Question

In language design circles there used to be a long-running debate over whether languages should use structural equivalence or name equivalence. Languages like ALGOL or ML or Modula-3 used structural equivalence while … well, pretty much most programming languages employ named equivalence (including Modula-2).

What are the typical arguments in favour of structural equivalence? What are the typical arguments in opposition to it? What are the typical arguments in favour of name equivalence? What are the typical arguments in opposition to it?

Answer 1

I think the advantage of structural type systems is that they encourage you to create fine-grained interfaces oriented towards what the user of the interface needs, rather than what the implementer provides.

In a nominative type system you need a common dependency on the interface. In a structural type system that requirement is eliminated: you can build a loosely coupled system without needing to create that common library where you put all the interfaces. Each client can independently declare the interface it expects from a collaborator.

The disadvantage of structural type systems is that they match up classes to interfaces which may not really implement the correct contract. For example, if you have this interface:

public interface IBananaProvider
{
   /// Returns a banana, never null.
   Banana GetBanana();
}

then the following class will implicitly be considered to implement IBananaProvider in a structural type system. However, the class violates the post condition that the returned banana is never null:

public class SomeBananaProvider
{
    // returns a banana or null if we're all out
    public Banana GetBanana()
    {
        if (bananas.Count > 0)
        {
            return bananas.RemoveLast();
        }
        else
        {
            return null;
        }
    }
} 

This could be fixed if the contract was somehow specified formally and considered part of the type structure. I think things are moving in that direction, e.g. System.Diagnostics.Contracts in .NET 4.0.