1 Introduction
Language models (LMs) predict a probability distribution over text, and are a fundamental technology widely studied in the natural language processing (NLP) community
(Bengio et al., 2003; Merity et al., 2018; Dai et al., 2019). Modern LMs are almost exclusively based on parametric recurrent (Mikolov et al., 2010; Sundermeyer et al., 2012) or selfattentional (Vaswani et al., 2017; AlRfou et al., 2019)neural networks. These models are of interest scientifically as one of the purest tests of our ability to capture the intricacies of human language mathematically (Linzen et al., 2016; Kuncoro et al., 2017; Petroni et al., 2019). They also have broad downstream applications in generating text in systems such as machine translation (Bahdanau et al., 2015), summarization (Rush et al., 2015), or dialog generation (Sordoni et al., 2015), as well as in the unsupervised representation learners that now power many applications in NLP (Devlin et al., 2018; Liu et al., 2019; Yang et al., 2019).However, there has been a recent move towards nonparametric neural LMs (Guu et al., 2018) that generate sentences in a twostep process of (1) selecting a prototype sentence from a large database, and (2) editing this prototype to the final desired output. Intuitively, these methods are attractive because they help remove some of the pressure on the parametric model to memorize the entirety of the language it must model. These intuitive advantages are also born out in superior performance on language modeling tasks (Guu et al., 2018), as well as downstream applications such as dialogue response generation (Weston et al., 2018; Wu et al., 2019), style transfer (Li et al., 2018), and code generation (Hashimoto et al., 2018; Hayati et al., 2018). In addition, the prototypes and continuous representations of the edits in such models lend an element of interpretability to the modeling process. On the down side, however, previous prototypedriven generation methods usually need to store and index a large prototype candidate pool (in general the whole training dataset), leading to significant issues with memory and speed efficiency at test time.
In this paper, we hypothesize that, in fact, a small set of prototypes is sufficient to achieve the great majority of the gains afforded by such nonparametric models. Intuitively, in a large corpus many sentences look very similar and may be represented by minor transformations of a single prototype sentence. For example, the sentence “I ordered a burger with fries” can serve as the prototype for data samples with the form “I ordered [NOUN PHRASE] with [NOUN PHRASE]”. This is evidenced by Guu et al. (2018)’s observation that 70 of the test set in the Yelp restaurant review corpus (Yelp, 2017) is within wordtoken Jaccard distance 0.5 of one training sentence.
To take advantage of this intuition, we propose a novel generative model that samples prototypes from a latent prototype distribution, which itself is sampled from a symmetric Dirichlet prior, as shown in Figure 1 (Section 3.1
). The Dirichlet prior with appropriate hyperparameters is able to encourage a
sparse prototype selection distribution, allowing us to reduce the prototype support set at test time to greatly improve efficiency. Moreover, we utilize amortized variational inference (Kingma and Welling, 2013) to train our model, which introduces a learnable prototype retriever to identify prototypes useful for generating each sentence (Section 3.2). This is different from (Guu et al., 2018) where prototypes for each sentence are fixed before training through edit distance heuristics.We evaluate our approach on the MSCOCO (Lin et al., 2014) and Yelp restaurant review (Yelp, 2017) corpora. Our method is able to improve perplexity over the neural language model baseline by up to 14 points and previous neural editor model by 6 points while achieving over 1000x memory savings and a 1000x speedup at test time (Section 4.2
). Interestingly, we find that the learned prototypes are able to represent different features when varying sparsity levels – a strong sparsity prior forces the model to share prototypes and the induced prototypes turn out to represent more generic features (e.g. syntactic form of the sentence). On the text generation side, our model is able to generate sentences that resemble the given prototype while allowing for smooth interpolation on the edit space as well (Section
4.3).2 Background
The prototypethenedit framework defines a nonparametric way to augment text generation models. Formally, given a corpus ,^{1}^{1}1Below, we sometimes ignore the subscript to simplify notation when there is no confusion. the model generates each observed example by: (1) retrieving a prototype sequence , (2) generating a continuous edit representation , and (3) generating conditioned on and . These intermediate prototype and edit representation variables can depend on extra context in conditional generation tasks (Hodosh et al., 2013; Gu et al., 2018), or are randomly sampled in unconditioned language modeling. In this paper, we focus on the latter, but our methods could likely be applied to the former as well.
