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Title: Syntax-Directed Variational Autoencoder for Structured Data

Paper content:
Deep generative models have been enjoying success in modeling continuous data.
However it remains challenging to capture the representations for discrete structures with formal grammars and semantics, e.g., computer programs and molecular structures.
How to generate both syntactically and semantically correct data still remains largely an open problem.
Inspired by the theory of compiler where syntax and semantics check is done via syntax-directed translation (SDT), we propose a novel syntax-directed variational autoencoder (SD-VAE) by introducing stochastic lazy attributes.
This approach converts the offline SDT check into on-the-fly generated guidance for constraining the decoder.
Comparing to the state-of-the-art methods, our approach enforces constraints on the output space so that the output will be not only syntactically valid, but also semantically reasonable.
We evaluate the proposed model with applications in programming language and molecules, including reconstruction and program/molecule optimization.
The results demonstrate the effectiveness in incorporating syntactic and semantic constraints in discrete generative models, which is significantly better than current state-of-the-art approaches.
Recent advances in deep representation learning have resulted in powerful probabilistic generative models which have demonstrated their ability on modeling continuous data, e.g., time series signals BID20 BID6 and images BID22 BID15 .
Despite the success in these domains, it is still challenging to correctly generate discrete structured data, such as graphs, molecules and computer programs.
Since many of the structures have syntax and semantic formalisms, the generative models without explicit constraints often produces invalid ones.Conceptually an approach in generative model for structured data can be divided in two parts, one being the formalization of the structure generation and the other one being a (usually deep) generative model producing parameters for stochastic process in that formalization.
Often the hope is that with the help of training samples and capacity of deep models, the loss function will prefer the valid patterns and encourage the mass of the distribution of the generative model towards the desired region automatically.Arguably the simplest structured data are sequences, whose generation with deep model has been well studied under the seq2seq BID25 framework that models the generation of sequence as a series of token choices parameterized by recurrent neural networks (RNNs).
Its widespread success has encourage several pioneer works that consider the conversion of more complex structure data into sequences and apply sequence models to the represented sequences.
BID11 (CVAE) is a representative work of such paradigm for the chemical molecule generation, using the SMILES line notation BID26 for representing molecules.
Figure 1: Illustration on left shows the hierarchy of the structured data decoding space w.r.t different works and theoretical classification of corresponding strings from formal language theory.
SD-VAE, our proposed model with attribute grammar reshapes the output space tighter to the meaningful target space than existing works.
On the right we show a case where CFG is unable to capture the semantic constraints, since it successfully parses an invalid program.However, because of the lack of formalization of syntax and semantics serving as the restriction of the particular structured data, underfitted general-purpose string generative models will often lead to invalid outputs.
Therefore, to obtain a reasonable model via such training procedure, we need to prepare large amount of valid combinations of the structures, which is time consuming or even not practical in domains like drug discovery.To tackle such a challenge, one approach is to incorporate the structure restrictions explicitly into the generative model.
For the considerations of computational cost and model generality, contextfree grammars (CFG) have been taken into account in the decoder parametrization.
For instance, in molecule generation tasks, BID18 proposes a grammar variational autoencoder (GVAE) in which the CFG of SMILES notation is incorporated into the decoder.
The model generates the parse trees directly in a top-down direction, by repeatedly expanding any nonterminal with its production rules.
Although the CFG provides a mechanism for generating syntactic valid objects, it is still incapable to regularize the model for generating semantic valid objects BID18 .
For example, in molecule generation, the semantic of the SMILES languages requires that the rings generated must be closed; in program generation, the referenced variable should be defined in advance and each variable can only be defined exactly once in each local context (illustrated in Fig 1b) .All
the examples require cross-serial like dependencies which are not enforceable by CFG, implying that more constraints beyond CFG are needed to achieve semantic valid production in VAE.In the theory of compiler, attribute grammars, or syntax-directed definition has been proposed for attaching semantics to a parse tree generated by context-free grammar. Thus
one straightforward but not practical application of attribute grammars is, after generating a syntactic valid molecule candidate, to conduct offline semantic checking. This
process needs to be repeated until a semantically valid one is discovered, which is at best computationally inefficient and at worst infeasible, due to extremely low rate of passing checking. As a
remedy, we propose the syntax-direct variational autoencoder (SD-VAE), in which a semantic restriction component is advanced to the stage of syntax tree generator. This
allows the generator with both syntactic and semantic validation. The
proposed syntax-direct generative mechanism in the decoder further constraints the output space to ensure the semantic correctness in the tree generation process. The
relationships between our proposed model and previous models can be characterized in Figure 1a .Our
method brings theory of formal language into stochastic generative model. The
contribution of our paper can be summarized as follows:• Syntax and semantics enforcement: We propose a new formalization of semantics that systematically converts the offline semantic check into online guidance for stochastic generation using the proposed stochastic lazy attribute. This
allows us effectively address both syntax and semantic constraints.• Efficient
learning and inference: Our approach has computational cost O(n) where n is the length of structured data. This is the
same as existing methods like CVAE and GVAE which do not enforce semantics in generation. During inference
, the SD-VAE runs with semantic guiding onthe-fly, while the existing alternatives generate many candidates for semantic checking.• Strong empirical
performance: We demonstrate the effectiveness of the SD-VAE through applications in two domains, namely (1) the subset of Python programs and (2) molecules. Our approach consistently
and significantly improves the results in evaluations including generation, reconstruction and optimization.
In this paper we propose a new method to tackle the challenge of addressing both syntax and semantic constraints in generative model for structured data.
The newly proposed stochastic lazy attribute presents a the systematical conversion from offline syntax and semantic check to online guidance for stochastic generation, and empirically shows consistent and significant improvement over previous models, while requiring similar computational cost as previous model.
In the future work, we would like to explore the refinement of formalization on a more theoretical ground, and investigate the application of such formalization on a more diverse set of data modality.

assistant: text: A new generative model for discrete structured data. The proposed stochastic lazy attribute converts the offline semantic check into online guidance for stochastic decoding, which effectively addresses the constraints in syntax and semantics, and also achieves superior performance