Robert Harper has (somewhat) recently released a pre-print of a book (PDF) that he has been working on, Practical Foundations for Programming Languages. I downloaded a copy when it initially came out, but I was guilty of putting off actually digging into the book’s 590-some pages. It was only until Harper successfully baited me with one of his most recent blog posts that I finally sat down and skimmed it a bit more thoroughly.
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The holy grail of web application development is a single language which runs on both the server side and the client side. The reasons for this are multifarious: a single language promotes reuse of components that no longer need to be reimplemented in two languages and allows for much more facile communication between the server and the client. Web frameworks that explicitly strive to handle both the server and client include Meteor, Ur/Web, Opa and Google Web Toolkit.
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Ur is a programming language, which among other things, has a rather interesting record system. Record systems are a topic of rather intense debate in the Haskell community, and I noticed that someone had remarked “[Ur/Web has a http://www.impredicative.com/ur/tutorial/tlc.html very advanced records system]. If someone could look at the UR implementation paper and attempt to distill a records explanation to a Haskell point of view that would be very helpful!” This post attempts to perform that distillation, based off my experiences interacting with the Ur record system and one of its primary reasons for existence: metaprogramming. (Minor nomenclature note: Ur is the base language, while Ur/Web is a specialization of the base language for web programming, that also happens to actually have a compiler. For the sake of technical precision, I will refer to the language as Ur throughout this article.)
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