PL/0

PL/0 is a small teaching programming language designed by Niklaus Wirth and introduced in his 1976 book Algorithms + Data Structures = Programs, where it serves as the worked example for compiler construction. It resembles Pascal but is far simpler: its only data type is the integer, its operators are arithmetic and comparison operations plus an odd test, and its control flow is limited to if-then, while-do and procedures that take no parameters. The original implementation had no input or output routines, because the compiler printed each variable as it changed. Its tiny grammar, written in extended Backus-Naur form, makes it easy for students to write a recursive descent parser, so PL/0 compilers are still built in university compiler courses, where students usually extend the language themselves. In later editions of his compiler book Wirth changed its syntax, and in the last edition he replaced it with Oberon-0.