2.36.2 Rationale

The feature symbols of the R7RS–small cond-expand syntax provide the ability to conditionally compile code based on particular properties of an implementation that it sees fit to publish. The features procedure, which returns the list of feature symbols of the implementation, provides run–time access to the same set of properties. Assuming that Rhinoceros Scheme provides the feature symbol rhinoceros but not diplodocus and Diplodocus Scheme does the opposite, programs can portably ask “Is this Rhinoceros Scheme or Diplodocus Scheme?” and behave differently at run time based on the answer. Similarly, a program can ask “Does this implementation have exact complex numbers?” by checking for the presence of the exact-complex feature symbol in the result of calling features.

However, a program using just the features procedure cannot report to its user “I am executing on X Scheme” for every value of X, because it does not know which symbol in the feature list represents the implementation name, nor does it have a comprehensive list of implementation names. Similarly, there are other properties that the feature list may allow testing for but not reporting on, such as the underlying OS and the CPU architecture. For the sake of logging and debugging, it is necessary or at least extremely useful to provide a standard way for Scheme applications, as well as Scheme users at the REPL, to report these things. In the Common Lisp world, bugs are often reported in a REPL transcript beginning with a call to lisp-implementation-version.