Establishing the requirements for your language

After you are sure you need a new programming language for what you are doing, take a few minutes to establish the requirements. This is open-ended. It is you defining what success for your project will look like. Wise language inventors do not create a whole new syntax from scratch. Instead, they define it in terms of a set of modifications to make to a popular existing language. Many great programming languages (Lisp, Forth, SmallTalk, and many others) had their success significantly limited by the degree to which their syntax was unnecessarily different from mainstream languages. Still, your language requirements include what it will look like, and that includes syntax.

More importantly, you must define a set of control structures or semantics where your programming language needs to go beyond existing language(s). This will sometimes include special support for an application domain that is not well-served by existing languages and their libraries. Such domain-specific languages (DSLs) are common enough that whole books are focused on that topic. Our goal for this book will be to focus on the nuts and bolts of building the compiler and runtime system for such a language, independent of whatever domain you may be working in.

In a normal software engineering process, requirements analysis would start with brainstorming lists of functional and non-functional requirements. Functional requirements for a programming language involve the specifics of how the end user developer will interact with it. You might not anticipate all the command-line options for your language upfront, but you probably know whether interactivity is required, or whether a separate compile step is OK. The discussion of interpreters and compilers in the previous section, and this book's presentation of a compiler, might seem to make that choice for you, but Python is an example of a language that provides a fully interactive interface, even though the source code you type in it gets crunched into bytecode rather than interpreted.

Non-functional requirements are properties that your programming language must achieve that are not directly tied to the end user developer's interactions. They include things such as what operating system(s) it must run on, how fast execution must be, or how little space the programs written in your language must run within.

The non-functional requirement regarding how fast execution must be usually determines the answer as to whether you can target a software (bytecode) machine or need to target native code. Native code is not just faster; it is also considerably more difficult to generate, and it might make your language considerably less flexible in terms of runtime system features. You might choose to target bytecode first, and then work on a native code generator afterward.

The first language I learned to program on was a BASIC interpreter in which the programs had to run within 4 KB of RAM. BASIC at the time had a low memory footprint requirement. But even in modern times, it is not uncommon to find yourself on a platform where Java won't run by default! For example, on virtual machines with configured memory limits for user processes, you may have to learn some awkward command-line options to compile or run even simple Java programs.

Many requirements analysis processes also define a set of use cases and ask the developer to write descriptions for them. Inventing a programming language is different from your average software engineering project, but before you are finished, you may want to go there. A use case is a task that someone performs using a software application. When the software application is a programming language, if you are not careful, the use cases may be too general to be useful, such as write my application and run my program. While those two might not be very useful, you might want to think about whether your programming language implementation must support program development, debugging, separate compilation and linking, integration with external languages and libraries, and so forth. Most of those topics are beyond the scope of this book, but we will consider some of them.

Since this book will present the implementation of a language called Jzero, here are some requirements for it. Some of these requirements may appear arbitrary. If it is not clear to you where one of them came from, it either came from our source inspiration language (plzero) or previous experience teaching compiler construction:

• Jzero should be a strict subset of Java. All legal Jzero programs should be legal Java programs. This requirement allows us to check the behavior of our test programs when we are debugging our language implementation.
• Jzero should provide enough features to allow interesting computations. This includes if statements, while loops, and multiple functions, along with parameters.
• Jzero should support a few data types, including Booleans, integers, arrays, and the String type. It only needs to support a subset of their functionality, as described later. These are enough types to allow input and output of interesting values into a computation.
• Jzero should emit decent error messages, showing the filename and line number, including messages for attempts to use Java features not in Jzero. We will need reasonable error messages to debug the implementation.
• Jzero should run fast enough to be practical. This requirement is vague, but it implies that we won't be doing a pure interpreter. Pure interpreters are a very retro thing, evocative of the 1960s and 1970s.
• Jzero should be as simple as possible so that I can explain it. Sadly, this rules out generating native code or even JVM bytecode; we will provide our own simple bytecode machine.

Perhaps more requirements will emerge as we go along, but this is a start. Since we are constrained for time and space, perhaps this requirements list is more important for what it does not say, rather than for what it does say. By way of comparison, here are some of the requirements that led to the creation of the Unicon programming language.