It is like Jony Ive wrote this:

  (set prims '((id join xar xdr wrb ops)
             (car cdr type sym nom rdb cls stat sys)
             (coin)))

  (def applyprim (f args s r m)
  (aif (some [mem f _] prims)
       (if (udrop (cdr it) args)
           (sigerr 'overargs s r m)
           (with (a (car args)
                  b (cadr args))
             (eif v (case f
                      id   (id a b)
                      join (join a b)
                      car  (car a)
                      cdr  (cdr a)
                      type (type a)
                      xar  (xar a b)
                      xdr  (xdr a b)
                      sym  (sym a)
                      nom  (nom a)
                      wrb  (wrb a b)
                      rdb  (rdb a)
                      ops  (ops a b)
                      cls  (cls a)
                      stat (stat a)
                      coin (coin)
                      sys  (sys a))
                    (sigerr v s r m)
                    (mev s (cons v r) m))))
       (sigerr 'unknown-prim s r m)))

I like the brevity in Arc. I don’t know why, but it works for me.