This acts sort of like a thread first. Instead of actually modifying the arguments list, I implicitly assign the result of each line in the body to `$_` then you can use `$_` in the body to refer to the result of the last line.

not-exactly-thread-first.js

macro thread {
  case { $name ($args (,) ...) { $body ... } } => {
    var result = makeIdent('$_', #{$name})
    return withSyntax($$_ = [result]) {
    console.log('here')
      return #{
        (function () {
          var $$_ = arguments.length > 1 ? [].slice.call(arguments) : arguments[0];
          thread_body $$_ $body ...;
          return $$_
        })($args (,) ...)
      }
    }
  }
}

macro thread_body {
  rule {
    $_ $first ...;
    $rest ...
  } => {
    $_ = $first ...;
    thread_body $_ $rest ...
  }
  rule {} => {}
}

var y = thread (1) {
  second(1, $_, 3);
  last(something, $_)
}