A small dialect of PL/I used for compiler writing from Stanford, 1967-69. XPL has one-dimensional arrays. I/O is achieved with character pseudo-variable INPUT and OUTPUT, e.g. OUTPUT = 'This is a line'; It has inline machine code. "Programmers are given all the rope they ask for. Novices tend to hang themselves fairly frequently." XPL has been implemented on IBM 360, {Univac 1100}, ICL System 4, CDC 6000 and Cyber series, {XDS Sigma-5} and Sigma-7 and DEC PDP-10. An optimising XPL compiler (version 1) by Robin Vowels <robin_vowels@rmit.edu.au> is a standard implementation of XPL and is based on McKeeman, Horning, and Wortman's improved XCOM (which employs hashed symbol table generation). It includes the extra built-in function COREHALFWORD. The following areas have been optimised: procedures calls when the argument and corresponding parameter are of the same type, and when the argument is a constant; constant subscripts; use of CORELHALFWORD and COREWORD; string constants of length one; iterative DO statements by transferring code to the end of the loop. String constants of length one do not require a descriptor, hence more descriptors are available for string variables. Comparison operations are treated as commutative, and an improved Commute algorithm is used. Halfword instructions are generated for BIT(16) variables. These areas have been improved or re-written: calls on OUTPUT, catenation, integer-to-string conversion, multiply, divide, and MOD. An emitter for SS-type instructions has been added. The compiler achieves an 11% reduction in object code compiling itself, an 11% increase in compilation rate, a 55% increase in compilation speed when the $E toggle is set. Special treatment for catenating a string to an integer substantially decreases consumption of the free string area, and decreases string moves. The latter improvement is most noticeable on small core machines. Core requirements: less than the improved XCOM on which it is based (approx. 98000 bytes). Symbol table size is 468. Ported to IBM System 370. The compiler is written in XPL. The code generators are machine-specific. ["A Compiler Generator," W.M. McKeeman et al, P-H 1970]. [JCC, AFIPS 1968]. (1993-08-07)

XPL is a programming language based on PL/I, a portable one-pass compiler written in its own language, and a parser generator tool for easily implementing similar compilers for other languages. XPL was designed in 1967 as a way to teach compiler design principles and as starting point for students to build compilers for their own languages.

xPL is an open protocol intended to permit the control and monitoring of home automation devices. The primary design goal of xPL is to provide a rich set of features and functionality, whilst maintaining an elegant, uncomplicated message structure.