I briefly discussed copy-paste errors in code earlier; though this is a rich area of defects that I will probably at some point go into more detail on, that’s not for today.
Though this is a trivial little issue, I think it is worthwhile to illustrate how to think about these sorts of defects.
What is the defect?
Last week Jon Skeet “tweeted” humorously that in his work on the ECMA committee that is standardizing C#, they had found a mistake in the specification that was probably my fault; some commenters suggested that perhaps hell had also frozen over.
Last time on FAIC I described how language designers and compiler writers use “lowering” to transform a high-level program written in a particular language into a lower-level program written using simpler features of the same language. Did you notice what this technique presupposes? To illustrate, let’s take a look at a line of the C# specification:
the operation e as T produces the same result as e is T ? (T)(e) : (T)null except that e is only evaluated once.
This seems bizarre; why does the specification say that a higher-level operation is exactly the same as a lower-level operation, except that it isn’t? Because there is no way to express the exact semantics of as in legal C# in any lowered form! The feature “evaluate a subexpression to produce its side effects and value once and then use that value several times throughout its containing expression” does not exist in C#, so the specification is forced to describe the lowering in this imprecise and roundabout way.
A nice principle of programming language design that C# does not adhere to is that all the higher-level features of the program can be expressed as lower-level features. Why is this nice to have? Well, not only is it a big convenience for writers of the specification, it also is a big convenience for writers of language analyzers. If you can programmatically transform a high-level language into an exactly equivalent program in a language with far fewer complicated concepts then your analyzer gets simpler. Continue reading →
Programming language designers and users talk a lot about the “height” of language features; some languages are considered to be very “high level” and some are considered to be very “low level”. A “high level” language is generally speaking one which emphasizes the business concerns of the program, and a low-level language is one that emphasizes the mechanisms of the underlying hardware. As two extreme examples, here’s a program fragment in my favourite high-level language, Inform7: Continue reading →
Thanks to everyone who came out to the on-the-web and in-person events last week; it was an exhausting week but I had a great time. I’ll post the links to the recorded versions of the talks once I have them.
Rachel Roumeliotis, who amongst other things edits C# books for O’Reilly, recently did an interview with me where I ramble on about async/await, Roslyn, performance analysis as an engineering discipline, and some broad-strokes ideas for future language research areas. If you have sixteen minutes to burn, check it out! The O’Reilly Radar blog post is here, and the video has also been posted to YouTube here.
A couple things to mention here; first, I say in the video that we’ve shipped one preview release of Roslyn; in fact we have shipped two. The video was recorded before we had announced the new release. And second, I want to re-emphasize that the end bit where you get more of Eric’s musings about ideas for future language research areas are for your entertainment. We have not announced any product beyond Roslyn, and we are certainly making no promises whatsoever about the feature sets of unannounced, entirely hypothetical products. Enjoy!
We decided early in the Roslyn design process that the primary data structure that developers would use when analyzing code via Roslyn is the syntax tree. And thus one of the hardest parts of the early Roslyn design was figuring out how we were going to implement syntax tree nodes, and what information they would proffer up to the user. We would like to have a data structure that has the following characteristics:
The form of a tree.
Cheap access to parent nodes from child nodes.
Possible to map from a node in the tree to a character offset in the text.