Higher Logics

Jeroen Frijters helpfully pointed out that the CLR implements some hard limits on nesting generics, which is 99 for .NET 4 based on my tests. My previous implementation of ThreadScoped<T> was thus limited to 99 instances. Not very useful! The solution is actually quite simple, which I briefly outlined on Jeroen's blog: add more type parameters and use a simple base-99 counting scheme to generate new instances. Each additional type parameters thus increases the permutations 99 fold. One type index parameter yields 99 1 instances, two type index parameters yields 99 2 instances, three type index parameters yields 99 3 , and so on. No one in the foreseeable future will require more than 99 3 , which is almost a million thread-local variables, so I've added two more type index parameters to make Ref<T0, T1, T2> . The instance allocation function is now: internal override ThreadScoped<T> Allocate() { // If 'next' is null, we are at the end of th...

Awhile ago, Jeroen Frijters blogged about .NET 4's new ThreadLocal<T>, and how it was implemented. It was based on a neat generics trick exploiting the fact that the CLR does not erase types. Indexing a generic type X with a type T, means X<string> has distinct static and thread-local fields from say, X<int>. I've actually used this trick before to implement a type of safe reflection. Jeroen's sample implementation exploiting this trick for thread-local instance variables seemed far too complicated however, and it suffered from a number of limitations that he notes in his post. If Microsoft's implementation was even more complicated as was claimed, I wasn't confident it would perform very well at all. I decided to do my own implementation, without conforming the ThreadLocal<T>'s interface since I have very specific applications in mind. ThreadScoped<T> As I suspected, my implementation was considerably simpler , coming in a...

There's a great post on implementing idioms with LINQ, and the example application was to implement formlets, as WebSharper does for F#. Tomas's post is well written, so if you're unclear on the above concepts I recommend reading it first before proceeding with this article. The claim in that post is that idioms can only be encoded via LINQ's 'join' operators. While strictly true if you stick to all the LINQ rules, because LINQ queries are just naive syntactic transforms you don't have to follow the rules. You can thus exploit this to hijack the signatures for the SelectMany overloads to yield idiom signatures. It's not all sunshine and roses though, as there are consequences. Overview LINQ is a standard set of methods one can implement that the C# compiler can use to provide "query patterns". This query: var foo = from x in SomeFoo from y in foo.Values select y; is translated by the C# compiler to: var foo = Some...

Equality and identity are often intermingled in the .NET framework. .NET delegates correctly implement a straightforward structural equality , where if two delegates are equal if they designate the same method and the same object, regardless of whether the delegate is a unique instance: class Bar { public void Foo(int i) { } } var bar = new Bar(); Action<int> a1 = bar.Foo; Action<int> a2 = bar.Foo; Console.WriteLine("Delegate Equality: {0}", a1 == a2); // prints: // Delegate Equality: True However, an IObservable<T> created from delegates does not respect structural equality, and reverts to an identity criterion: // use a1 and a2 from previous code sample var o1 = Observer.Create<int>(a1); var o2 = Observer.Create<int>(a2); Console.WriteLine("Observable Equality: {0}", o1 == o2 || o1.Equals(o2) || EqualityComparer<IObserver<int>>.Default.Equals(o1, o2)); //prints: // Obs...