PL/I

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PL/I ("Programming Language One", pronounced "pee el one") is an imperative computer programming language designed for scientific, engineering, and business applications. It is undoubtedly one of the most feature-rich programming languages that has ever been created and was one of the very first in the highly-feature-rich category. It has been used by various academic, commercial and industrial users since it was introduced in the early 1960s, and is still actively used today. See the newsletter[1].

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.

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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 evaluation strategy
  • 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)
  • Very powerful built-in macro (preprocessor) facility that allows adding new features to the language, allowing brief shorthand notation to expand automatically into complicated operations
  • Automatic garbage collection
  • Built-in coprocessing facility

An unusual feature is that PL/I has no reserved words. This makes an oddity like the following perfectly legal, even though it is hardly a model of clarity:

IF IF = THEN THEN IF = IF + 1 ELSE IF = ELSE - 1;

A more practical benefit is that a programmer need only learn the applicable language subset without concern about variable names clashing with keywords. This must have made writing a compiler challenging, though. The compiler would have to interpret the symbols from the context instead of looking for the keywords with tools like lex and yacc.

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History of PL/I

PL/I was developed by IBM as part of the development of System/360. Prior to 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 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. The 'NPL' name was in effect between March 1 and November 30 in 1964. MPL and MPPL were considered before settling on PL/I.

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, used Early PL/I (EPL), a subset dialect of PL/I, as their implementation language in 1964. EPL was developed at Bell Labs and MIT by Douglas McIlroy, Robert Morris, and others.

Although PL/I did not have immense universal popularity on all computing platforms of that era, that is not to say that it did not enjoy success on isolated islands within the 1970s computer industry. It received significant use in business data processing, and also for more general programming use, especially for authoring operating systems on certain processors. PL/M, a subset of PL/I distributed by Intel, was used to write the CP/M operating system proper and much application software running on CP/M and MP/M; Digital Research sold a PL/I compiler which was itself written in PL/M. PL/M was used to write much other software at Intel for the 8080, 8085, and Z-80 processors during the 1970s. The SAS System was initially written in PL/I, and still carries the Proc syntax. The XPL dialect of PL/I was used to write HAL/S as used on the Space Shuttle. A subset of PL/I, PL/P, was used to write new portions of PRIMOS at all rings of protection in its later revisions. 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.

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Retrospective

PL/I was not as successful as originally hoped for a number of reasons.

Perhaps most important was that, even though the language was easy to learn and use, implementing a PL/I compiler was difficult and time-consuming. This was at least partially a result of having been 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 and its co-processing facilities required a 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 (i.e. Fortran) programmers of the time had the opinion that it was a business language, while business (i.e. 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' exception handling 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. In addition, the PL/I macro preprocessor was unusual, perhaps unique, in using its target language's syntax and semantics (e.g. as compared to the C preprocessor's "#" directives). 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. This is called bootstrapping. The experimental language NELIAC achieved this at least five years prior, and there may have been others.

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Sample programs

Example_Program: 

Test: procedure options(main);

   declare My_String char(20) varying initialize('Hello, world!');
   
   put skip list(My_String);
   
end Test;
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Hello World 2 

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


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Standards

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

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External links

  • PL/I Resources: [4]
  • PL/I home: [5]
  • PL/I newsletter: [6]
  • 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
  • Languages Related to PL/I: [7]
  • Kednos PL/I for OpenVMS Compiler Documentation: [8]

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