2.1.4 Specifications

Syntax:

<command or definition>
    --> <command>
      | <definition>
      | <syntax definition>
      | (begin <command or definition>+)
      | <conditional expansion form>
<conditional expansion form>
    --> (cond-expand <cond-expand clause>+)
      | (cond-expand <cond-expand clause>*
                     (else <command or definition>*))
<cond-expand clause>
    --> (<feature requirement> <command or definition>*)
<feature requirement>
    --> <feature identifier>
      | (and <feature requirement>*)
      | (or <feature requirement>*)
      | (not <feature requirement>)
<feature identifier>
    --> a symbol which is the name or alias of a SRFI

The cond-expand form tests for the existence of features at macro–expansion time. It either expands into the body of one of its clauses or signals an error during syntactic processing. cond-expand expands into the body of the first clause whose feature requirement is currently satisfied (the else clause, if present, is selected if none of the previous clauses is selected).

A feature requirement has an obvious interpretation as a logical formula, where the <feature identifier> variables have meaning TRUE if the feature corresponding to the feature identifier, as specified in the SRFI registry, is in effect at the location of the cond-expand form, and FALSE otherwise. A feature requirement is satisfied if its formula is true under this interpretation.

Examples:

(cond-expand
  ((and srfi-1 srfi-10)
   (write 1))
  ((or srfi-1 srfi-10)
   (write 2))
  (else))

(cond-expand
  (command-line
   (define (program-name) (car (argv)))))

The second example assumes that command-line is an alias for some feature which gives access to command line arguments. Note that an error will be signaled at macro–expansion time if this feature is not present.