Comparative Notes on ALGOL 68 and PL/I

An ALGOL 68 expert described PL/I, not long ago, as a 'hotch-potch of obsolete ideas'. The PL/I people tend not to know much about ALGOL 68, and not to care much either. In fact the two languages have many features in common, which makes it convenient to consider them together. We begin with the lexical aspect, that is, how statements are actually written. Here ALGOL 68 has followed on from ALGOL 60, but PL/I has taken features from several different languages. In ALGOL 68 language words are 'basic tokens' and are distinguished from identifiers by writing them in a different type face, which may be represented in a particular implementation by use of the underline character or by enclosing the word between quote marks, exactly as in ALGOL 60. Spaces and card or line boundaries are ignored in all contexts, except perhaps within basic tokens. In PL/I there is only one alphabet. The fact that a word is a keyword, however, does not prevent its being used as an identifier. The usage may always be deduced from the context. Spaces are significant where they separate one word from the next. In general extra spaces are allowed. Card or line boundaries are not significant. The semicolon is used to show where each statement finishes. For characters in general, PL/I offers either a 48-character set or a 60-character set. Alternatives are provided for the extra 12 characters when the smaller set is in use. ALGOL 68 is defined in terms of abstract symbols, but the Report does stoop to suggesting representations for them, which turn out to be on similar lines to the PL/I set, but hopeful of 80-odd characters if available—or well over 100 if the different type fount is to be provided. Here one feels that PL/I was designed with punched cards and line printers in mind, while ALGOL 68 is envisaged for 7 or 8 channel paper tape as the main input/output medium.