Problem Statement

Name and briefly demonstrate any support your language has for metaprogramming. Your demonstration may take the form of cross-references to other tasks on Rosetta Code. When possible, provide links to relevant documentation. For the purposes of this task, "support for metaprogramming" means any way the user can effectively modify the language's syntax that's built into the language (like Lisp macros) or that's conventionally used with the language (like the C preprocessor). Such facilities need not be very powerful: even user-defined infix operators count. On the other hand, in general, neither operator overloading nor eval count. The task author acknowledges that what qualifies as metaprogramming is largely a judgment call.

Let's start with the solution:

(defmacro whil ((condition : result) . body)
  (let ((cblk (gensym "cnt-blk-"))
        (bblk (gensym "brk-blk-")))
    ^(macrolet ((break (value) ^(return-from ,',bblk ,value)))
       (symacrolet ((break (return-from ,bblk))
                    (continue (return-from ,cblk)))
         (block ,bblk
           (for () (,condition ,result) ()
             (block ,cblk ,*body)))))))

(let ((i 0))
  (whil ((< i 100))
    (if (< (inc i) 20)
      continue)
    (if (> i 30)
      break)
    (prinl i)))

(prinl
  (sys:expand
    '(whil ((< i 100))
       (if (< (inc i) 20)
         continue)
       (if (> i 30)
         break)
       (prinl i))))