  Efficiently building an adversarial attacker for natural language processing
(NLP) tasks is a real challenge. Firstly, as the sentence space is discrete, it
is difficult to make small perturbations along the direction of gradients.
Secondly, the fluency of the generated examples cannot be guaranteed. In this
paper, we propose MHA, which addresses both problems by performing
Metropolis-Hastings sampling, whose proposal is designed with the guidance of
gradients. Experiments on IMDB and SNLI show that our proposed MHA outperforms
the baseline model on attacking capability. Adversarial training with MAH also
leads to better robustness and performance.
