#?(FEATURE CONSEQUENT ALTERNATE)
Perform an if-then test at read-time on FEATURE, expanding to CONSEQUENT if FEATURE is present or ALTERNATE if absent. FEATURE may be any feature expression permitted in a cond-expand, such as windows or (and windows macosx).
#? can be used inside macros because it is expanded when the macro form is read, prior to macroexpansion. It expands correctly irrespective of Chicken version.
#? is similar to the Common Lisp idiom
#+FEATURE CONSEQUENT #-FEATURE ALTERNATE
and, in Chicken versions >= 4.6.7 it is exactly equivalent, even inside macro bodies. However, in previous versions #+ and #- will not work properly inside macros; see #+ for further explanation.
An example of #? which is essentially equivalent to cond-expand:
(define af/unix #?(AF_UNIX _af_unix #f)) ;; is basically the same as (define af/unix (cond-expand (AF_UNIX _af_unix) (else #f)))
A more powerful example of #?:
(cond ((eq? x _af_inet) "internet address family") #?(AF_UNIX ((eq? x _af_unix) "unix address family") (#f)) (else "unknown address family")
which, if AF_UNIX is a registered feature, expands into
(cond ((eq? x _af_inet) "internet address family") ((eq? x _af_unix) "unix address family") (else "unknown address family")
and if not, expands into
(cond ((eq? x _af_inet) "internet address family") (#f) (else "unknown address family")
In the latter case, the false clause cannot succeed and is hopefully optimized out by the compiler. #+ would be more appropriate, but requires Chicken >= 4.6.7. This technique doesn't work with every macro, but you do what you can.