For unconditioned language modeling, Guu et al. (2018) define the data likelihood as:
(1) 
where
is the prior distribution over prototypes and defined as a uniform distribution over
all training examples,is a continuous distribution over the edit vector, and
represents a sequencetosequence model parameterized by . Guu et al. (2018)’s stated goal of this prototypedriven model is to take direct advantage of training examples to improve language modeling performance while capturing interpretable semantic or syntactic properties in the latent prototype and edit vector variables. However, because the prototypes are selected from the entire training dataset, such a formulation sacrifices memory and speed efficiency due to the necessity of indexing and searching every training example at test time. In the following section, we detail our approach to mitigate this issue through the learning of sparse prototypes.3 Method
First we present our our proposed generative model, then we describe the learning and inference techniques for this model class.
3.1 Model Structure
In the previous formulation, Eq. 1, maintaining the entire training dataset at test time is necessary due to assuming a uniform prior over prototypes . Motivated by the hypothesis that a much smaller prototype set would suffice to achieve comparable performance, however, we believe that can be a sparse distribution where the probability mass concentrates on only a few representative prototypes. Since which training examples are representative as prototypes is unknown in advance, we propose to model the prototype distribution as a latent variable, endowing the model with freedom to infer a sparse prototype posterior automatically. We define and further assume that the latent prototype distribution is sampled from a prior distribution (detailed below) which is able to encourage a sparse probability distribution, given appropriate hyperparameters. The graphical model is depicted in Figure 1, which gives the following joint likelihood:
(2) 
The log marginal likelihood of the data, which we will approximate during training is:
(3) 
This is a general framework for learning sparse prototypes, and in this work we specifically experiment with the following parameterization to instantiate this model class:
Prior over prototype distribution :
We employ the Dirichlet distribution as : . The support of Dirichlet distribution is the standard probability simplex. Here we use the symmetric Dirichlet distribution which has the same value for all components since we have no prior knowledge favoring one component over another. is positive and also referred to as the concentration parameter, where smaller prefers a sparser prototype distribution , with equivalent to a uniform distribution over the probability simplex. In our experiments, we often choose to encourage sparsity.
Prior over edit vector :
We follow Guu et al. (2018) and utilize a vonMises Fisher (vMF) distribution to model . The vMF distribution places mass on the surface of the unit sphere, and is parameterized by the mean direction vector and concentration parameter as . Thus, information about the edit is captured through the directions of different unit vector samples. Xu and Durrett (2018) shows that the vMF distribution has the advantage of being free of the issue of posterior collapse that plagues a large amount of previous work in latent variable models of text (Bowman et al., 2016; He et al., 2019). While Guu et al. (2018) add additional randomness on the norm of edit vectors by multiplying the vMF distribution with another uniform distribution, we sample edit vectors from a uniform vMF distribution directly, which simplifies the model but we nonetheless found sufficient to obtain competitive results. Formally, we define .
The editor :
Generally can be parameterized by any standard Seq2Seq model with the edit vector incorporated. To compare with Guu et al. (2018) directly in the experiments, in this work we adopt the same attentional LSTM architecture (Hochreiter and Schmidhuber, 1997). is utilized to predict the initial hidden state of the decoder and concatenated to the input for the decoder.
3.2 Learning and Inference
Ideally the log marginal likelihood in Eq. 3 should be optimized during training. However, computation is intractable due to marginalization of latent variables, and we resort to amortized variational inference (Kingma and Welling, 2013), optimizing its evidence lower bound (ELBO) instead:
(4) 
where represents the variational distribution to approximate the model posterior distribution and admits the following factorization form:
(5) 
Note that we make conditional independence assumption between and other latent variables in to simplify the approximate posterior, following common practice in traditional mean field variational inference. The inference diagram is depicted in Figure 1. The optimal to maximize Eq. 4 is a Dirichlet distribution parameterized by (proof is in Appendix A), i.e., .^{2}^{2}2 is not symmetric and is a vector. And , the prototype retriever, is a multinomial distribution over training examples parameterized by a neural network . We assume , the inverse neural editor, is a vMF distribution where the mean direction parameter is from an encoder that encodes and parameterized by , and the scalar concentration parameter is a hyperparameter. Prefixing results in a constant KL divergence term associated with and proves to be effective to mitigate the posterior collapse issue (Xu and Durrett, 2018) where and become independent. Yet there might be still posterior collapse on , in practice we follow (Li et al., 2019) to combine annealing (Bowman et al., 2016) and freebits techniques (Kingma et al., 2016) on to mitigate this issue.
