Introduction to Pic Programming

Pic: A Domain-Specific Programming Language for Diagram Specification

In the world of programming languages, the term “domain-specific language” (DSL) refers to a language tailored for a specific application domain rather than for general-purpose programming. One such DSL that has left a lasting impact on the way diagrams are specified and represented in documents is Pic, a language developed by renowned computer scientist Brian Kernighan. Initially designed to facilitate the inclusion of diagrams in text documents, Pic has become an integral part of the Unix ecosystem, primarily used within the troff document processing system.

The evolution of Pic, its features, and its relevance in modern software systems offers a fascinating glimpse into the intersection of computing and technical documentation. This article delves into the history of Pic, its syntax, usage, and its significance in both the Unix environment and the wider context of document processing.

History and Origins of Pic

Pic was created in 1988 by Brian Kernighan, a notable figure in the development of Unix and programming languages. The primary purpose of Pic was to allow users to specify diagrams through a simple textual syntax. These diagrams could then be transformed into graphical representations by the Pic compiler, which generated concrete drawing commands to be rendered by drawing tools.

The language was designed with the goal of being both procedural and declarative, offering users the ability to define objects such as boxes and arrows and arrange them in a structured way. As such, Pic emerged as a powerful tool for programmers and document creators, especially within the Unix ecosystem, which was heavily focused on text-based processing.

Pic was initially introduced as a preprocessor for the troff document formatting system, which was developed as part of the Unix operating system at Bell Labs. Troff is a typesetting system, and Pic acted as a powerful addition to it, enabling users to include complex diagrams directly within their documents without requiring sophisticated graphics software. The preprocessor would parse the Pic source code and replace diagram definitions with corresponding graphical output, while leaving the rest of the document unaltered.

The Syntax and Structure of Pic

Pic’s syntax is simple yet expressive, making it an appealing choice for users who need to describe diagrams without having to learn complex graphic design tools. In its essence, Pic allows users to define objects, relationships, and actions using basic text descriptions.

Basic Elements of Pic

• Objects: The basic building blocks in Pic are objects like boxes, lines, circles, and arrows. These objects can be drawn and manipulated through Pic’s commands, and they represent the core components of any diagram.
• Variables: Pic supports variable assignment, enabling users to store values and refer to them in subsequent commands. This adds flexibility to the language, allowing for the dynamic construction of diagrams.
• Macros: Pic allows users to define macros, which are reusable code blocks. This feature is particularly useful for defining common patterns or structures within diagrams.
• Conditionals and Loops: As a procedural language, Pic supports conditional statements (e.g., if-else) and loops (e.g., for and while), which enable more dynamic control over the drawing process.

Example Code

A simple Pic program might look like this:

vbnet
Copy code
box "Process" from 0,0 to 3,2
arrow from 3,1 to 5,1
box "Next Step" from 5,0 to 8,2

This snippet creates a diagram with two boxes labeled “Process” and “Next Step”, connected by an arrow. The commands are simple and intuitive, highlighting the ease of use that Pic offers.

Features of Pic

Pic offers a variety of features that make it particularly suitable for technical and scientific documentation, where the need for precise diagramming and integration with text is paramount.

• Comments: Pic supports both line comments and block comments. Line comments are denoted by the # symbol, allowing users to annotate their code and improve readability. This is particularly useful in collaborative environments or for future revisions.

  Example:

  bash
  Copy code
  # This is a comment in Pic
  

• Semantic Indentation: Unlike many modern programming languages, Pic does not emphasize semantic indentation. This can make it less visually structured, but it also simplifies parsing and reduces the cognitive load for users who are more focused on diagramming than strict code formatting.

• Preprocessing with Troff: One of Pic’s most unique features is its role as a preprocessor. When embedded within a troff document, Pic processes diagram descriptions and replaces them with corresponding graphical elements. This feature has been particularly useful in technical writing, where diagrams often need to be included alongside text.

• Text Formatting: Pic also allows for text formatting within the diagrams. This is essential for creating labels and annotations that are associated with specific elements of a diagram.

• Output Flexibility: The output from a Pic program is typically directed towards a document processing system, and various post-processors may be applied to further refine the output. The language can generate output in various formats, including PostScript and SVG, depending on the implementation.

Pic in the Unix Ecosystem

Pic was initially implemented as a preprocessor for the troff system, which was widely used within the Unix environment. Troff itself is a powerful typesetting system used for creating documents with a high degree of control over layout and formatting.

• Troff Integration: Pic’s ability to work seamlessly with troff made it an invaluable tool for technical writers, engineers, and scientists who needed to generate precise and professional-quality documents. With Pic, these professionals could easily integrate complex diagrams into their text-based documents without relying on external graphic design software.

• GNU Pic: Over the years, various implementations of Pic have been created, with the most prominent being GNU Pic, which is part of the GNU Groff package. This version of Pic is open-source and has gained popularity among Unix-based systems, including Linux distributions.

• DPIC: Another important implementation of Pic is DPIC, created by Dwight Aplevich. DPIC adds additional features to the original Pic language, including the ability to generate PostScript or SVG images directly. It can also serve as a preprocessor for TeX documents, generating TPIC specials for graphical rendering in TeX-based environments.

Pic’s Role in Modern Computing

Although the use of Pic has declined with the advent of more sophisticated graphical tools, it still holds relevance in certain specialized contexts, especially where text-based diagramming is required. Pic is still widely used in technical documentation and academic publishing, where the need to embed accurate diagrams in text documents remains paramount.

• Open Source: The open-source nature of Pic, particularly through implementations like GNU Pic and DPIC, ensures that the language remains accessible to a broad community of users. These tools are available on many Unix-based systems and are often bundled with Linux distributions.

• Legacy and Influence: Pic has influenced the development of other diagramming languages, such as MetaPost and DOT, which share some of its key features. However, Pic’s simplicity and procedural nature set it apart, and it remains an important part of the history of computer-based diagramming.

Conclusion

Pic, the domain-specific language developed by Brian Kernighan, stands as a testament to the power of simplicity in the world of computing. As a procedural language for generating diagrams, it strikes a balance between ease of use and flexibility, making it an ideal tool for integrating graphical elements into text-based documents. While its usage has become less widespread with the rise of modern graphical tools, Pic remains an essential part of the Unix ecosystem and continues to serve the needs of technical writers and researchers.

Whether working within the troff system or using one of its modern implementations like GNU Pic or DPIC, Pic’s legacy endures in the world of technical documentation, offering a simple yet powerful means of diagramming that has stood the test of time.

As computing continues to evolve, Pic serves as an important reminder that sometimes, the most effective tools are those that prioritize clarity, simplicity, and precision.