Fortran 90 Handbook

1212.1 Procedure Terms and ConceptsThe procedure paradigm of Fortran 90 may be viewed as a straightforwardextension of Fortran 77, including the ability to explicitly specify procedureinterfaces as needed or desired. A procedure interface is explicit when it isknown in detail to the calling program. Both internal and module procedureshave explicit interfaces by nature. Explicit procedure interfaces not onlyprovide a tool to eliminate one of the most serious problem areas of Fortran 77(undetected procedure argument mismatches), but also set the scene forprofound improvements in the engineering of Fortran software.With explicit interfaces, the integrity of information flow among different partsof the program is automatically enforced. Such interfaces relieve theprogrammer (and maintainer) of the considerable mechanics of ensuring suchintegrity and allow software development resources to be concentrated insteadon the functional design of this information flow. The net effect is that aFortran program becomes more of an integrated, highly reliable, cohesivewhole, rather than simply an aggregation of separate program units. Thus, theadvantages of modular design are retained, while the effects of a change in oneplace on other parts of the program are tracked automatically. The result ismore productive application development and maintenance—in short, betterengineered software.Often a sophisticated technical area gets surrounded and mystified with“jargon”—short and often esoteric terms or phrases that represent key conceptsin the technical area. Understanding that jargon is an important part ofassimilating an understanding of the technical area. This section is an attempt,at the outset, to describe some of the basic terms and concepts associated withFortran procedures.12.1.1 Procedure TermsThere are two basic forms procedures take in a Fortran program: one is asubroutine; the other is a function. These two forms are very similar except inthe way they are invoked.522 Fortran 90 HandbookCopyright © J. Adams, W. Brainerd, J. Martin, B. Smith, and J. Wagener