Implementing a Subset of PostScript in JavaScript
This article explores the process of implementing a simplified subset of the PostScript programming language using JavaScript, focusing on its syntax and execution model. The implementation includes parsing, data…
PostScript is primarily designed for describing printed documents but can also function as a general-purpose programming language. Programs are composed of whitespace-delimited tokens and several special characters, such as %, /, ( ), < >, [ ], and { }, which serve as delimiters or comments. For example, the PostScript program fragment % This is a line comment /double {2 mul} def 3 double is parsed into tokens ['/double','{','2','mul','}','def','3','double'] (source ID 1). [1]
The PostScript execution model relies on several stacks, notably the operand stack for passing arguments between operators and the dictionary stack for associating names with values. In the example, the name /double is pushed onto the operand stack, and the procedure {2 mul} is associated with the name through the def operator. The number 3 is then pushed, and the procedure is executed by retrieving the associated procedure from the dictionary stack (source ID 1). [1]
To implement a subset of PostScript in JavaScript, data types are represented using JavaScript equivalents, such as numbers, booleans, names, dictionaries, arrays, and procedures. The parse function uses a JavaScript regular expression to split the input program into tokens, while the operand stack (o) and dictionary stack (d) are represented as global arrays. Simple routines manipulate these stacks, such as push, pop, and popmark (source ID 1). [1]
Primitive operators, such as add, mul, and def, are defined as JavaScript functions and installed into the dictionary stack for use during execution. An initialization function sets up the stacks with the primitives, and the interpreter executes programs by processing tokens sequentially, pushing values onto the operand stack or invoking operators. A REPL interface can be added to allow interactive use of the interpreter (source ID 1). [1]
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The PostScript interpreter discussed in the source is implemented in JavaScript and is designed to be concise, following a style influenced by Forth. This interpreter is limited in scope, focusing on a subset of PostScript's features, and omits certain elements such as string handling and more complex error-checking mechanisms. The interpreter translates PostScript syntax into JavaScript data structures, using objects and arrays to represent PostScript values like numbers, booleans, names, dictionaries, arrays, and procedures. The core of the interpreter includes global arrays for the operand stack (`o`) and the dictionary stack (`d`), which are used to manage the execution model of PostScript programs. The interpreter is written in a functional style, with many short routines that perform specific tasks, such as pushing values onto the stack, popping values, and executing procedures. The source provides a complete, albeit simplified, implementation of the interpreter, which can be run in environments like Node.js or QuickJS. [1]
The interpreter includes a set of primitive operators implemented as JavaScript functions, such as `add`, `mul`, `div`, `dup`, `exch`, `pop`, and others, which mirror the behavior of their PostScript counterparts. These operators manipulate the operand and dictionary stacks according to the rules of PostScript, and they are installed into the dictionary stack during initialization. The interpreter also includes a `parse` function that splits PostScript programs into tokens, handling special characters and comments. The source explicitly states that the interpreter is not a full-featured PostScript implementation and omits many advanced features of the language, such as string manipulation and more complex error-handling. The implementation is focused on demonstrating the mechanics of PostScript as a general-purpose programming language, rather than on replicating all of its capabilities. [1]
The interpreter is designed to be interactive, with support for a Read-Eval-Print-Loop (REPL) that allows users to input PostScript code and see the results. The source includes example code for setting up the REPL in both Node.js and QuickJS environments. The interpreter also includes utility functions for formatting and displaying values on the operand stack, such as the `format` function, which converts JavaScript data structures back into PostScript-like representations. The source provides several examples of using the interpreter, such as executing simple arithmetic operations and manipulating arrays. However, it explicitly notes that the interpreter is not intended for production use and is more of an educational tool for understanding the structure and execution model of PostScript programs. The interpreter is limited in scope and is not a complete replacement for a full-featured PostScript interpreter like Ghostscript. [1]
Sources
Prepared with AI assistance from the cited sources.
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