PL/I Guide, Meaning , Facts, Information and Description

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

With full support for pointers to all data types (including pointers to structures of course), 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.

History of PL/I

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.

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 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.

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.

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.

That is not to say that PL/I was not used. It received significant use in business data processing, and also for more general programming use. The Multics project, one of the first to develop an operating system in a high level language, used PL/I. 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 from 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 C programming language is closely related to PL/I and PL/S.

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.)

The pioneering online airline reservation system SABRE is believed to be written in PL/I.

Mostly used on mainframes, PL/I also has versions for Microsoft Windows, AIX, OpenVMS, and Unix.

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 DATA('HELLO WORLD!');
         END LOOP;
 
END HELLO;

Hello2: proc options(main);
    put list ('Hello, world!');
end Hello2;

In 1987 ANSI published ANSI X3.74-1987 (R1998), entitled Information Systems - Programming Language - PL/I General-Purpose Subset.

External links

• The C programming language was heavily modelled after PL/I. The following link makes a side-by-side comparison of the two languages: Power vs. Adventure - PL/I and C

This is an Article on PL/I. Page Contains Information, Facts Details or Explanation Guide About PL/I

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