F＃瞭解歧視工會

Question

我有點問過這個問題，所以很抱歉再次提出類似的問題。但不幸的是我不能真正理解如何設計一個歧視的工會。

，所以我有一堆看起來像

 
 

type Artist(artistId : int, name : String) = do if name = null then nullArg String.Empty new(artistId: int) = Artist(artistId) member x.ArtistId = artistId member x.Name = name
 

and Genre() = let mutable name = String.Empty let mutable genreId : int = 0 let mutable description = String.Empty let mutable albums = List.empty member x.Description with get() = description and set(value) = description <- value
 member x.Albums with get() = albums and set (value) = albums <- value
 

and Album() = let mutable title = String.Empty let mutable albumId = 0 let mutable genreId = 0 let mutable artistId = 0 let mutable price : decimal = Decimal.Zero let mutable albumArtUrl = String.Empty let mutable genre = new Genre() let mutable artist = new Artist(artistId) member x.Title with get() = title and set (value) = title <- value member x.Genre with get() = genre and set (value) = genre <- value member x.AlbumId with get() = albumId and set (value) = albumId <- value member x.GenreId with get() = genreId and set (value) = genreId <- value member x.ArtistId with get() = artistId and set (value) = artistId <- value member x.Price with get() = price and set (value) = price <- value member x.AlbumArtUrl with get() = albumArtUrl and set (value) = albumArtUrl <- value member x.Artist with get() = artist and set (value) = artist <- value
 

enter code here
 

I tried defining the above as a Discriminated union based on suggestions by some of F# guru's
 

which i defined like below
 

 
 

type Name = string type AlbumId = int
 

type Artist = | ArtistId of int | Artist of Name
 

type Album = | Title of string | Price of decimal | Album of AlbumId * Artist | AlbumArtUrl of string
 

type Genre = | GenreId of int | Genre of Name * Album list 
 

enter code here 

數據結構，但現在我無法弄清楚如何將我填充我識別聯合同樣地，我與這只是性質我簡單的F＃類型做什麼？ 。

有人可以幫我解釋一下嗎？我一直在閱讀歧視工會，但不會說我完全理解他們。

Answer 1

判別聯合用於表示具有多種不同情況的類型，它們大致對應於面嚮對象語言中的類層次結構。例如，一個基類Shape有兩個繼承類Circle和Rectangle可能是這樣定義的：你定義了識別聯合

type Shape = 
    | Rectangle of (float * float) * (float * float) // Carries locations of two corners 
    | Circle of (float * float) * float    // Carries center and diameter 

的方式並沒有真正做什麼你可能打算。您的類型Album，Artist和Genre僅代表一種具體類型。

可以代表這些與任何記錄（這就像輕量級的班，只是性質），或者使用可識別聯合有一個案例，這相當於一個類，但有一個非常輕量級語法，這是主要好處。例如：

type Name = string 
type Price = decimal 
type AlbumId = int 
type ArtistId = int 

type Artist = Artist of ArtistId * Name 
type Album = Album of AlbumId * Name * Price * Artist 

要帶幾張專輯一起構建一個藝術家，你可以這樣寫：

let pinkFloyd = Artist(1, "Pink Floyd") 

let darkSide = Album(1, "The Dark Side of the Moon", 12.0M, pinkFloyd) 
let finalCut = Album(2, "The Final Cut", 11.0M, pinkFloyd) 

如果然後創建一個流派，將包含專輯列表和可能列表藝術家，所以你可以寫這樣的事情：

type Genre = Genre of Name * Artist list * Album list 

let rock = Genre("Rock", [pinkFloyd], [darkSide; finalCut]) 

現在的問題是，你怎麼竟要填充的類型。你的數據源是什麼？如果您要從數據庫或XML文件加載數據，則可能需要編寫一個函數，該函數需要部分數據源並返回Artist或Album，並且在加載所有專輯和藝術家後，將它們包裹在Genre並將其作爲最終結果返回。

PS：回答你的問題有點困難，因爲你並沒有真正給出你想要做什麼的更大圖景。如果你可以給出一個小的，但具體的例子（包括數據加載和使用），那麼有人可以幫助你從更實用的角度來看問題。