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shouldn't use reference equal?
f#

Suppose creating some manager class which is registed and unregisted some entries.

In c#,like below

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class Scheduler{
  List<Schedule> _scheduleList;
  public void Add(Schedule schedule){
    _scheduleList.Add(schedule);
  }

  public void Remove(Schedule schedule){
    _scheduleList.Remove(schedule);
  }
}

(Suppose Schdule is class)

The code above use reference equal to identify each instance.

In f# ,how should I do? Especially if I wanna use discriminated union for Schdule instead of class?

Should I add some unique identity id like SQL database?

Or I shoudn't write the code like this?

.

I am unclear about what the question/issue is.

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open System.Collections.Generic 
 
let li = new List<int>()
li.Add(3)
li.Remove(3) |> ignore
printfn "%d" li.Count 
 
let o = new obj()
let o2 = new obj()
let o3 = new obj()
let lo = new List<obj>()
lo.Add(o)
lo.Add(o2)
lo.Remove(o) |> ignore
lo.Remove(o3) |> ignore
printfn "%d" lo.Count 
 
type MyDU = Foo of int
let f = Foo(3)
let f2 = Foo(3)
let f3 = Foo(3)
let ld = new List<MyDU>()
lo.Add(f)
lo.Add(f2)
lo.Remove(f) |> ignore
lo.Remove(f3) |> ignore
printfn "%d" lo.Count
By on 8/9/2010 6:40 PM ()

My quiestion is how to treate identity of objects.

In C# and C++,I'm used to use pointer(a kind of) value as object's identity.

In F#, most of type values are treated like value type,so I'm confused.

In most of the cases,it may work well if I change my mind to treate most of them as just a value.

But if there are two list view of Schedule(suppose Schedule is discriminated union) for one collection of Schedule and I remove one Schedule from first view and want to reflect this deletion to second view,which row of second view should I delete?

In SQL,every rows have to have some unique identifier,but collection of f#'s value dosen't.

This makes me feel strange...

Should I add some fields for revealing unique identity?

By on 8/9/2010 8:07 PM ()

F# runs on the same CLR that C# does - objects have the same representation.

What may be confusing you is that F# types (which are just normal object references/pointers like C# objects) such as lists, tuples, etc., for your convenience have automatic structural equalty functionality defined so that they are equal (and compare) in "obvious" ways - so that (1,"xx") = (1,"xx"), (2,"xx") > (1,"xx"), etc. (Just as if you had defined Equals/HashCode/etc. on your class yourself with deep semantics.) If you don't want this behavior, you can tell F# not to do this for you by using attributes.

Some data-structures such as lists are immutable, but in no way are these "value types" from the CLR persective - they are still reference types, unless you define a struct - which is the same as C#.

By on 8/9/2010 9:20 PM ()

BTW, if your question was equality vs. equivalance (same object) and you want to be sure you have the *exact same object* you can use either one of these:

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let inline (==) a b = LanguagePrimitives.PhysicalEquality a b
let inline (==) a b : System.Object.ReferenceEquals(a, b)
By on 8/9/2010 9:24 PM ()

Thxs for your reply.

I heared the book ,Expert F# ,explains how to treate identity of values and objects.

I'll read it.

By on 8/11/2010 7:53 PM ()

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