Extended 'if' syntax, allowing 'else' and 'elif' clauses

extended-if.scm

(define-module (elmord extended-if)
  #:export (if*))

(define-syntax if*
  (syntax-rules (else elif)
    ;; Subsume the standard 'if'.
    [(if* condition form1) (if condition form1)]
    [(if* condition form1 form2) (if condition form1 form2)]
    ;; If more forms present, use extra syntax.
    [(if* condition forms ...)
     (if/sub condition () forms ...)]))

(define-syntax if/sub
  (syntax-rules (else elif)
    [(if/sub condition (then-forms ...) else else-forms ...)
     (if condition (begin then-forms ...) (begin else-forms ...))]
    [(if/sub condition1 (then-forms ...) elif condition2 rest ...)
     (if condition1
         (begin then-forms ...)
       (if/sub condition2 () rest ...))]
    [(if/sub condition (then-forms ...) then-form rest ...)
     (if/sub condition (then-forms ... then-form) rest ...)]))

;;; Example.

(define (signum x)
  (if* (> x 0)
      (display "positive\n")
      +1
   elif (< x 0)
      (display "negative\n")
      -1
   else
      (display "zero\n")
      0))

(signum 42)
(signum 0)
(signum -42)