Usage
PL/I implementations were developed for mainframes from the late 1960s, mini computers in the 1970s, and Personal Computers in the 1980s and 1990s. Although its main use has been on mainframes, there are PL/I versions for DOS, Microsoft Windows, OS/2, AIX, OpenVMS, and Unix.
It has been widely used in business data processing and for system use for authoring operating systems on certain platforms. Very complex and powerful systems have been built with PL/I:
The SAS System was initially written in PL/I; the SAS data step is still modeled on PL/I syntax.
The pioneering online airline reservation system Sabre was originally written for the IBM 7090 in assembler. The S/360 version was largely written using a purpose built subset PL/I compiler for a dedicated control program.
PL/I was used to write an executable formal definition to interpret IBM's System Network Architecture
PL/I did not fulfill its supporters' hopes that it would displace Fortran and COBOL and become the major player on mainframes. It remained a minority but significant player. There cannot be a definitive explanation for this, but some trends in the 1970s and 1980s militated against its success by progressively reducing the territory on which PL/I enjoyed a competitive advantage.
First, the nature of the mainframe software environment changed. Application subsystems for Data Base and Transaction processing (CICS and IMS and Oracle on System 370) and application generators became the focus of mainframe users' application development. Significant parts of the language became irrelevant because of the need to use the corresponding native features of the subsystems (such as tasking and much of input/output). Fortran was not used in these application areas, confining PL/I to COBOL’s territory; most users stayed with COBOL. But as the PC became the dominant environment for program development Fortran, COBOL and PL/I all became minority languages overtaken by C++, Java and the like.
Second, PL/I was overtaken in the Systems Programming field. The IBM system programming community was not ready to use PL/I; instead, IBM developed and adopted a proprietary dialect of PL/I for system programming. – PL/S. With the success of PL/S inside IBM, and of C outside IBM, the unique PL/I strengths for system programming became less valuable.
Third, the development environments grew capabilities for interactive software development that, again, made the unique PL/I interactive and debugging strengths less valuable.
Fourth, COBOL and Fortran added features such as structured programming, character string operations, and object orientation, that further reduced PL/I's relative advantages.
On mainframes there were substantial business issues at stake too. IBM’s hardware competitors had little to gain and much to lose from success of PL/I. Compiler development was expensive, and the IBM compiler groups had an in-built competitive advantage. Many IBM users wished to avoid being locked into proprietary solutions. With no early support for PL/I by other vendors it was best to avoid PL/I.
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