Communications of the ACM

Review articles

Learning Executable Semantic Parsers For Natural Language Understanding

By Percy Liang
Communications of the ACM, September 2016, Vol. 59 No. 9, Pages 68-76
10.1145/2866568
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A long-standing goal of artificial intelligence (AI) is to build systems capable of understanding natural language. To focus the notion of "understanding" a bit, let us say the system must produce an appropriate action upon receiving an input utterance from a human. For example:

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Key Insights

We are interested in utterances such as the ones listed here, which require deep understanding and reasoning. This article focuses on semantic parsing, an area within the field of natural language processing (NLP), which has been growing over the last decade. Semantic parsers map input utterances into semantic representations called logical forms that support this form of reasoning. For example, the first utterance listed previously would map onto the logical form max(primes ∩ (−∞, 10)). We can think of the logical form as a program that is executed to yield the desired behavior (for example, answering 7). The second utterance would map onto a database query; the third, onto an invocation of a calendar API.

Semantic parsing is rooted in formal semantics, pioneered by logician Montague,²⁵ who famously argued that there is "no important theoretical difference between natural languages and the artificial languages of logicians." Semantic parsing, by residing in the practical realm, is more exposed to the differences between natural language and logic, but it inherits two general insights from formal semantics: The first idea is model theory, which states that expressions (for example, primes) are mere symbols that only obtain their meaning or denotation (for example, {2, 3, 5, ...}) by executing the expression with respect to a model, or in our terminology, a context. This property allows us to factor out the understanding of language (semantic parsing) from world knowledge (execution). Indeed, one can understand the utterance "What is the largest prime less than 10?" without actually computing the answer. The second idea is compositionality, a principle often attributed to Gottlob Frege, which states that the denotation of an expression is defined recursively in terms of the denotations of its subexpressions. For example, primes denotes the set of primes, (−∞, 10) denotes the set of numbers smaller than 10, and so primes ∩ (−∞, 10) denotes the intersection of those two sets. This compositionality is what allows us to have a succinct characterization of meaning for a combinatorial range of possible utterances.

Early systems. Logical forms have played a foundational role in natural language understanding systems since their genesis in the 1960s. Early examples included LUNAR, a natural language interface into a database about moon rocks,³⁴ and SHRDLU, a system that could both answer questions and perform actions in a toy blocks world environment.³² For their time, these systems were significant achievements. They were able to handle fairly complex linguistic phenomena and integrate syntax, semantics, and reasoning in an end-to-end application. For example, SHRDLU was able to process "Find a block which is taller than the one you are holding and put it into the box." However, as the systems were based on handcrafted rules, it became increasingly difficult to generalize beyond the narrow domains and handle the intricacies of general language.

Rise of machine learning. In the early 1990s, influenced by the successes of statistical techniques in the neighboring speech recognition community, the field of NLP underwent a statistical revolution. Machine learning offered a new paradigm: Collect examples of the desired input–output behavior and then fit a statistical model to these examples. The simplicity of this paradigm coupled with the increase in data and computation allowed machine learning to prevail.

What fell out of favor was not only rule-based methods, but also the natural language understanding problems. In the statistical NLP era, much of the community's attention turned to tasks—documentation classification, part-of-speech tagging, and syntactic parsing—which fell short of full end-to-end understanding. Even question answering systems relied less on understanding and more on a shallower analysis coupled with a large collection of unstructured text documents,¹⁰ typified by the TREC competitions.

Statistical semantic parsing. The spirit of deep understanding was kept alive by researchers in statistical semantic parsing.^{19,24,33,36,37} A variety of different semantic representations and learning algorithms were employed, but all of these approaches relied on having a labeled dataset of natural language utterances paired with annotated logical forms, for example:

Weak supervision. Over the last few years, two exciting developments have really spurred interest in semantic parsing. The first is reducing the amount of supervision from annotated logical forms to answers:^13,21

This form of supervision is much easier to obtain via crowdsourcing. Although the logical forms are not observed, they are still modeled as latent variables, which must be inferred from the answer. This results in a more difficult learning problem, but Liang et al.²¹ showed it is possible to solve it without degrading accuracy.

Scaling up. The second development is the scaling up of semantic parsers to more complex domains. Previous semantic parsers had only been trained on limited domains such as U.S. geography, but the creation of broad-coverage knowledgebases such as Freebase⁸ set the stage for a new generation of semantic parsers for question answering. Initial systems required annotated logical forms,¹¹ but soon, systems became trainable from answers.^4,5,18 Semantic parsers have even been extended beyond fixed knowledgebases to semi-structured tables.²⁶ With the ability to learn semantic parsers from question-answer pairs, it is easy to collect datasets via crowdsourcing. As a result, semantic parsing datasets have grown by an order of magnitude.

