PL/0 - Compiler Construction

Compiler Construction

In December 1976, Wirth wrote a small booklet about compiler construction, containing the full source code of the PL/0 compiler. The syntax rules above were taken from this first edition of Wirth's book Compilerbau. In later editions of this book (under the influence of his ongoing research) Wirth changed the syntax of PL/0. He changed the spelling of keywords like const and procedure to uppercase. This change made PL/0 resemble Modula-2 more closely. At the same time, Wirth's friend and collaborator C. A. R. Hoare was working on his influential communicating sequential processes concept, which used the exclamation mark ! and the question mark ? to denote communication primitives. Wirth added both symbols to the PL/0 language, but he did not mention their semantics in the book.