module: rpn

define function rpn()
  let (#rest s) = split("[ \t]",read-line(*standard-input*));
  if (~s.empty?)  process-line(s);  end;
  rpn();
end;

define function process-line(s)
  block()
    local method process-term(r,s)
      let op = block()
                 assert(1 = s.size);
                 $ops[s[0]];
               exception(<condition>) #f;  end;
      add!(r, if (op)
                let (z,y) = values(r.pop,r.pop);
                op(y,z);
              else
                let (x,i) = string-to-float(s);
                assert(i = s.size);
                x;
              end);
    end;
    let r = reduce(process-term,make(<deque>),s);
    assert(1 = r.size);
    format-out("%d\n",r.pop);
  exception(<condition>) format-out("error\n");  end;
  force-output(*standard-output*);
end;

define table $ops =
  {'+' => \+, '-' => \-, '*' => \*, '/' => \/};

block()
  rpn();
exception(<end-of-stream-error>)  end;