In addition, semantic parsers have been applied to a number of applications outside question answering: robot navigation,^2,29 identifying objects in a scene,^15,23 converting natural language to regular expressions,¹⁷ and many others.

Outlook. Today, the semantic parsing community is a vibrant field, but it is still young and grappling with the complexities of the natural language understanding problem. Semantic parsing poses three types of challenges:

• Linguistic: How should we represent the semantics of natural language and construct it compositionally from the natural language?
• Statistical: How can we learn semantic parsers from weak supervision and generalize well to new examples?
• Computational: How do we efficiently search over the combinatorially large space of possible logical forms?

In the remainder of this article, we present a general framework for semantic parsing, introducing the key components. The framework is pleasantly modular, with different choices of the components corresponding to existing semantic parsers in the literature.

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Framework

Natural language understanding problem. In this article, we focus on the following natural language understanding problem: Given an utterance x in a context c, output the desired action y. Figure 1 shows the setup for a question answering application, in which case x is a question, c is a knowledgebase, and y is the answer. In a robotics application, x is a command, c represents the robot's environment, and y is the desired sequence of actions to be carried by the robot.²⁹ To build such a system, assume that we are given a set of n examples . We would like to use these examples to train a model that can generalize to new unseen utterances and contexts.

Semantic parsing components. This article focuses on a statistical semantic parsing approach to the above-mentioned problem, where the key is to posit an intermediate logical form z that connects x and y. Specifically, z captures the semantics of the utterance x, and it also executes to the action y (in the context of c). In our running example, z would be max(primes ∩ (−∞, 10)). Our semantic parsing framework consists of the following five components (see Figure 2):

1. Executor: computes the denotation (action) y = ⟦z⟧_c given a logical form z and context c. This defines the semantic representation (logical forms along with their denotations).
2. Grammar: a set of rules G that produces D(x, c), a set of candidate derivations of logical forms.
3. Model: specifies a distribution p_θ(d|x, c) over derivations d parameterized by θ.
4. Parser: searches for high probability derivations d under the model p_θ.
5. Learner: estimates the parameters θ (and possibly rules in G) given training examples .

We now instantiate each of these components for our running example: "What is the largest prime less than 10?"

Executor. Let the semantic representation be the language of mathematics, and the executor is the standard interpretation, where the interpretations of predicates (for example, primes) are given by c. With c(primes) = {2, 3, 5, 7, 11, ...,}, the denotation is ⟦primes ∩ (-∞, 10) ⟧_c = {2, 3, 5, 7}.

Grammar. The grammar G connects utterances to possible derivations of logical forms. Formally, the grammar is a set of rules of the form a ⇒ β.^a Here is a simple grammar for our running example:

We start with the input utterance and repeatedly apply rules in G. A rule α ⇒ β can be applied if some span of the utterance matches α, in which case a derivation over the same span with a new syntactic category and logical form according to β is produced. Equation (1) illustrates one possible derivation (call it d₁) for our running example.

For example, applying (R3) produces category QP and logical form (−∞, 10) over span [5:7] corresponding to "less than 10." We stop when we produce the designated ROOT category over the entire utterance. Note that we could have also applied (R6) instead of (R5) to generate the (incorrect) logical form min(primes ∩ (−∞, 10)); let this derivation be d₂. We have D(x, c) = {d₁, d₂} here, but in general, there could be exponentially many derivations, and multiple derivations can generate the same logical form. In general, the grammar might contain nonsense rules (R6) that do not reflect ambiguity in language but are rather due to model uncertainty prior to learning.

Model. The model scores the set of candidate derivations generated by the grammar. A common choice used by virtually all existing semantic parsers are log-linear models (generalizations of logistic regressions). In a log-linear model, define a feature vector ϕ(x, c, d) ∈ ℝ^F for each possible derivation d. We can think of each feature as casting a vote for various derivations d based on some coarse property of the derivation. For example, define F = 7 features, each counting the number of times a given grammar rule is invoked in d, so that ϕ(x, c, d₁) = [1, 1, 1, 1, 1, 0, 1] and ϕ(x, c, d₂) = [1, 1, 1, 1, 0, 1, 1].

