[英]What is the difference between member val and member this in F#?
When I created a class containing a generic, mutable .NET Stack in F# like the example below, that stack ignores anything I push onto it. 当我在F#中创建一个包含通用的,可变的.NET Stack的类时,如下例所示,该堆栈忽略了我推送它的任何东西。
open System.Collections.Generic
type Interp(code: int array) =
member val PC = 0 with get, set
member this.stack: Stack<int> = new Stack<int>()
member this.Code = code
let interp = Interp([|1;2;3|])
interp.stack.Push(1)
printfn "%A" interp.stack // prints "seq[]" WAT?!
Yet if I make the stack mutable via a property: 然而,如果我通过属性使堆栈可变:
open System.Collections.Generic
type Interp(code: int array) =
member val PC = 0 with get, set
member val stack: Stack<int> = new Stack<int>() with get, set
member this.Code = code
let interp = Interp([|1;2;3|])
interp.stack.Push(1)
printfn "%A" interp.stack // prints "seq[1]"
Everything magically works like I'd expect. 一切都像我期望的那样神奇地起作用。
What on earth is going on here? 这到底是怎么回事? My understanding of immutability from previous languages (C# mostly) would say that even though the stack in the first example is an immutable member, that immutablity should only go as far the reference (aka I shouldn't be able to reassign Stack itself). 我对以前语言(大多数是C#)的不变性的理解会说,即使第一个例子中的堆栈是一个不可变成员,这个不可变性应该只是参考(我不应该重新分配Stack本身)。 I should still be able to push values to/from it. 我仍然可以将值推送到/从中推送。 What am I missing, and if trying to mutate that stack is the wrong thing, why doesn't it throw an exception or a compile error? 我错过了什么,如果试图改变该堆栈是错误的,为什么不抛出异常或编译错误?
If you try to compile the first version, and then use eg Reflector to decompile it to C#, you'll see that the stack member is defined like this: 如果您尝试编译第一个版本,然后使用例如Reflector将其反编译为C#,您将看到堆栈成员的定义如下:
public class Interp
{
public Stack<int> stack
{
get { return new Stack<int>(); }
}
// Other members omitted for clarity...
}
As you can see, this is also valid C# code, but obviously not what you want. 如您所见,这也是有效的C#代码,但显然不是您想要的。
The second version cross-compiles to something like this: 第二个版本交叉编译成这样的东西:
public class Interp
{
internal int[] code;
internal Stack<int> stack@;
public Interp(int[] code) : this()
{
this.code = code;
this.stack@ = new Stack<int>();
}
public Stack<int> stack
{
get { return this.stack@; }
set { this.stack@ = value; }
}
// Other members omitted for clarity...
}
This seems more like what you'd want a property to do. 这看起来更像是你想要财产做的事情。
A more idiomatic way to do what you want is this: 做一个你想要的更惯用的方法是:
open System.Collections.Generic
type Interp(code: int array) =
let stack = Stack<int>()
member val PC = 0 with get, set
member this.Stack = stack
member this.Code = code
If you don't need to expose the stack externally, omit the next to last line. 如果您不需要在外部公开堆栈,请省略最后一行。
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