— special form: cond-expand clause clause dots

Each clause has the form

          (feature-requirement expression ...)
     

where feature-requirement can have one of the following forms:

          feature-identifier
          (and feature-requirement ...)
          (or feature-requirement ...)
          (not feature-requirement)
          else
     

(Note that at most one else clause may be present, and it must always be the last clause.)

The cond-expand special 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 (an 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.

          (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.

Note that MIT/GNU Scheme allows cond-expand in any context where a special form is allowed. This is an extension of the semantics defined by SRFI 0, which only allows cond-expand at top level.