GoKawiilSoftware Gems: The Computer History Museum Historical Source Code Series
Thousands of programming languages were invented in the first 50 years of the age of computing. Many of them were similar, and many followed a traditional, evolutionary path from their predecessors.
But some revolutionary languages had a slant that differentiated them from their more general-purpose brethren. LISP was for list processing. SNOBOL was for string manipulation. SIMSCRIPT was for simulation. And APL was for mathematics, with an emphasis on array processing.
What eventually became APL was first invented by Harvard professor Kenneth E. Iverson in 1957 as a mathematical notation, not as a computer programming language. Although other matrix-oriented symbol systems existed, including the concise tensor notation invented by Einstein, they were oriented more towards mathematical analysis and less towards synthesis of algorithms. Iverson, who was a student of Howard Aiken’s, taught what became known as “Iverson Notation” to his Harvard students to explain algorithms.
Iverson was hired by IBM in 1960 to work with Adin Falkoff and others on his notation. In his now famous 1962 book “A Programming Language” 1, he says the notation is for the description of “procedures…called algorithms or programs”, and that it is a language because it “exhibits considerable syntactic structure”. But at that point it was just a notation for people to read, not a language for programming computers. The book gives many examples of its use both as a descriptive tool (such as for documenting the definition of computer instruction sets) and as a means for expressing general algorithms (such as for sorting and searching). Anticipating resistance to something so novel, he says in the preface, “It is the central thesis of this book that the descriptive and analytical power of an adequate programming language amply repays the considerable effort required for its mastery.” Perhaps he was warning that mastering the language wasn’t trivial. Perhaps he was also signaling that, in his view, other notational languages were less than “adequate”.
The team, of course, soon saw that the notation could be turned into a language for programming computers. That language, which was called APL starting in 1966, emphasized array manipulation and used unconventional symbols. It was like no other computer program language that had been invented.
APL became popular when IBM introduced “APL\360” for their System/360 mainframe computer. Unlike most other languages at the time, APL\360 was also a complete interactive programming environment. The programmer, sitting at an electromechanical typewriter linked to a timeshared computer, could type APL statements and get an immediate response. Programs could be defined, debugged, run, and saved on a computer that was simultaneously being used by dozens of other people.
Written entirely in 360 assembly language, this version of APL took control of the whole machine. It implemented a complete timesharing operating system in addition to a high-level language.
With the permission of IBM, the Computer History Museum is pleased to make available the source code to the 1969-1972 “XM6” version of APL for the System/360 for non-commercial use.
The text file contains 37,567 lines, which includes code, macros, and global definitions. The 90 individual files are separated by ‘./ ADD” commands. To access this material, you must agree to the terms of the license displayed here, which permits only non-commercial use and does not give you the right to license it to third parties by posting copies elsewhere on the web.
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