Forth

Forth is a stack-based, extensible programming language and environment created in the 1970s by Charles H. Moore. It is designed to be simple and compact enough to run on small computers and embedded systems. The name Forth is commonly believed to be a contraction of the word “fourth” in Moore’s sequence of experiments, though the exact origin is uncertain.

The language centers on a data stack and a return stack. A word is the basic unit of code, defined in the language’s dictionary. Programs are built by defining new words with colon definitions and ending them with a semicolon. Forth programs are typically read and executed in a mix of interpretive and compilation modes, with code assembled from existing words to create higher-level functionality. Postfix (reverse Polish) notation is typical, meaning operands precede operators.

A key feature of Forth is its extensibility. Users can add new words at compile time or run time, effectively extending the language without modifying a separate compiler. The core is small, while libraries provide much of the functionality needed for specific domains. The language also supports immediate words, which execute during compilation, enabling sophisticated compiling behavior.

Standardization and implementations: the most widely referenced standard is ANSI Forth (ANSI X3J14), established in the 1990s to promote portability. Numerous implementations exist, including the open-source GForth, as well as commercial variants such as SwiftForth and VFX Forth. Forth runs on many architectures and is particularly popular in resource-constrained or real-time environments.

Applications and usage: Forth has seen widespread use in embedded systems, hardware control, and spaceflight payloads, as well as education and rapid prototyping. It is valued for its small footprint, interactive development, and the ability to tailor the language to a project through custom words, at the expense of a steeper initial learning curve for some users.