Notably, the variational distribution family defined in Eq. 5 admits tractable closedform expressions of all three KL divergence terms in Eq. 4 (detailed derivations and expressions are in Appendix A). To compute the reconstruction log likelihood , expectations over can be efficiently approximated by the reparameterization trick for the vMF distribution (Guu et al., 2018). However, the prototype is discrete and nondifferentiable, and summing over all prototypes to compute is infeasible due to the evaluation burden of . Thus, we use the REINFORCE algorithm (Williams, 1992) to compute the gradients of contributed from as:
(6) 
where are samples from . We use an average reward from samples as the baseline . The neural parameters
are updated with stochastic gradient descent to maximize Eq.
4. With respect to the posterior Dirichlet parameter , we found in preliminary experiments that classic gradient descent was unable to update it effectively – was updated too slowly and the Dirichlet prior became decoupled with the model. Thus, we instead update with stochastic variational inference (SVI, Hoffman et al. (2013)) based on the formula of the optimal given (derivations can be found in Appendix A):(7) 
It is infeasibly expensive to keep optimal under current at each training step, as it would involve summing over all training examples. Thus we perform SVI, which uses a batch of examples to approximate Eq. 7, leading to the following update form:
(8) 
where is the batch size, is the stepsize at iteration , is the forgetting rate, and is the delay parameter to downweight early iterations.
We note that our training algorithm is different from Guu et al. (2018) in that we use a learnable prototype retriever to derive a lower bound as the objective while Guu et al. (2018) directly approximate marginalization over . They use heuristics to fix the prototype set for each to be examples similar to in terms of edit distance, which might produce suboptimal prototypes for the generative model and also does not permit the learning of sparse prototype support.
Sparsity and scalability:
After training we expect to be able to infer a sparse prototype distribution with most components being almost zero, based on which we can prune and store the entries over a particular probability threshold only, improving memory and timeefficiency at test time. Specifically, we compute mean of under the Dirichlet posterior: , and then take the largest entries that occupy of the probability mass. At test time, we only maintain these prototypes and the prototype retriever is renormalized accordingly. One issue present during training is that cannot fit into memory when dealing with large datasets since it is a categorical distribution over all training examples. In this work, we randomly downsample a subset of training data as our prototype library before training if memory is unable to fit all training examples, and learn the sparse prototypes on top of this subsampled corpus. This acts like a rough prefiltering and in Section 4 we show that it suffices to learn good prototypes and achieve competitive language modeling performance. We leave more advanced techniques to address this issue (e.g. dynamically updating the prototype library) as future work.
Architectures:
We now describe the neural architectures we use for the prototype retriever and inverse neural editor . is defined as:
(9) 
where we prevent selecting the data example itself as the prototype during training to avoid overfitting. is a pretrained sentence encoder,
is a linear transformation matrix, and
is a temperature hyperparameter to control the entropy of , which is critical to stabilize the Reinforce algorithm at the initial training stage. To ease the computation of encoding all training examples at each update step, we fix the parameters of and update only, which proves to be sufficient in our experiments. We use the average embeddings of the last layer in pretrained BERT (Devlin et al., 2018)^{3}^{3}3We use pretrained uncased BERT base from the transformers library (Wolf et al., 2019). as our sentence encoder.While Guu et al. (2018) uses sum of inserted/deleted word vectors as the mean direction parameter of vMF distribution , we choose a more powerful encoder following recent advances on representing edits (Yin et al., 2019). Specifically, a standard diffing algorithm is run to compute an alignment of tokens in and , and produces two aligned sequences and an additional edit sequence that indicates edit operation (insertion), (deletion), (substitution), and (equal) at each position. We use
to denote padding. This is illustrated in Figure
2. Word embeddings of all three sequences are concatenated and fed into a singlelayer LSTM to obtain the edit representation.4 Experiments
Our experiments below are designed to (1) examine the efficacy of the proposed method on language modeling, (2) examine the efficiency of the proposed method on memory savings and speedup at test time, and (3) demonstrate the interpretable semantics/syntax captured by prototypes and edit vectors.
