PL/I ("Programming Language One", pronounced "pee el one") is a computer programming language designed for scientific, engineering, and business applications. It is undoubtedly one of the most powerful programming languages that has ever been created.
It has been used by various academic, commercial and industrial users since it was introduced in the early 1960s, and in fact is still actively used today. See the newsletter[1]. It is no longer considered a mainstream computer programming language.
PL/I's principal domain is data processing; it supports recursion and structured programming. The language syntax is English-like and suited for describing complex data formats, with a wide set of functions available to verify and manipulate them.
Details
PL/I has a very large vocabulary of built-in functions. In fact, there is probably no one compiler that has the full standard of keywords available. PL/I compilers are normally subsets of the language that specialize in various fields.
Partial list of features:
- Preprocessing
- Free form syntax
- Full support for pointers
- Case-insensitive keywords
- Variable-length arrays
- Call by reference is the default
- Supports complex structure declarations with unions
- Built-in support for a slew of data types, including two types of strings
- Four classes of storage: Automatic, Static, Controlled (dynamic) and Based (anonymous dynamic)
- Automatic garbage collection
- Built-in coprocessing facility
History of PL/I
PL/I was developed by IBM as part of the development of the System 360. Prior to the System 360, IBM made several different incompatible models of mainframes for different purposes: some were designed for business use, others for scientific use. The goal of the System 360 project was to develop one series of compatible models to replace all the previous models, and which could be used equally well for commercial or scientific use.
Not only did business and scientific users use different machines; they also used different languages. Business users mainly used COBOL, while scientific users used Fortran. The goal of PL/I was to develop a single language usable for both business and scientific purposes. Another goal was to add structured programming constructs derived from ALGOL, which neither COBOL nor Fortran supported (at the time). PL/I was designed by a committee drawn from IBM programmers and users drawn from across the United States, working over several months. IBM originally wanted PL/I to be ready for use by the launch of the System 360; but unfortunately this deadline could not be met.
The language was originally to be called NPL, for "New Programming Language"; but that abbreviation could not be used because it was the name of the National Physical Laboratory in the UK. So PL/I was chosen instead.
Compilers were implemented by several groups in the early 1960s; the Multics project at MIT, one of the first to develop an operating system in a high level language, decided to make PL/I their implementation language in 1964.
Although PL/I did not have great success, that is not to say that it was not used. It received significant use in business data processing, and also for more general programming use. A subset of PL/I, PL/M, was used to write CP/M, while the XPL dialect was used to write HAL/S as used on the Space Shuttle. A subset compiler named CORC, developed at Cornell University for teaching a dialect called PL/C, had the unusual capability of never failing to compile any program, through the use of extensive automatic correction of many syntax errors and by converting any remaining syntax errors to output statements.
Another variant of PL/I was PL/S, the IBM systems programming language. IBM used PL/S to write the MVS operating system in the early 1970s. IBM uses an improved and renamed PL/S for internal work on current operating systems, OS/390 and now z/OS.
The pioneering online airline reservation system Sabre, primarily written in 7090 assembler, later used PL/I and a custom compiler for some high-level tasks.
Although mostly used on mainframes, there are also PL/I versions for DOS, Microsoft Windows, AIX, OpenVMS, and Unix.
Retrospective
PL/I was not as successful as originally hoped for, due to a number of factors.
Perhaps most important was that the language was very complex, making it difficult to implement, especially in a timely fashion. This complexity was probably because it was designed by a committee, and the desire to supply the needs of very different types of users (business and scientific). Such delays, its complexity, and the low quality of early versions of IBM's PL/I compiler discouraged users from switching from COBOL or Fortran. It contained many rarely used features, such as multithreading support, which added corresponding cost and complexity to the compiler. (Its co-processing facilities requires multi-programming environment with support for non-blocking multiple threads for processes by the operating system.)
Another major problem was that instead of noticing features that would make their job easier, scientific (Fortran) programmers of the time had the opinion that it was a business language, while business (COBOL) programmers looked on it as a scientific language. In addition, COBOL programmers viewed it as a "bigger COBOL" and Fortran programmers viewed it as a "bigger Fortran," both somewhat intimidated by the language and disinclined to adopt it.
Compiler complexity was another issue that was perhaps underestimated during the initial design of the language. Optimization was unusually complex due to the need to handle asynchronous modification of variables (for example in the 'on error' construct) making it difficult to reliably predict how certain variables might be modified at runtime.
With full support for pointers to all data types (including pointers to structures), recursion, co-processing, extensive built-in functions and many other facilities, PL/I was indeed quite a leap forward compared to the programming languages of its time. Even by today's standards, a correct and full PL/I implementation would be a very hard contestant to compete with for other programming languages.
PL/I was probably the first commercial language where the compiler was written in the language to be compiled. (The experimental language NELIAC achieved this at least five years prior, and there may have been others.)
Sample programs
Example_Program:
Test: procedure options(main);
declare My_String char(20) varying initialize('Hello, world!');
put skip list(My_String);
end Test;
Hello World
HELLO: PROCEDURE OPTIONS (MAIN);
FLAG = 0;
LOOP: DO WHILE (FLAG = 0);
PUT SKIP LIST('HELLO WORLD!');
END LOOP;
END HELLO;
Hello World 2
Hello2: proc options(main);
put list ('Hello, world!');
end Hello2;
Standards
In 1987 ANSI published ANSI X3.74-1987 (R1998), entitled Information Systems - Programming Language - PL/I General-Purpose Subset.
External links
- The pl1gcc project is an attempt to create a native PL/I compiler using the GNU Compiler Collection. The project is looking for more people to join the development and testing: PL/I for GCC
