What is the difference between val member and this member in F #?
When I created a class that contains a generic, mutable .NET Stack in F # like the example below, that stack ignores whatever I click on it.
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?!
However, if I make the stack mutable via a property:
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 as I expected.
What's going on here? My understanding of the immutability of previous languages (mainly C #) might say that even if the stack in the first example is an immutable member, that immutability must only go by reference (otherwise I won't be able to reassign the stack itself). I would still have to click values on it. What am I missing, and if trying to mutate this stack is wrong, why isn't it throwing an exception or compilation error?
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If you try to compile the first version and then use for example Reflector to decompile it to C #, you will see that the stack element is defined like this:
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.
Second version cross-compile to something like this:
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 is more like what you need to make a property.
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A more idiomatic way to do what you want is this:
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 from the outside, omit the next line.
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