4.1 Setup
We perform experiments on three differentscale datasets to test our method in different scenarios:

[leftmargin=*]

MSCOCO (Lin et al., 2014): MSCOCO is an image caption dataset and we only focus on its captions as our data. The average length of captions is 12.6. The sentences do not have complex variations and are easy to find similar sentences as prototypes. We randomly sample 40K sentences as our training data and 4K as validation and test set respectively. This dataset represents a simple and smallscale setting to test our approach.

Yelp Medium/Yelp Large: These datasets consist of sentences from Yelp restaurant reviews (Yelp, 2017) preprocessed by Guu et al. (2018), allowing us to perform a direct comparison with their method. The medium and large datasets consist of 1.5M and 17M sentences respectfully, allowing us to test in moderate and relatively large data settings respectively. Note that Yelp Medium is obtained by further filtering Yelp Large to keep sentences that are generally shorter and have less variations, but the test sets for these two are the same.
Baselines:
We mainly consider neural language models (NLM) and the neural editor (Guu et al., 2018) as our baseline. It is worth noting that the neural editor model does not have likelihood defined on test sentences that are not similar to any example in the prototype library, thus it is necessary to interpolate with another NLM at test time for smoothing purposes, while our model is able to be used on its own. Note that we only report the neural editor baseline on Yelp Large since their public code is not ready to run on other datasets due to the required prebuilt index to retrieve prototypes.
Evaluations:
We evaluate language modeling performance with perplexity (PPL). For our model, we approximate the log marginal data likelihood through 1000 importanceweighted latent variable samples (Burda et al., 2015) and compute PPL based on this likelihood. At test time our approach prunes and has access to only prototypes that occupy probability mass of the posterior prototype distribution as described in Section 3.2. We report as “prototypes”. To have an intuitive notion about how similar the prototype and data examples are, we compute average of smoothed sentencelevel BLEU scores (Lin and Och, 2004) of the data examples on the validation set with their most likely prototype as the reference. We also report BLEU scores based on partofspeech sequences (POSBLEU)^{4}^{4}4POS tagging is performed using the Stanza library (Qi et al., 2020). to view the similarity from a more syntactic perspective. Test speed is evaluated on a single Nvidia 1080 Ti GPU.
Hyperparameters:
We try different Dirichlet prior parameters to control the sparsity and report different sparsity settings for all the datasets. The temperature parameter in the prototype retriever is tuned on the MSCOCO validation data and set as for all datasets. The concentration parameter of the vMF distribution is tuned and set as 30 for MSCOCO and Yelp Medium and 40 for Yelp Large. The number of Reinforce samples is set as 10 across all datasets. We sample 50K examples for Yelp Medium and 100K examples for Yelp Large as our training prototype library to address the training memory issue discussed in Section 3.2. We employ the same attentional LSTM Seq2Seq model as in Guu et al. (2018) to parameterize for a direct comparison. Our implementation is based on the fairseq toolkit (Ott et al., 2019) and complete hyperparameter settings can be found in Appendix B.
4.2 Results
Dataset  Model  PPL  prototypes  test speed (sent/s)  BLEU  POSBLEU 

MSCOCO  random retrieval  –  –  –  10.9  31.6 
NLM  20.0  –  3714  –  –  
Ours ()  18.9  25 (0)  388  13.2  38.8  
Ours ()  18.6  778 (2)  313  17.2  42.5  
Ours ()  19.0  16K (40)  250  20.9  46.7  
Ours ()  19.2  22K (56)  217  22.2  47.9  
Yelp Medium  random retrieval  –  –  –  8.1  17.8 
NLM  74.7  –  236  –  –  
Ours ()  63.6  77 (0)  157  12.3  24.7  
Ours ()  61.9  1.5K (0.1)  107  21.6  38.4  
Ours ()  63.2  31K (2.1)  95  29.9  48.3  
Yelp Large  random retrieval  –  –  –  6.6  16.0 
NLM  34.2  –  272  –  –  
Ours ()  30.2  2K (0.01)  108  10.5  24.8  
Ours ()  30.3  5.5K (0.03)  98  10.8  25.3  
Interpolated w/ NLM  
Neural Editor (Guu et al., 2018)  26.9  17M (100)  –  –  –  
Neural Editor (our runs)  31.2  17M (100)  0.1  –  –  
Ours ()  20.2  2K (0.01)  108  10.5  24.8 
Results are shown in Table 1. Our approach outperforms the NLM baseline across all datasets in terms of PPL, often by a large margin. When interpolated with an NLM at test time, our method outperforms the NLM baseline by 14 PPL points and neural editor by 6.7 PPL points.^{5}^{5}5For a fair comparison, we interpolate with the same pretrained NLM from (Guu et al., 2018). This effect is also observed in (Guu et al., 2018) – prototypedriven language models are especially strong at modeling test sentences that have similar prototypes but relatively weak at modeling others, thus interpolation with a normal NLM is likely to help. Furthermore, in line with the goal of this work to learn a sparse prototype set, our method is able to achieve superior language modeling performance while utilizing only a small fraction of training examples as prototypes. This verifies our hypothesis that a sparse prototype set suffices for such nonparametric language modeling. Also, sparsity learned in our model allows for over a 1000x memory savings and 1000x speedup^{6}^{6}6We include the time to retrieve prototypes for new test sentences when computing test speed. We use Guu et al. (2018)’s public implementation of the neural editor, where the computation of edit distance between all training examples and test sentences to find nearest neighbors accounts for much of the runtime. More efficient implementation of this operation, for example through tries, may speed this to some extent. at test time on Yelp Large compared with a previous neural editor that memorizes all training examples.
