瑣。

F#
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://learn.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: {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 {contactAlternate.PrintedContactCard}"

您也可以將記錄表示為結構。 若要這麼做,請使用 [<Struct>] 屬性:

F#
[<Struct>]
type ContactCardStruct =
    { Name     : string
      Phone    : string
      Verified : bool }

差異聯集 (DU) 是可能為數個具名表單或案例的值。 儲存在型別中的資料有可能是數個相異值之一。

F#
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 $"{showPlayingCard card}"

您也可以使用 DU 作為單一案例差異聯集,以利基本類型的領域模型化。 字串和其他基本類型通常用於表示某物,因此具有特定意義。 然而,如果只使用資料的基本表示法,可能會導致系統錯誤指派不正確的值! 將每種資訊型別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。

F#
// 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: {address |> unwrapAddress}, Name: {name |> unwrapName}, and SSN: {ssn |> unwrapSSN}"

如上述範例所示,若要在單一案例的差異聯集中取得基礎值,您必須明確解除它。

此外,DU 也支援遞迴定義,可供您輕鬆表示樹狀結構及遞迴性質的資料。 例如,以下示範如何使用 existsinsert 函式來表示二進位搜尋的樹狀結構。

F#
/// 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.
///
/// Note 'T here is a type parameter, indicating that 'BST' is a generic type.
/// More on generics later.
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 DU 可供您表示資料型別中樹狀結構的遞迴結構,因此在這種遞迴結構上操作相當直接明瞭,並可確保正確性。 模式比對也支援這項功能,如下所示。

模式比對

模式比對是 F# 的一種功能,可供您確保在 F# 型別上運作的正確性。 在上述範例中,您或許有注意到一些 match x with ... 語法。 此建構可供編譯器了解資料型別的「圖形」,以強制您在透過「詳盡模式比對」的資料使用資料型別時,考慮所有可能的案例。 以正確性而言,這項功能非常強大,而且可以巧妙地用來將一般的執行階段問題「增益」為編譯時間問題。

F#
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

您可能也注意到了模式 _ 的使用方式。 這是萬用字元模式,是一種表達「我不在乎某事」的方式。 雖然方便,但如果您使用 _ 時不夠小心,可能會不小心略過詳盡模式比對,且不再享有編譯時間強制執行的好處。 若您希望在模式比對時忽略分解類型的某些部分,或是希望在模式比對運算式中列舉所有具意義的案例時忽略最後一個子句,最適合使用這個功能。

在下列範例中,剖析作業失敗時,會使用 _ 案例。

F#
/// Find all managers/executives named "Dave" who do not have any reports.
/// This uses the 'function' shorthand to as a lambda expression.
let 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.

/// You can also use the shorthand function construct for pattern matching,
/// which is useful when you're writing functions which make use of Partial Application.
let private parseHelper (f: string -> bool * 'T) = f >> function
    | (true, item) -> Some item
    | (false, _) -> None

let parseDateTimeOffset = parseHelper DateTimeOffset.TryParse

let result = parseDateTimeOffset "1970-01-01"
match result with
| Some dto -> printfn "It parsed!"
| None -> printfn "It didn't parse!"

// Define some more functions which parse with the helper function.
let parseInt = parseHelper Int32.TryParse
let parseDouble = parseHelper Double.TryParse
let parseTimeSpan = parseHelper TimeSpan.TryParse

現用模式是另一種強大的建構,可與模式比對搭配使用。 它們可供您將輸入資料分割至多個自訂表單,並在模式比對呼叫網站上進行分解。 它們也可以參數化,因此可將分割定義為函式。 展開前一個範例以支援作用中的模式,就像這樣:

F#
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 $"%O{d}"
    | TimeSpan t -> printfn $"%O{t}"
    | _ -> 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!"

選項。

差異聯集型別的一個特殊案例是選項型別,由於非常實用,因此屬於 F# 核心程式庫的一部分。

選項型別是一種型別,代表兩種案例之一:一個值,或完全沒有值。 其可用於值來自或並未來自特定作業的任何情況。 如此一來,這會強制您考量兩種情況,也就是使其成為編譯時間問題,而不是執行階段問題。 這些通常用於 API 中,null 用於表示「無」,因此您在許多情況下不需要擔心 NullReferenceException

F#
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

F# 的型別系統包含透過測量單位提供數值常值內容的能力。 測量單位可供您建立數值型別與單位之間的關聯 (例如計量),並讓函式在單位 (而不是數值常值) 上執行工作。 這讓編譯器得以驗證傳入的數值常數值型別在特定內容下具有意義,進而消除與該類型工作相關聯的執行階段錯誤。

F#
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{sampleValue1} race I would walk %f{sampleValue2} miles which would be %f{sampleValue3} meters"

F# 核心程式庫會定義許多 SI 單元型別和單位轉換。 如需更多資訊,請參閱 FSharp.Data.UnitSystems.SI.UnitSymbols 命名空間

物件程式設計

F# 完全支援透過類別、介面抽象類別繼承等所進行的物件程式設計。

類別是表示 .NET 物件的型別,其有可能將屬性、方法和事件作為其成員

F#
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{vector1.Length}\nLength of vector2: %f{vector2.Length}"

泛型類別的定義也相當直接明瞭。

F#
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

若要實作介面,您可使用 interface ... with 語法或物件運算式

F#
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

類別、記錄、差異聯集和元組的存在導致了一個重要問題:您應該使用哪一個? 如同生活中大多數的事物,答案端視您的情況而定。

元組非常適合用於從函式傳回多個值,並使用臨機操作的值彙總作為值本身。

記錄是元組的「升級」,有具名標籤且支援選擇性成員。 它們非常適合用於非正式表示資料傳輸中。 由於它們結構相等,因此可輕易與比較搭配使用。

差異聯集有許多用途,但主要好處是能夠與模式比對搭配使用,以將資料可能具有的所有可能「圖形」都納入考量。

類別用途很廣,例如當您需要表示資訊,以及將該資訊繫結至功能時,這非常實用。 根據經驗法則,當您具有概念上繫結至某些資料的功能時,使用類別和物件導向程式設計原則可帶來重大好處。 類別也是與 C# 和 Visual Basic 交互操作時慣用的資料型別,因為這些語言進行幾乎所有工作時都會使用類別。

後續步驟

現在您已了解語言的一些主要功能,應該已經準備好要寫入第一套 F# 程式了! 請參閱使用者入門,以了解如何設定開發環境及寫入程式碼。

此外,請參閱 F# 語言參考,以查看 F# 概念性內容的完整集合。