Next, let θ ∈ ℝ^F denote the parameter vector, which defines a weight for each feature representing how reliable that feature is. Their weighted combination score(x, c, d) = ϕ(x, c, d) and θ represents how good the derivation is. We can exponentiate and normalize these scores to obtain a distribution over derivations as shown in Equation (2).

If θ = [0, 0, 0, 0, +1, −1, 0], then p_θ would assign probability to d₁ and ≈ 0.12 to d₂.

Parser. Given a trained model p_θ, the parser (approximately) computes the highest probability derivation(s) for an utterance x under p_θ. Assume the utterance x is represented as a sequence of tokens (words). A standard approach is to use a chart parser, which recursively builds derivations for each span of the utterance. Specifically, for each category A and span [i:j] (where 0 ≤ i < j ≤ length(x)), we loop over the applicable rules in the grammar G and apply each one to build new derivations of category A over [i:j]. For binary rules—those of the form B C ⇒ A such as (R4), we loop over split points k (where i < k ≤ j), recursively compute derivations B[z₁] over [i:k] and C[z₂] over [k:j], and combine them into a new derivation A[z] over [i:j], where z is determined by the rule; for example, z = z₁ ∩ z₂ for (R4). The final derivations for the utterance are collected in the ROOT category over span [0:length(x)].

This procedure would generate all derivations, which could be exponentially large. Generally, we only wish to compute the derivations with high probability under our model p_θ. If the features of p_θ were to decompose as a sum over the rule applications in d—that is, ϕ (x, c, d) = Σ_{(r, i, j)∈d} ϕ_rule (x, c, r, i, j), then we could use dynamic programming: For each category A over [i:j], compute the highest probability derivation. However, in executable semantic parsing, feature decomposition isn't sufficient, since during learning, we also need to incorporate the constraint that the logical form executes to the true denotation ([⟦d.z⟧_c = y]); see Equation (6). To maintain exact computation in this setting, the dynamic programming state would need to include the entire logical form d.z, which is infeasible, since there are exponentially many logical forms. Therefore, beam search is generally employed, where we keep only the K sub-derivations with the highest model score based on only features of the sub-derivations. Beam search is not guaranteed to return the K highest scoring derivations, but it is often an effective heuristic.

Learner. While the parser turns parameters into derivations, the learner solves the inverse problem. The dominant paradigm in machine learning is to set up an objective function and optimize it. A standard principle is to maximize the likelihood of the training data . An important point is that we do not observe the correct derivation for each example, but only the action y_i, so we must consider all derivations d whose logical form d.z satisfy ⟦d.z⟧_ci = y_i. This results in the log-likelihood of the observed action y_i:

The final objective is then simply the sum across all n training examples:

The simplest approach to maximize O(θ) is to use stochastic gradient descent (SGD), an iterative algorithm that takes multiple passes (for example, say 5) over the training data and makes the following update on example i:

where η is a step size that governs how aggressively we want to update parameters (for example, η = 0.1). In the case of log-linear models, the gradient has a nice interpretable form:

where is the model distribution p_θ over derivations d, but restricted to ones consistent with y_i. The gradient pushes θ to put more probability mass on q and less on p_θ. For example, if p_θ assigns probabilities [0.2, 0.4, 0.1, 0.3] to four derivations and the middle two derivations are consistent, then q assigns probabilities [0, 0.8, 0.2, 0].

The objective function O(θ) is not concave, so SGD is at best guaranteed to converge to a local optimum, not a global one. Another problem is we cannot enumerate all derivations D(x_i, c_i) generated by the grammar, so we approximate this set with the result of beam search, which yields K candidates (typically K = 200); p_θ is normalized over this set. Note this candidate set depends on the current parameters θ, resulting a heuristic approximation of the gradient ∇ O_i.

Summary. We have covered the components of a semantic parsing system. Observe that the components are relatively loosely coupled: The executor is concerned purely with what we want to express independent of how it would be expressed in natural language. The grammar describes how candidate logical forms are constructed from the utterance but does not provide algorithmic guidance nor specify a way to score the candidates. The model focuses on a particular derivation and defines features that could be helpful for predicting accurately. The parser and the learner provide algorithms largely independent of semantic representations. This modularity allows us to improve each component in isolation.

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Refining the Components

Having toured the components of a semantic parsing system, we now return to each component and discuss the key design decisions and possibilities for improvement.