Table 1 demonstrates the trend that a smaller Dirichlet hyperparameter leads to a sparser prototype set, which agrees with our expectation. BLEU scores are also improved as increases, implying that the less sparse the prototype set the closer the match between the sentence and its prototype. Interestingly, the BLEU score on Yelp Large is a bit low and the model tends to generally favor sparse prototypes. We suspect that this is because it is difficult to learn prototypes that capture finegrained semantic features and large sentence variations among 17M examples with a limited prototype memory budget, thus the model has to learn prototypes that represent more generic shared features among examples to reach an optimum – for example, the syntactic feature as somewhat reflected by the decent POSBLEU scores.
We want to emphasize that different sparsity may lead to different notions of prototypes and it is hard to judge which one should be preferred – memorizing more prototypes pays cost on language modeling performance and does not necessarily produce better PPL. Also, prototypes that are “less similar” to the examples on the superficial level but capture coarsegrained features may have potentially interesting application on sentence generation since the model is able to generate more diverse output conditioned on prototypes.
4.3 Analysis
Model  Overall  NOUN  DET  AUX  PRON  ADJ  VERB  CCONJ 

Ours (31K prototypes)  91.2K  14.4K  9.6K  9.3K  9.0K  7.2K  6.4K  5.5K 
Ours (1.5K prototypes)  74.7K  9.9K  8.5K  8.2K  7.3K  5.6K  4.4K  5.0K 
Relative Change  18.1  31.3  11.5  11.8  18.9  22.2  31.3  9.1 
Data Examples  Prototypes 

the best corned beef hash i ’ve ever had !  (dense) the best real corned beef hash i ’ve had . 
(sparse) the chicken satay is the best i ’ve ever had .  
the grilled chicken was flavorful , but too flavorful .  (dense) the chicken was moist but it lacked flavor . 
(sparse) my sandwich was good but the chicken was a little plain .  
i asked her what time they close and she said  (dense) i asked what time they closed <date> , and was told <cardinal> . 
(sparse) we asked how long the wait was and we were informed it  
<cardinal> o’clock .  would be <time> . 
Prototype: A man is using a small laptop computer  Prototype: A cat sitting on a sidewalk behind a bush 

A man is using his laptop computer with his hands on the keyboard  A cat laying on top of a wooden bench 
A man is using a laptop computer with his hands on the keyboard  A cat standing next to a tree in a park 
A man is using a laptop computer while sitting on a bench  Two cats sitting on a bench near a park bench 
A man is using a laptop computer in the middle of a room  A dog sitting on a bench near a park bench 
A young man is using a laptop computer in the middle of a room  A dog sitting on a bench near a park bench 
How do prototypes change when they grow sparser?
In Table 1 we notice the BLEU scores usually drop when the prototype set is sparser, implying the learned prototypes change in some way under different sparsity settings. Here we take the Yelp Medium dataset as an example to analyze how prototypes are trained to capture sentence attributes differently in the sparse () and relatively dense () situations. Specifically, we align prototype and example sequences to minimize edit distance, and focus on words that were aligned between the two sequences. This allows us to obtain a notion of what kind of words are more likely to be kept the same as the prototype during the model’s editing process and how this pattern changes in different sparsity settings. We cluster these matched words in terms of POS tag and report the most common ones.^{7}^{7}7Note that alignment is performed on word sequences instead of POS sequences.
