我试图将别名和字段传递给pig宏：

define mymac (a1, f, a2) returns void {
  foreach (join $a1 on $f, $a2 on field) generate
    $a1::$f as $f, $a2::field as field;
};

我出错了

Unexpected character '$'

在第二个 $ 在 $a1::$f 当我把宏称为

mymac(x,foo,y);

我也试过了

define mymac (a1, f, af, a2) returns void {
  foreach (join $a1 on $f, $a2 on field) generate
    $af as $f, $a2::field as field;
};
mymac(x,foo,x::foo,y);

还有更神秘的错误

org.apache.pig.PigServer - exception during parsing: Error during parsing. Pig script failed to parse: at expanding macro

那么，我该怎么办？如何将别名与宏中的字段组合？