Skip to main content

On Confluence and Type Parameter Unification in C#

Awhile back I had written about a a type unification nuisance I had run into. In a nutshell, the problem occurs when a class with two type parameters tries to implement the same interface twice, once for each type parameter:

// compiler error:
// 'Foo<T0,T1>' cannot implement both 'IEnumerable<T0>' and
// 'IEnumerable&;lt;T1>' because they may unify for some type
// parameter substitutions
public class Foo<T0, T1> : IEnumerable<T0>, IEnumerable<T1>

As Doug McClean pointed out in the comments, the reason behind this error is because the two implementations of the interfaces may not be confluent, ie. behaviourally identical, in which case there's no legitimate way to choose between the two.

The application I had in mind at the time used marker interfaces, ie. interfaces with no methods or properties, so they were guaranteed to be confluent. I also had a sneaking suspicion that C# already permitted this structure elsewhere, but they just try to superficially enforce this rule like some other annoying aesthetic constraints.

This turns out to be exactly the case, and it is possible to implement the same interface twice for two type parameters. All you need to do is implement the interfaces at two separate classes in the same inheritance hierarchy, and C# lets this pass with nary a whimper. Here's the sample code:

public interface IFoo<T>
    void Bar(T value);
public abstract class FooBase<T0, T1> : IFoo<T1>
    public void Bar(T1 value)
        Console.WriteLine("T1 Bar");
public sealed class Foo<T0, T1> : FooBase<T0, T1>, IFoo<T0>
    public void Bar(T0 value)
        Console.WriteLine("T0 Bar");
public static class Example
    public static void Main(string[] args)
        var core = new Foo<int, int>();
        var ifoo = core as IFoo<int>;
        var foob = core as FooBase<int, int>;
        var ifoo2 = foob as IFoo<int>;

        core.Bar(2);  // output: T0 Bar
        ifoo.Bar(2);  // output: T0 Bar
        foob.Bar(2);  // output: T1 Bar
        ifoo2.Bar(2); // output: T0 Bar

So reduction is still confluent because all views of IFoo<T> go through the most recent implementation in the inheritance hierarchy. The only way to call the Bar method on FooBase is by explicitly casting to an instance of FooBase and invoking Bar.

This recently bit me since I was implementing an IObserver<T> that was observing two differently typed streams, but because the interfaces were declared at different levels of the inheritance hierarchy, the compiler never complained. Not very common I agree, but for consistency, I'd suggest that either this structure too be ruled out, or that type unification on type parameters be permitted via some convention just like it's allowed via inheritance. For instance, select the implementation for the first (or last) type parameter.


Popular posts from this blog

async.h - asynchronous, stackless subroutines in C

The async/await idiom is becoming increasingly popular. The first widely used language to include it was C#, and it has now spread into JavaScript and Rust. Now C/C++ programmers don't have to feel left out, because async.h is a header-only library that brings async/await to C! Features: It's 100% portable C. It requires very little state (2 bytes). It's not dependent on an OS. It's a bit simpler to understand than protothreads because the async state is caller-saved rather than callee-saved. #include "async.h" struct async pt; struct timer timer; async example(struct async *pt) { async_begin(pt); while(1) { if(initiate_io()) { timer_start(&timer); await(io_completed() || timer_expired(&timer)); read_data(); } } async_end; } This library is basically a modified version of the idioms found in the Protothreads library by Adam Dunkels, so it's not truly ground bre

Easy Automatic Differentiation in C#

I've recently been researching optimization and automatic differentiation (AD) , and decided to take a crack at distilling its essence in C#. Note that automatic differentiation (AD) is different than numerical differentiation . Math.NET already provides excellent support for numerical differentiation . C# doesn't seem to have many options for automatic differentiation, consisting mainly of an F# library with an interop layer, or paid libraries . Neither of these are suitable for learning how AD works. So here's a simple C# implementation of AD that relies on only two things: C#'s operator overloading, and arrays to represent the derivatives, which I think makes it pretty easy to understand. It's not particularly efficient, but it's simple! See the "Optimizations" section at the end if you want a very efficient specialization of this technique. What is Automatic Differentiation? Simply put, automatic differentiation is a technique for calcu

Building a Query DSL in C#

I recently built a REST API prototype where one of the endpoints accepted a string representing a filter to apply to a set of results. For instance, for entities with named properties "Foo" and "Bar", a string like "(Foo = 'some string') or (Bar > 99)" would filter out the results where either Bar is less than or equal to 99, or Foo is not "some string". This would translate pretty straightforwardly into a SQL query, but as a masochist I was set on using Google Datastore as the backend, which unfortunately has a limited filtering API : It does not support disjunctions, ie. "OR" clauses. It does not support filtering using inequalities on more than one property. It does not support a not-equal operation. So in this post, I will describe the design which achieves the following goals: A backend-agnostic querying API supporting arbitrary clauses, conjunctions ("AND"), and disjunctions ("OR"). Implemen