Results are shown in Table 2. While the overall number of matching tokens decreases as the prototype set becomes sparser, the content words exhibit a more drastic change (e.g. the nouns, adjectives, and verbs). In contrast, the function words experience a moderate decrease only (e.g. the determiners, auxiliaries, and coordinating conjunctions). This shows that the model tends to learn prototypes that drop finegrained semantic distinctions but keep the same general syntax when a sparsity constraint is enforced, which is not surprising since it is difficult for a limited number of prototypes to capture large semantic variations in a large dataset. We list qualitative examples in Table 3 to demonstrate this phenomenon.
Interpolation on the edit vector space:
We take our model with 1.5K prototypes on the MSCOCO dataset (i.e. ) and perform sentence interpolation in edit vector space. Specifically, we sample two edit vectors from the uniform vMF prior to produce two sentences for each prototype with beam search decoding, then we perform spherical linear interpolation of the two edit vectors to generate interpolated sentences inbetween. Qualitative examples in Table 4 (more examples in Appendix C) show that the edit vectors are able to smoothly capture minor edits over the given prototypes.
5 Conclusion
In this work, we propose a novel generative model that discovers a sparse prototype set automatically by optimizing a variational lower bound of the log marginal data likelihood. We demonstrate its effectiveness on language modeling and its efficiency advantages over previous prototypedriven generative models. The framework proposed here might be generalized to automatically discover salient prototypes from a large corpus. New kinds of prototype structure in text might be discovered through either injecting different biases into the model (e.g. sparsity biases in this paper), or incorporating prior knowledge into the prototype library before training. Finally, the approach might be easily extended to conditional generation (e.g. with the edit vectors depending on other data input), and we envision that inducing a sparse prototype set in this case may potentially facilitate controlling text generation through prototypes. We leave exploration in this direction as our future work.
Acknowledgements
This work was supported in part by the DARPA GAILA project (award HR00111990063), and a gift of computation credits from Amazon AWS. The views and conclusions contained in this document are those of the authors and should not be interpreted as representing the official policies, either expressed or implied, of the U.S. Government or Amazon.
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Appendix A Derivations of Variational Inference and ELBO
a.1 Derivation of optimal
Here we try to show that the optimal variational distribution over , , is a Dirichlet distribution and derive its optimal as given in Eq. 7. According to (Bishop, 2006) (Chapter 10.1.1), we have:
(10) 
where denotes expectation over all latent variables except for . We expand Eq. 10 as:
(11) 
where is the indicator function. We conclude that has the form of Dirichlet distribution and the optimal Dirichlet parameter .
a.2 Derivation of Three KL Divergence Terms
There are three KL divergence terms in our training objective ELBO (Eq. 4). Now we show that all three KL divergence terms can be computed exactly and efficiently at training time and we derive their expressions respectively:
(1). :
As shown in (Xu and Durrett, 2018), the KL divergence between any vMF distribution with fixed concentration parameter and a uniform vMF distribution is a constant:
(12) 
where is the number of dimensions, stands for the modified Bessel function of the first kind at order . Therefore,
(13) 
(2). :
(14) 
where is the digamma function, and step computes the expectation of over Dirichlet variable by using the general fact that the derivative of the log normalization factor with respect to the natural parameter is equal to the expectation of the sufficient statistic.
(3). :
(15) 
where is the multivariate beta function and is the normalization factor for Dirichlet distribution parameterized by .
Appendix B Experimental Details
On MSCOCO dataset, we use a singlelayer attentional LSTM seq2seq architecture with word embedding size 100 and hidden state size 400 as , the latent edit vector dimension is 50. This configuration follows the hyperparameters for vMFVAE (Xu and Durrett, 2018). On Yelp Medium and Yelp Large datasets, we follow (Guu et al., 2018) to use a threelayer attentional LSTM seq2seq architecture as with word embedding size 300 and hidden state size 256, the edit vector dimension is 128. Skip connections are also used between adjacent LSTM layers. In the inverse editor , we use a singlelayer LSTM to encode three sequences – the aligned prototype, aligned data example, and the edit operation sequence, of which the word embedding size for text sequences and the hidden state size are the same as in , and the word embedding size for edit operation sequence is 10 (since the vocobulary size of edit operations is very small). Across all datasets, we initialize word embeddings in (both encoder and decoder sides) and with GloVe word embeddings (Pennington et al., 2014) following (Guu et al., 2018). All NLM baselines use the same architecture as in our model for a fair comparison.
