Looks interesting, but if I'm understanding correctly, this OO type class pattern is already widely used in Java/C# class libraries [1, 2]. The main thrust of the paper is then pointing out the connections of this pattern to type classes, and how to extend this pattern with implicit instantiation to encompass Haskell's implicit type class resolution. It took me a couple of reads to understand these two points.
C# suffers from a similar problem as Haskell here, re: numerous sort/sortBy and equals overloads for IComparable<T>/delegate/IComparer<T> instances. They've tried to mitigate this to some extent in recent releases by providing standard factory methods which perform runtime type tests on the type parameters and cache the resulting instances in static fields [3]. They then provide two overloads, one with an explicit IEqualityComparer<T> param, and one without that uses the implicit equality comparer obtained via EqualityComparer<T>.Default. This is obviously just a pattern to overcome the absence of implicits. However, while reading this paper I remembered that C# 4.0 is getting named and optional parameters with default values. Assuming default values could be specified using general static terms, ie. access static fields/properties or invoke static methods, we get some of the power of implicits. Only one overload would then be needed, with the default value being the term: EqualityComparer<T>.Default Unfortunately, default parameters are currently limited to constants [4], since they are embedded as attribute metadata. Named params with default values could generalize explicit type class instances by making all instances explicit, and specifying a standard path-dependent type as a default. There is quite a bit of hand-waving going on here of course, so take with a grain of salt. Sandro [1] IEqualityComparer<T>, http://msdn.microsoft.com/en-us/library/system.collections.iequalitycomparer.aspx [2] IComparer<T>, http://msdn.microsoft.com/en-us/library/system.collections.icomparer.aspx [3] http://msdn.microsoft.com/en-us/library/ms132123.aspx [4] http://msdn.microsoft.com/en-us/library/dd264739%28VS.100%29.aspx On 26/03/2010 4:33 AM, Bruno Oliveira wrote: > Hi, > > Adriaan Moors, Martin Odersky and I finally managed to submit the paper > that we have been working on. I think this is of interest to the > discussion, so here is a link to it: > > http://ropas.snu.ac.kr/%7Ebruno/papers/TypeClasses.pdf > > A quick comment below, but I am still not entirelly freed from the paper. > Still have a couple of hours to try to spot typos :). > _______________________________________________ bitc-dev mailing list bitc-dev@coyotos.org http://www.coyotos.org/mailman/listinfo/bitc-dev
