8.1. The Road Home

So far, we've covered the parsing and translation of arithmetic expressions, Boolean expressions, and combinations connected by relational operators. We've also done the same for control constructs. In all of this we've leaned heavily on the use of top-down, recursive descent parsing, BNF definitions of the syntax, and direct generation of assembly-language code. We also learned the value of such tricks as single-character tokens to help us see the forest through the trees. In the last installment we dealt with lexical scanning, and I showed you simple but powerful ways to remove the single-character barriers.

Throughout the whole study, I've emphasized the KISS philosophy … Keep It Simple, Sidney … and I hope by now you've realized just how simple this stuff can really be. While there are for sure areas of compiler theory that are truly intimidating, the ultimate message of this series is that in practice you can just politely sidestep many of these areas. If the language definition cooperates or, as in this series, if you can define the language as you go, it's possible to write down the language definition in BNF with reasonable ease. And, as we've seen, you can crank out parse procedures from the BNF just about as fast as you can type.

As our compiler has taken form, it's gotten more parts, but each part is quite small and simple, and very much like all the others.

At this point, we have many of the makings of a real, practical compiler. As a matter of fact, we already have all we need to build a toy compiler for a language as powerful as, say, Tiny BASIC. In the next couple of installments, we'll go ahead and define that language.

To round out the series, we still have a few items to cover. These include:

These will all be covered in future chapters. When we're finished, you'll have all the tools you need to design and build your own languages, and the compilers to translate them.

I can't design those languages for you, but I can make some comments and recommendations. I've already sprinkled some throughout past installments. You've seen, for example, the control constructs I prefer.

These constructs are going to be part of the languages I build. I have three languages in mind at this point, two of which you will see in installments to come:

TINY
A minimal, but usable language on the order of Tiny BASIC or Tiny C. It won't be very practical, but it will have enough power to let you write and run real programs that do something worthwhile.
KISS
The language I'm building for my own use. KISS is intended to be a systems programming language. It won't have strong typing or fancy data structures, but it will support most of the things I want to do with a higher-order language (HOL), except perhaps writing compilers.

I've also been toying for years with the idea of a HOL-like assembler, with structured control constructs and HOL-like assignment statements. That, in fact, was the impetus behind my original foray into the jungles of compiler theory. This one may never be built, simply because I've learned that it's actually easier to implement a language like KISS, that only uses a subset of the CPU instructions. As you know, assembly language can be bizarre and irregular in the extreme, and a language that maps one-for-one onto it can be a real challenge. Still, I've always felt that the syntax used in conventional assemblers is dumb … why is

MOVE.L A,B

better, or easier to translate, than

B=A

I think it would be an interesting exercise to develop a “compiler” that would give the programmer complete access to and control over the full complement of the CPU instruction set, and would allow you to generate programs as efficient as assembly language, without the pain of learning a set of mnemonics. Can it be done? I don't know. The real question may be, “Will the resulting language be any easier to write than assembly?” If not, there's no point in it. I think that it can be done, but I'm not completely sure yet how the syntax should look.

Perhaps you have some comments or suggestions on this one. I'd love to hear them.

You probably won't be surprised to learn that I've already worked ahead in most of the areas that we will cover. I have some good news: Things never get much harder than they've been so far. It's possible to build a complete, working compiler for a real language, using nothing but the same kinds of techniques you've learned so far. And that brings up some interesting questions.