Paper:
Improving Neural Program Synthesis with Inferred Execution Traces
The task of program synthesis, or automatically generating programs that are consistent with a provided specification, remains a challenging task in artificial intelligence. As in other fields of AI, deep learning-based end-to-end approaches have made great advances in program synthesis. However, compared to other fields such as computer vision, program synthesis provides greater opportunities to explicitly exploit structured information such as execution traces. While execution traces can provide highly detailed guidance for a program synthesis method, they are more difficult to obtain than more basic forms of specification such as input/output pairs. Therefore, we use the insight that we can split the process into two parts: infer traces from input/output examples, then infer programs from traces. Our application of this idea leads to state-of-the-art results in program synthesis in the Karel domain, improving accuracy to 81.3% from the 77.12% of prior work.

Review:
Summary: This paper provides a novel program synthesis approach by inferring the intermediate states: execution trace. Unlike conventional methods that treat program synthesis as an end-to-end task based on input/output (i/o) examples, in this manuscript, the authors attempt to infer the execution trace from i/o examples, and further synthesize the program given the i/o and execution trace. The motivation is clear and straightforward since generating programs from execution traces should be easier. Experiments on Karel program synthesis task show the effectiveness of the proposed approach. The research question is significant and very hard. It is good to see more program characteristics are included in the design of the neural network.   Strengths: 1.    Though straightforward, the proposed framework for program synthesis is novel. It splits the program synthesis into two steps by introducing the execution traces.  2.    The experiments are well-designed and the results are superior to the competing methods greatly. 3.    This paper is well-organized and the writing is good. Weaknesses: 1.    A few parts of this paper are not very clear or not sufficiently provided, such as the model details. Section 4.3 should be addressed more to make it clearer since some concepts/statements are misleading or confusing. 2.    The trace to code model is complex, which requires as many LSTMs as the number of input/output pairs, and it may be hard to be applied to other program synthesis scenarios.  3.    Apart from the DSL, more experiments on dominant PL (e.g., Python) would be appreciated by people in this research field.  Details are described as below: 1.    For a given program, how to generate diverse execution traces that can be captured by the I/O -> Trace model? Since execution traces are generated by running the program on N I/O pairs, it is possible that some execution traces have a large overlap. For example, in the extreme case, two execution traces may be the same (or very similar) given different I/O pairs.  2.    The authors involve the program interpreter in their approach, which is a good trial and it should help enhance the performance. However, I am curious about is it easy to be integrated with the neural network during training and testing? 3.    The concept of state is not very clear, from my understanding, it represents the grid status (e.g., agent position) and it is obtained after applying an action of the trace. Line 186-line 187, is the “elements” equivalent to “states”? or “actions”? More should be elaborated. 4.    Line 183-line 184, is it necessary to use embedding for only four conditionals (of Boolean type)? only 16 possible combinations.  5.    As depicted in Figure 3, if more I/O pairs are provided, the Trace->Code should be very complex since each i/o example requires such an LSTM model. How to solve this issue? 6.    In essence, the Trace->Code structure is a Sequence to Sequence model with attention, the only differences are the employment of I/O pair embedding and the max pooling on multiple LSTM. How are the I/O pair embeddings integrated into the computation? Some supplementary information should be provided.  7.    It is interesting to find that model trained on gold traces perform poorly on inferred traces, the authors do not give a convincing explanation. More exploration should be conducted for this part. 8.    It would be better if some synthesized program samples are introduced in an appendix or other supplementary documents.