F#理解歧视工会 [英] F# understanding discriminated union
问题描述
我早些时候曾问过这个问题，很抱歉再次提出类似的问题.但不幸的是，我无法真正理解如何设计一个受歧视的工会.
所以我有一堆像这样的数据结构
type Artist(artistId:int，name:String)=做如果名称= null，则为nullArg String.Emptynew(artistId:int)=艺术家(artistId)成员x.ArtistId = artistId成员x.Name =名称
和Genre()=让可变名称= String.Empty让可变的genreId:int = 0让可变描述= String.Empty让可变相册= List.empty成员x.说明使用get()=说明和set(value)=说明<-value
成员x.相册与get()=相册并设置(value)=相册<-值
和Album()=让可变标题= String.Empty让可变的albumId = 0让可变的genreId = 0让可变的artistId = 0让可变价格:十进制= Decimal.Zero让mutable albumArtUrl = String.Empty让可变类型= new Genre()让可变艺术家=新艺术家(artistId)成员x.Title使用get()= title和set(value)= title<-valuex.Genre成员使用get()=类型并设置(value)=类型<-value成员x.AlbumId与get()= albumId和set(value)= albumId<-value成员x.GenreId使用get()= genreId并设置(value)= genreId<-value成员x.ArtistId与get()= artistId并设置(value)= artistId<-value会员x.Price与get()=价格并设置(value)= price<-value成员x.AlbumArtUrl与get()= albumArtUrl和set(value)= albumArtUrl<-value成员x.艺术家使用get()= artist并设置(value)= artist<-value
在此处输入代码
我尝试根据F#专家的一些建议将以上内容定义为有区别的联合"
我定义如下的类型名称=字符串输入AlbumId = int
类型Artist =|int的ArtistId|姓名艺术家
type Album =|字符串标题|小数价格|AlbumId *艺术家的相册|字符串的AlbumArtUrl
类型流派=|int的GenreId|名称类型*相册列表
在这里输入代码 但是现在我无法弄清楚我将如何像对待简单的F#类型(只是属性)那样填充受歧视的工会?
有人可以帮我解释一下吗?我一直在阅读有歧视的工会，但不会说我完全理解他们.
区分联合用于表示具有多种不同情况的类型，这些情况大致对应于面向对象语言中的类层次结构.例如，可以这样定义基类 Shape ，其中两个继承的类分别是 Circle 和 Rectangle :
type Shape =|(float * float)*(float * float)的矩形//携带两个角的位置|(浮动*浮动)*浮动的圆//携带中心和直径 您定义受歧视的工会的方式并没有真正实现您可能想要的目的.您的类型专辑，艺术家和流派仅代表一种具体类型.
您可以使用记录(就像带有属性的轻量级类一样)或使用区分大小写的联合带有单个大小写(对应于单个类，但语法非常轻量)来表示它们，这是主要好处.例如:
类型名称=字符串类型价格=小数类型AlbumId = int类型ArtistId = int类型Artist = ArtistId的艺术家*名称类型专辑=专辑ID的专辑*名称*价格*艺术家 要与一些专辑一起构建艺术家，您可以编写:
让pinkFloyd = Artist(1，"Pink Floyd")let darkSide = Album(1，月球的黑暗面"，12.0M，pinkFloyd)let finalCut = Album(2，"The Final Cut"，11.0M，pinkFloyd) 如果您随后创建一个流派，其中将包含一个专辑列表，可能还包括一个艺术家列表，因此您可以编写如下内容:
类型类型=名称类型*艺术家列表*专辑列表let rock = Genre("Rock"，[pinkFloyd]，[darkSide; finalCut]) 现在的问题是，您实际上如何填充这些类型.您的数据源是什么?如果要从数据库或XML文件加载数据，则可能要编写一个函数，该函数占用数据源的一部分，并返回 Artist 或 Album ，然后在加载所有专辑和歌手之后，将它们包装在 Genre 中，并将其作为最终结果返回.
PS:回答您的问题有点困难，因为您并没有真正了解要尝试的工作.如果您可以举一个小而具体的示例(包括数据的加载及其使用)，那么有人可以帮助您从更实用的角度看问题.
I've kind of asked this question earlier so sorry for asking a bit similar question again. But unfortunately im not able to really understand how to design a discriminated unions.
so i have bunch of data structures which look like
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
But now i unable to figure out how would i populate my discriminated union similarly i was doing with my simple F# types which are just properties ?.
Can someone help me to explain this ?. I have been reading on discriminated unions but wont say i fully understand them .
Discriminated unions are used to represent types with multiple different cases, which roughly corresponds to class hierarchies in object oriented langauges. For example, a base class Shape with two inherited classes for Circle and Rectangle might be defined like this:
type Shape =
| Rectangle of (float * float) * (float * float) // Carries locations of two corners
| Circle of (float * float) * float // Carries center and diameter
The way you defined your discriminated unions does not really do what you probably intended. Your types Album, Artist and Genre represent just a single concrete type.
You can represent these with either records (which are just like lightweight classes with just properties) or using discriminated unions with a single case, which corresponds to a single class, but has a pretty lightweight syntax, which is the main benefit. For example:
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
To construct an artist together with a few albums, you can write:
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)
If you then create a genre, that will contain a list of albums and possibly a list of artists, so you could write something like this:
type Genre = Genre of Name * Artist list * Album list
let rock = Genre("Rock", [pinkFloyd], [darkSide; finalCut])
The question now is, how do you actually want to populate the types. What is your data-source? If you're loading data from a database or from a XML file, you're probably want to write a function that takes some part of the data source and returns Artist or Album and after you load all albums and artists, wrap them inside a Genre and return that as a final result.
PS: It is a bit difficult to answer your questions, because you're not really giving a bigger picture of what you're trying to do. If you can give a small, but concrete example (including the loading of data and their use), then someone can help you to look at the problem from a more functional perspective.
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