ABC (programming language)

ABC is an imperative general-purpose programming language and integrated development environment developed at Centrum Wiskunde & Informatica, in Amsterdam, Netherlands by Leo Geurts, Lambert Meertens, and Steven Pemberton. It is interactive, structured, high-level, and intended to be used instead of BASIC, Pascal, or AWK. It is intended for teaching or prototyping, but not as a systems-programming language. ABC was developed from the B language, an earlier creation by Meertens and Pemberton which was the first language to use indentation for block structure.
ABC had a major influence on the design of the Python language, whose creator, Guido van Rossum, had worked for several years on the ABC system in the mid-1980s.

Features

Its designers claim that ABC programs are typically around a quarter the size of the equivalent Pascal or C programs, and more readable. Key features include:

Only five basic data types
No required variable declarations
Explicit support for top-down programming
Statement nesting is indicated by indentation, via the off-side rule
Infinite precision arithmetic, unlimited-sized lists and strings, and other features supporting orthogonality and ease of use by novices
Polymorphic commands and functions
Interactive environment with command completion, persistent workspaces, and no separate file handling

ABC was originally a monolithic implementation, leading to an inability to adapt to new requirements, such as creating a graphical user interface. ABC could not directly access the underlying file system and operating system.
The full ABC system includes a programming environment with a structure editor, suggestions, static variables, and multiple workspaces, and is available as an interpreter–compiler., the latest version is 1.05.02, for Unix, MS-DOS, Atari ST, and MacOS.

Example

An example function to collect the set of all words in a document:
HOW TO RETURN words document:
PUT IN collection
FOR line IN document:
FOR word IN split line:
IF word not.in collection:
INSERT word IN collection
RETURN collection

Implementations

ABC has been through multiple iterations, with the current version being the 4th major release. Implementations exist for Unix-like systems, MS-DOS/Windows, Macintosh, and other platforms. The source code was made available via Usenet in the late 1980s/early 1990s.