How are objects declared and defined in F#?

Question

I need clarity on how objects are declared and assigned a definition in F#.

What's happening in this code?

let service = { 
    new IService with
        member this.Translate(_) = raise error }

My guess is we're creating an object that will implement some interface on the fly even though there is no actual class that's backing this object. Hence, we're removing the ceremony involved with creating an object by not having to declare a separate class to use it. In this case, we're minimizing the ceremony involved for implementing a mock object that could be used within a unit test.

Is my understanding accurate?

I tried to research my question and found the specification for F# 3.0 (Section - 6.3.8 Object Expressions)

6.3.8 Object Expressions An expression of the following form is an object expression: { new ty0 args-expropt object-members interface ty1 object-members1 … interface tyn object-membersn } In the case of the interface declarations, the object-members are optional and are considered empty if absent. Each set of object-members has the form: with member-defns endopt Lexical filtering inserts simulated $end tokens when lightweight syntax is used. Each member of an object expression members can use the keyword member, override, or default. The keyword member can be used even when overriding a member or implementing an interface.

For example:

let obj1 = 
    { new System.Collections.Generic.IComparer<int> with         
          member x.Compare(a,b) = compare (a % 7) (b % 7) }

I found this link to be helpful: msdn.microsoft.com/en-us/library/dd233237.aspx — Scott Nimrod

Tomas Petricek Tomas Petricek · Accepted Answer · 2015-12-01T18:41:02

You can get a pretty good picture of what is happening behind the scenes if you look at the generated IL using a decompiler like ILSpy. For the example involving IComparer, it generates a hidden class, which implements the interface:

internal sealed class obj1@2 : IComparer<int> {
  public obj1@2() : this() { }
  int IComparer<int>.System-Collections-Generic-IComparer(int x, int y) {
    int num = x % 7;
    int num2 = y % 7;
    if (num < num2) { return -1; }
    return (num > num2) ? 1 : 0;
  }
}

Inside the body of the method, it then creates a new instance:

IComparer<int> obj1 = new obj1@2();

How are objects declared and defined in F#?

1 Answers