Executor. By describing an executor, we are really describing the language of the logical form. A basic textbook representation of language is first-order logic, which can be used to make quantified statements about relations between objects. For example, "Every prime greater than two is odd." would be expressed in first-order logic as ∀x.prime(x) ∧ more(x, 2) → odd(x). Here, the context c is a model (in the model theory sense), which maps predicates to sets of objects or object pairs. The execution of this logical form with respect to the standard mathematical context would be true. Blackburn and Bos⁷ gives a detailed account on how first-order logic is used for natural language semantics.

First-order logic is reasonably powerful, but it fails to capture some common phenomena in language. For example, "How many primes are less than 10?" requires constructing a set and manipulating it and thus goes beyond the power of first-order logic. We can instead augment first-order logic with constructs from lambda calculus. The logical form corresponding to the preceding question would be count (λx.prime(x) ∧ less(x, 10)), where the λ operator can be thought of as constructing a set of all x that satisfy the condition; in symbols, ⟦λx.f(x)⟧_c = {x : ⟦f(x)⟧_c = true}. Note that count is a higher-order function that takes a function as an argument.

Another logical language, which can be viewed as syntactic sugar for lambda calculus, is lambda dependency-based semantics (DCS).²⁰ In lambda DCS, the logical form would be count(prime (less. 10)), where the constant 10 represent λx.(x = 10), the intersection operator z₁ z₂ represents λx.z₁ (x) ∧ z₂(x), and the join operator r.z represents λx.∃y.r(x, y) ∧ z(y).

Lambda DCS is "lifted" in the sense that operations combine functions from objects to truth values (think sets) rather than truth values. As a result, lambda DCS logical forms partially eliminate the need for variables. Noun phrases in natural language (for example, "prime less than 10") also denote sets. Thus, lambda DCS arguably provides a transparent interface with natural language.

From a linguistic point of view, the logical language seeks primarily to model natural language phenomena. From an application point of view, the logical language dictates what actions we support. It is thus common to use application-specific logical forms, for example, regular expressions.¹⁷ Note that the other components of the framework are largely independent of the exact logical language used.

Grammar. Recall that the goal of the grammar in this article is just to define a set of candidate derivations for each utterance and context. Note that this is in contrast to a conventional notion of a grammar in linguistics, where the goal is to precisely characterize the set of valid sentences and interpretations. This divergence is due to two factors: First, we will learn a statistical model over the derivations generated by the grammar anyway, so the grammar can be simple and coarse. Second, we might be interested in application-specific logical forms. In a flight reservation domain, the logical form we wish to extract from "I'm in Boston and would like to go to Portland" is flight from.Boston to.Portland, which is certainly not the full linguistic meaning of the utterance, but suffices for the task at hand. Note the connection here between language and logic is less direct compared to "prime less than 10" ⇒ prime (less.10).

CCG. One common approach to the grammar in semantic parsing is combinatory categorial grammar (CCG),²⁸ which had been developed extensively in linguistics before it was first used for semantic parsing.³⁷ CCG is typically coupled with logical forms in lambda calculus. Equation (7) shows an example of a CCG derivation:

Syntactic categories in CCG include nouns (N) denoting sets and NP denoting objects. Composite categories ((NN)/NP) represent functions of multiple arguments, but where the directionality of the slashes indicate the location of the arguments. Most rules in CCG are lexical (for example, [prime ⇒ N[λx.prime(x)]), which mention particular words. The rest of the rules are glue; for example, we have a forward application (>) and backward application (<) rule:

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From a linguistic point of view, the logical language seeks primarily to model natural language phenomena. From an application point of view, the logical language dictates what actions we support.

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It is common to use other combinators that both handle more complex linguistic phenomena such as NP coordination "integers that divide 2 or 3" as well as ungrammatical language,³⁸ although these issues can also be handled by having a more expansive lexicon.¹⁹

Crude rules. In CCG, the lexical rules can become quite complicated. An alternative approach that appears mostly in work on lambda DCS is to adopt a much cruder grammar whose rules correspond directly to the lambda DCS constructions, join and intersect. This results in the derivation shown in Equation (8):

The lack of constraints permits the derivation of other logical forms such as 10 less.prime, but these incorrect logical forms can be ruled out statistically via the model. The principle is to keep the lexicon simple and lean more heavily on features (whose weights are learned from data) to drive the selection of logical forms.

Floating rules. While this new lexicon is simpler, the bulk of the complexity comes from having it in the first place. Where do the rules [prime ⇒ N|N:prime] and [less than ⇒ N|N:less] come from? We can reduce the lexicon even further by introducing floating rules [⇒N|N:prime] and [ ⇒N|N:less], which can be applied in the absence of any lexical trigger. This way, the utterance "primes smaller than 10" would also be able to generate prime less.10, where the predicates prime and less are not anchored to any specific words.

