chickadee » coerce » case-coerce

Returns a coercion procedure for use with extend-coerce.

CASE is of the form ((TYPE-TAG ...) EXPRESSION ...), or (else EXPRESSION ...).

The variable object is bound to the object to coerce within the scope of CASE ....

The variable on-error is bound to a thunk within the scope of CASE ...} to invoke upon failure.

(define (unspecified? obj) (eq? (void) obj))

(define unspecified-coerce
  (case-coerce
    ((char)     #\nul)
    ((number)   +nan)
    ((string)   "")
    ((symbol)   '||)
    ((list)     (list))
    ((vector)   (vector))))

(coerce-extend! 'unspecified unspecified? unspecified-coerce)

(type-of (void)) ;=> unspecified

(coerce-extend! 'string
  string?
  (case-coerce ((unspecified) (if (string=? "" object) (void) (on-error)))))

(coerce "" 'unspecified) ;=> (void)