Introduction
1.1 Related Surveys  
1.2 Structure of the Survey

Machine Reading Comprehension
2.1 Classical MRC  
2.2 Novel MRC  

Open-Domain Question Answering
3.1 Curated TREC  
3.2 Web Questions  
3.3 WikiMovies  
3.4 OpenSQuAD  
3.5 Qusar-T  
3.6 Open Trivia QA  
3.7 Open Natural Questions  
3.8 KILT benchmarks  

Commonsense Question Answering
4.1 SWAG  
4.2 CODAH  
4.3 Commonsense QA  
4.4 ReCoRD  
4.5 CosmosQA  

Metrics
5.1 Accuracy  
5.2 Exact Match  
5.3 F1  
5.4 ROUGE  
5.5 BLEU  
5.6 Meteor  
5.7 HEQ  

Current Trends and Future Directions
6.1 Current Trends  
6.2 Future Directions  

Conclusion