Editing 754: Dependencies

{{comic
| number    = 754
| date      = June 16, 2010
| title     = Dependencies
| image     = dependencies.png
| titletext = The prereqs for CPSC 357, the class on package management, are CPSC 432, CPSC 357, and glibc2.5 or later.
}}

==Explanation==
A compiler is a program that converts code written in a high-level programming language into an executable program. A section of code is said to be dependent on a second segment of code if the results of the first segment are potentially impacted by the second segment. Dependency resolution is part of compiler design, and is the study of determining and correcting dependencies which result in an unwanted, ambiguous, or impossible definition of the dependent section. Requiring that an action occurs if and only if the action has already occurred, like the prerequisite in this comic, is one type of potentially unwanted dependency.

The comic envisions a college computer science course (CPSC432) focusing on "compiler design with dependency resolution" which has itself as a prerequisite. The joke is that the prerequisite is an unresolved dependency, as you must complete this course before you can enroll in it, a phenomenon called {{w|Catch-22 (logic)|Catch-22}}.

This dependency would send a poorly designed compiler into an infinite loop. In real life, the problem is solved by allowing an object to satisfy itself as a prerequisite. This stops the compiler's infinite loop, but may not produce the desired functionality in the program. Another layer of the joke may be that any student who successfully enrolls in the class already knows this solution because they must have employed it in order to get past the apparent infinite recursion in the class prerequisites.

Managing dependencies is useful in other areas of computer science, e.g. {{w|package management}}. Collections of files are known as "packages". A software package might require that a particular operating system patch (a type of package) be installed first. That package might in turn require other packages be installed, and so on. Therefore, a package installer must know the dependencies of a package, and be able to figure out whether any required packages are missing before continuing with the installation. 

The title text envisions a course on package management which has itself as a prerequisite, as well as the compiler design course with the impossible prerequisite presented in the main comic (CPSC 432), and glibc2.5 or greater. By looking at the course number it can be observed that CPSC 432 is a fourth year course, and this package management course (CPSC 357) is a third year course. Glibc is a commonly used package on Unix systems, and therefore should be taught in the course. This continues the joke since this course has the following unresolved dependencies:
*Requiring that the course be a prerequisite to itself (CPSC 357).
*Requiring that a course with an unresolved dependency (CPSC 432) be a prerequisite, as CPSC 432 can not be taken.
*Requiring that a fourth year course (CPSC 432) be a prerequisite to a third year course (CPSC 357), as the student should be in their fourth year while taking CPSC 432, and should be in their third year while taking CPSC 357. This is analogous to a lower-numbered package requiring a higher-numbered package.
*Requiring that the student knows part or all of the course material (glibc2.5 or greater) before taking the course, as the student is supposed to learn this information from the course.

==Transcript==
:[A portion of a page from an imaginary course catalog.]
:Page 3

:[A table with four columns]

:;Department
::Computer Science

:;Course
::CPSC 432

:;Description
::Intermediate compiler design, with a focus on dependency resolution.

:;Prereqs
::CPSC 432

:[The very top of the text for the next course in the table is visible but unreadable.]

==Trivia==
The letter code "CPSC" is the letter code Christopher Newport University, [[Randall]]'s alma mater, uses for Computer Science.

{{comic discussion}}
[[Category:Programming]]
[[Category:Recursion]]