Languages used in the examples
This book provides code examples in two languages using a parallel translations model. The first language is Java because that language is ubiquitous. Hopefully, you know Java (or C++, or C#) and will be able to read the examples with intermediate proficiency. The second example language is the author’s own language, Unicon. While reading this book, you can judge for yourself which language is better suited to building programming languages. As many examples as possible are provided in both languages, and the examples in the two languages are written as similarly as possible. Sometimes, this will be to the advantage of Java, which is a bit lower level than Unicon. There are sometimes fancier or shorter ways to write things in Unicon, but our Unicon examples will stick as close to Java as possible. The differences between Java and Unicon will be obvious, but they are somewhat lessened in importance by the compiler construction tools we will use.
This book uses modern descendants of the venerable Lex and YACC tools to generate our scanner and parser. Lex and YACC are declarative programming languages that solve some of our hard problems at a higher level than Java or Unicon. It would have been nice if a modern descendant of Lex and YACC (such as ANTLR) supported both Java and Unicon, but such is not the case. One of the very cool parts of this book is this: by choosing tools for Java and Unicon that are very compatible with the original Lex and YACC and extending them a bit, we have managed to use the same lexical and syntax specifications of our compiler in both Java and Unicon!
While Java and Unicon are our implementation languages, we need to talk about one more language: the example language we are building. It is a stand-in for whatever language you decide to build. Somewhat arbitrarily, this book introduces a language called Jzero for this purpose. Niklaus Wirth invented a toy language called PL/0 (programming language zero; the name is a riff on the language name PL/1) that was used in compiler construction courses. Jzero is a tiny subset of Java that serves a similar purpose. I looked pretty hard (that is, I googled Jzero and then Jzero compiler) to see whether someone had already posted a Jzero definition we could use and did not spot one by that name, so we will just make it up as we go along.
The Java examples in this book will be tested using Java 21; maybe other recent versions of Java will work. You can get OpenJDK from http://openjdk.org, or if you are on Linux, your operating system probably has an OpenJDK package that you can install. Additional programming language construction tools (Jflex and byacc/j) that are required for the Java examples will be introduced in subsequent chapters as they are used. The Java implementations we will support might be more constrained by which versions will run these language construction tools than anything else.
The Unicon examples in this book work with Unicon version 13.3, which can be obtained from http://unicon.org. To install Unicon on Windows, you must download a .msi file and run the installer. To install on Linux, you should follow the instructions found on the unicon.org site.
Having gone through the basic organization of a programming language and the implementation that this book will use, perhaps we should take another look at when a programming language is called for, and when building one can be avoided by developing a library instead.