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7.5.2 SRFI-0 - cond-expand

   This SRFI lets a portable Scheme program test for the presence of certain
   features, and adapt itself by using different blocks of code, or fail if the
   necessary features are not available. There's no module to load, this is in the
   Guile core.

   A program designed only for Guile will generally not need this mechanism, such a
   program can of course directly use the various documented parts of Guile.

   syntax: cond-expand (feature body...) ...[12] ¶
          Expand to the body of the first clause whose feature specification is
          satisfied. It is an error if no feature is satisfied.

          Features are symbols such as srfi-1, and a feature specification can use
          and, or and not forms to test combinations. The last clause can be an
          else, to be used if no other passes.

          For example, define a private version of alist-cons if SRFI-1 is not
          available.

(cond-expand (srfi-1
              )
             (else
              (define (alist-cons key val alist)
                (cons (cons key val) alist))))

          Or demand a certain set of SRFIs (list operations, string ports, receive
          and string operations), failing if they're not available.

(cond-expand ((and srfi-1 srfi-6 srfi-8 srfi-13)
              ))

   The Guile core has the following features,
guile
guile-2   ;; starting from Guile 2.x
guile-2.2 ;; starting from Guile 2.2
guile-3   ;; starting from Guile 3.x
guile-3.0 ;; starting from Guile 3.0
r5rs
r6rs
r7rs
exact-closed ieee-float full-unicode ratios ;; R7RS features
srfi-0
srfi-4
srfi-6
srfi-13
srfi-14
srfi-16
srfi-23
srfi-30
srfi-39
srfi-46
srfi-55
srfi-61
srfi-62
srfi-87
srfi-105

   Other SRFI feature symbols are defined once their code has been loaded with
   use-modules, since only then are their bindings available.

   The `--use-srfi' command line option (see [13]Invoking Guile) is a good way to
   load SRFIs to satisfy cond-expand when running a portable program.

   Testing the guile feature allows a program to adapt itself to the Guile module
   system, but still run on other Scheme systems. For example the following demands
   SRFI-8 (receive), but also knows how to load it with the Guile mechanism.
(cond-expand (srfi-8
              )
             (guile
              (use-modules (srfi srfi-8))))

   Likewise, testing the guile-2 feature allows code to be portable between Guile
   2.x and previous versions of Guile. For instance, it makes it possible to write
   code that accounts for Guile 2.x's compiler, yet be correctly interpreted on 1.8
   and earlier versions:
(cond-expand (guile-2 (eval-when (compile)
                        ;; This must be evaluated at compile time.
                        (fluid-set! current-reader my-reader)))
             (guile
                      ;; Earlier versions of Guile do not have a
                      ;; separate compilation phase.
                      (fluid-set! current-reader my-reader)))

   It should be noted that cond-expand is separate from the *features* mechanism
   (see [14]Feature Tracking), feature symbols in one are unrelated to those in the
   other.
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References

   1. http://www.gnu.org/software/guile/manual/html_node/index.html
   2. http://www.gnu.org/software/guile/manual/html_node/Concept-Index.html
   3. http://www.gnu.org/software/guile/manual/html_node/index.html#SEC_Contents
   4. http://www.gnu.org/software/guile/manual/html_node/SRFI-Support.html
   5. http://www.gnu.org/software/guile/manual/html_node/SRFI_002d1.html
   6. http://www.gnu.org/software/guile/manual/html_node/About-SRFI-Usage.html
   7. http://www.gnu.org/software/guile/manual/html_node/SRFI_002d1.html
   8. http://www.gnu.org/software/guile/manual/html_node/About-SRFI-Usage.html
   9. http://www.gnu.org/software/guile/manual/html_node/SRFI-Support.html
  10. http://www.gnu.org/software/guile/manual/html_node/index.html#SEC_Contents
  11. http://www.gnu.org/software/guile/manual/html_node/Concept-Index.html
  12. http://www.gnu.org/software/guile/manual/html_node/SRFI_002d0.html#index-cond_002dexpand
  13. http://www.gnu.org/software/guile/manual/html_node/Invoking-Guile.html
  14. http://www.gnu.org/software/guile/manual/html_node/Feature-Tracking.html
  15. http://www.gnu.org/software/guile/manual/html_node/SRFI_002d1.html
  16. http://www.gnu.org/software/guile/manual/html_node/About-SRFI-Usage.html
  17. http://www.gnu.org/software/guile/manual/html_node/SRFI-Support.html
  18. http://www.gnu.org/software/guile/manual/html_node/index.html#SEC_Contents
  19. http://www.gnu.org/software/guile/manual/html_node/Concept-Index.html