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 →