The following table lists a few languages with repositories for contributed software. The "Autochecks" column describes the routine checks done.
Very few people have the ability to test their software under multiple operating- systems with different versions of the core code and with other contributed packages they may use. For the [[R (programming language)|R programming language]], the [[CRAN (R programming language)|Comprehensive R Archive Network (CRAN)]] runs tests routinely.
To seeunderstand how this is valuable, supposeimagine a situation with two developers, Sally and John. Sally contributes a package A. Sally only runs the current version of the software under one version of Microsoft Windows, and has only tested it in that environment. At more or less regular intervals, CRAN tests Sally's contribution under a dozen combinations of operating systems and versions of the core R language software. If one of them generates an error, she gets that error message. With luck, that error message details may sufficeprovide enough input to allow herenable toa fix for the error, even if she cannot replicate it with theher current hardware and software she has. Next, suppose John contributes to the repository a package B that uses a package A. Package B passes all the tests and is made available to users. Later, Sally submits an improved version of A, which unfortunately, breaks B. The autochecks make it possible to provide information to John so he can fix the problem.
This example exposes both a strength and a weakness in the R contributed-package system: CRAN supports this kind of [[automated testing]] of contributed packages, but packages contributed to CRAN need not specify the versions of other contributed packages that they use. Procedures for requesting specific versions of packages exist, but contributors might not use those procedures.
