Building a Toy PostScript Interpreter in JavaScript

PostScript, a language traditionally used for describing printed documents, operates on a stack-based execution model where programs are processed as a sequence of whitespace-delimited tokens. By mapping these mechanics to JavaScript types, a developer can build a functional 'toy' interpreter that handles basic arithmetic, array manipulation, and procedure definitions.

The Stack-Based Execution Model

The core of PostScript's logic relies on two primary structures: an operand stack and a dictionary stack. In this model, operators pop values from the operand stack, perform a calculation, and push the result back onto the stack. Names, such as `/double`, are pushed onto the stack as literals, while other names are looked up in the dictionary to find associated procedures.

For example, the command `3 double` first pushes the number 3. The interpreter then looks up 'double' in the dictionary stack, finds its associated procedure (e.g., `{2 mul}`), and executes it. This procedure pops the 3, multiplies it by 2, and pushes the result, 6, back onto the stack.

“PostScript is primarily designed for describing printed documents: it has a rich vocabulary of features for creating and manipulating raster and vector graphics.”

— Let's Write a PostScript Interpreter

Mapping PostScript to JavaScript

To build a simplified interpreter, PostScript data types must be translated into JavaScript equivalents. Numbers and booleans map directly to JavaScript's `Number` and `Boolean` types, while PostScript arrays and dictionaries are represented as JavaScript `Array` and `Map` objects. A unique PostScript concept, the `mark`, is implemented as a singleton object to track stack positions for array construction.

The parsing process uses a regular expression to break a program into tokens, filtering out line comments (starting with `%`). The interpreter then iterates through these tokens. If a token is a name without a `/` prefix, the system retrieves its procedure from the dictionary; if it is a procedure, the interpreter executes the sequence of tokens contained within the curly braces.

This code listing illustrates the translation of PostScript types into JavaScript objects and the execution of procedures.

Implementation and Limitations

The interpreter's machinery is largely contained in primitive operator definitions. These include standard arithmetic (add, sub, mul, div), stack manipulations (dup, exch, roll), and control flow (ifelse, for, forall). By installing these primitives into the dictionary stack during initialization, the interpreter can execute complex nested expressions like `[11 22 33] {10 mul} forall`.

This visual provides a concrete example of the interpreter's machinery and how it handles the sequence of tokens.

This implementation serves as a 'human-scale' exercise to understand language mechanics rather than a production-ready tool. It intentionally elides complex features like string handling and error-handling for simplicity. A full-scale PostScript engine, such as Ghostscript, involves significant additional complexity regarding high-fidelity graphics rendering and font management that is outside the scope of a basic interpreter.