With respect to hyperparameter tuning, we tune the temperature parameter in the prototype retriver on the MSCOCO validation data in the range of , and set as for all datasets. The concentratioon parameter of the vMF distribution is tuned in the range of for all datasets. is set as 30 for MSCOCO and Yelp Medium and 40 for Yelp Large. We run different as for each dataset to obtain prototype set with varying sparsity. On MSCOCO dataset we also add an additional run with since is a large value on this dataset already (90 of training examples are selected as the prototype set when ). We apply annealing and freebits techniques following (Li et al., 2019) to the KL term on prototype variable, , to mitigate posterior collapse. Specifically, in our training objective becomes in practice. This objective means that we can downweight this KL term with and optimize it only when this KL is larger than a threshold value . We increase from 0 to 1 linearly in the first epochs (annealing). is tuned in the range of for MSCOCO and for Yelp Medium and Yelp Large.^{8}^{8}8It is unreasonable to set a large on Yelp dataset since there are tens of thousands update steps per epoch on Yelp, the annealinig process would be too slow if is large which usually hurts the language modeling performance (He et al., 2019). is tuned in the range of . To obtain the reported results in Section 4, is set as 5 for MSCOCO, 2 for Yelp Medium and 3 for Yelp Large. is set as 5 for MSCOCO, 6 for Yelp Medium and 8 for Yelp Large. We use Adam (Kingma and Ba, 2014) to optimize the training objective with learning rate 0.001.
Appendix C Qualitative Results on Interpolation
As in Section 4.3, here we show more generated examples through interpolation on MSCOCO dataset.
Prototype: a horse drawn carriage on the side of a city street  Prototype: A baseball pitcher on the mound having just threw a pitch 

Two horses drawn carriage on a city street  a baseball player swinging a bat at home plate 
Two horses standing next to each other on a city street  a man about to hit a ball with his bat 
Two horses on the side of a city street  a man swinging a bat at the ball during a game 
Two horses on the side of a city street  a person swinging a bat at the ball during a game 
A brown and white horse drawn carriage on a city street  A person swinging a bat during a baseball game 
Prototype: A man walking on the beach carrying a surfboard  Prototype: A group of people are raising an umbrella on a beach 
Two people standing next to each other on a beach  A group of people are walking on the beach with umbrellas 
A person standing on the beach holding a surfboard  A group of people are walking on the beach next to each other 
A man walking along the beach with a surfboard  A group of people are walking on the beach with umbrellas 
A man walking on the beach with a surfboard  A group of people are holding umbrellas on the beach 
A young man walking on the beach with a surfboard  A group of people are walking on the beach 
Prototype: there is a white truck that is driving on the road  Prototype: A couple of bags of luggage sitting up against a wall 
there are many cows that are standing in the dirt  A large pile of luggage sitting on top of a wall 
there are many cows that are standing in the dirt  A pile of luggage sitting on top of a wall 
the truck is driving down the road in the rain  Two bags of luggage sitting on the ground 
this truck is driving down the road in the rain  Two bags of luggage sitting in a room 
This truck is pulled up to the side of the road  A couple of bags of luggage on a wooden floor 
Prototype: A man riding a sailboat in the ocean next to a shore  Prototype: A beer bottle sitting on a bathroom sink next to a mirror 
A man on a boat in a body of water  A white cell phone sitting next to a toilet in a bathroom 
A man riding a boat on a body of water  A white bottle of wine sitting next to a toilet 
A man riding a boat in a body of water  A glass of wine sitting next to a toilet in a bathroom 
A man riding a small boat on a body of water  A pair of scissors is placed next to a toilet 
A man riding a wave on top of a boat  A pair of scissors sitting next to each other on a toilet 
Prototype: A little boy sitting on a mattress holding a stuffed animal  Prototype: A giraffe has its nose pressed against the trunk of a tree 
A little girl playing with a stuffed animal  Two giraffes look at a wire fence to eat 
A little girl playing with a stuffed animal  Two giraffes look at a fence to eat 
A little boy holding a stuffed animal in his mouth  a couple of giraffes are standing by a fence 
A little girl sitting on a bed with stuffed animals  a close up of a giraffe is eating a carrot 
A little girl sitting on a bed with stuffed animals  a close up of a giraffe has its mouth open 
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