Let’s Write a PostScript Interpreter

In this article, we will explore the process of implementing a simplified version of the PostScript programming language using JavaScript. PostScript is primarily used for describing printed documents and provides a rich set of features for creating and manipulating graphics. For the purposes of this article, we will focus on its mechanics as a general-purpose programming language.

PostScript programs consist of whitespace-delimited tokens, such as % for line comments, / for literal names, ( ) for strings, [, ] for arrays, and { } for procedures. The execution model involves two stacks: the operand stack for passing arguments between operators and the dictionary stack for associating names with values.

To build our PostScript interpreter, we will use a concise style of JavaScript influenced by the Forth programming language. We will represent PostScript datatypes using JavaScript data types and define helper routines to construct, access, and identify various PostScript values.

The interpreter will tokenize the input program, splitting it into line comments, delimiters, names, and numbers. We will then manipulate the operand and dictionary stacks to execute the postScript program. Names will be looked up in the dictionary stack and the corresponding operators or procedures will be called. Anything else will be pushed onto the operand stack.

The interpreter will consist of primitive operator definitions, which are individually simple in nature. However, the for operator will require additional helper routines for generating stepped ranges. Some of these operators will be pure JavaScript functions that do not directly access or modify the stacks.

To provide interactive use, we can create a Read-Eval-Print-Loop (REPL) that allows users to input PostScript programs and observe their effects on the stacks. For Node.js, we can use an event-driven style, while for QuickJS, a more straightforward infinite loop can be used.

Our interpreter will offer a small subset of the primitive operators found in standard PostScript. We can enhance it by implementing additional operators such as abs to compute the absolute value of a number and min to find the minimum of two values on the stack. Additionally, we can explore classic functional programming exercises like recursively computing the Fibonacci sequence.

While there are more idiomatic iterative approaches, the exploration of these concepts can be a fun and educational exercise in implementing a PostScript interpreter.