lazy evaluation with sweet.js

gistfile1.js

macro lazy {
  rule { $e:expr } => {
    new function() {
      this.done = false;
      this.valueOf = function() {
        if(!this.done) {
          this.done  = true;
          this.value = $e;
        }
        return this.value;
      };
    }
  }
}