While this relaxation may seem linguistically blasphemous, recall the purpose of the grammar is to merely deliver a set of logical forms, so floating rules are quite sensible provided we can keep the set of logical forms under control. Of course, since the grammar is so flexible, even more nonsensical logical forms are generated, so we must lean heavily on the features to score the derivations properly. When the logical forms are simple and we have strong type constraints, this strategy can be quite effective.^5,26,30

The choice of grammar is arguably the most important component of a semantic parser, as it most directly governs the trade-off between expressivity and statistical/computational complexity. We have explored three grammars, from a tightly constrained CCG to a very laissez faire floating grammar. CCG is natural for capturing complex linguistic phenomena in well-structured sentences, whereas for applications where the utterances are noisy but the logical forms are simple, more flexible grammars are appropriate. In practice, one would probably want to mix and match depending on the domain.

Model. At the core of a statistical model is a function score(x,c,d) that judges how good a derivation d is with respect to the utterance x and context c. Previously, we described a simple log-linear model (2) in which score(x,c,d) = ϕ(x,c,d) · θ, and with simple rule features. There are two ways to improve the model: use more expressive features and use a nonlinear scoring function.

Most existing semantic parsers stick with a linear model and leverage more targeted features. One simple class of features {ϕ_a,b} is equal to the number of times word a appears in the utterance and predicate b occurs in the logical form; if the predicate b is generated by a floating rule, then a is allowed to range over the entire utterance; otherwise, it must appear in the span of b.

These features are quite numerous, which provides flexibility but require a lot of data. Note that logical predicates (for example, birthplace) and the natural language (for example, born in) often share a common vocabulary (English words). One can leverage this structure by defining a few features based on statistics from large corpora, external lexical resources, or simply string overlap. For example, ϕ_match might be the number of word-predicate pairs that match almost exactly. This way, the lexical mapping need not be learned from scratch using only the training data.^4,18

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Going forward, the two big questions are: How to represent the semantics of language and what supervision to use to learn the semantics from.

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A second class of useful features is based on the denotation ⟦d.z⟧_c of the predicted logical form. For example, in a robot control setting, a feature would encode whether the predicted actions are valid in the environment c (picking up a cup requires the cup to be present). These features make it amply clear that semantic parsing is not simply a natural language problem but one that involves jointly reasoning about the nonlinguistic world. In addition, we typically include type constraints (features with weight −∞) to prune out ill-formed logical forms such as birthplace4 that are licensed the grammar.

One could also adopt a nonlinear scoring function, which does not require any domain knowledge about semantic parsing. For example, one could use a simple one-layer neural network, which takes a weighted combination of m nonlinear basis functions: score . The parameters of the model we would learn are then the m top-level weights {α_i} and the mF bottom-level weights {w_ij}; recall that F is the number of features (ϕ(s,c,d) ∈ ℝ^F). With more parameters, the model becomes more powerful, but also requires more data to learn.

Parsing. Most semantic parsing algorithms are based on chart parsing, where we recursively generate a set of candidate derivations for each syntactic category (for example, NP) and span (for example, [3:5]). There are two disadvantages of chart parsing: First, it does not easily support incremental contextual interpretation: The features of a span [i:j] can only depend on the sub-derivations in that span, not on the derivations constructed before i. This makes anaphora (resolution of "it") difficult to model. A solution is to use shift-reduce parsing rather than chart parsing.⁴⁰ Here, one parses the utterance from left to right, and new sub-derivations can depend arbitrarily on the sub-derivations constructed thus far.

A second problem is the chart parsing generally builds derivations in some fixed order—for example, of increasing span size. This causes the parser to waste equal resources on non-promising spans which are unlikely to participate in the final derivation that is chosen. This motivates agenda-based parsing, in which derivations that are deemed more promising by the model are built first.⁶

Learner. In learning, we are given examples of utterance-context-response triples (x, c, y). There are two aspects of learning: inducing the grammar rules and estimating the model parameters. It is important to remember that practical semantic parsers do not do everything from scratch, and often the hard-coded grammar rules are as important as the training examples. First, some lexical rules that map named entities (for example, [paris ⇒ ParisFrance]), dates, and numbers are generally assumed to be given,³⁷ though we need not assume these rules are perfect.²¹ These rules are also often represented implicitly.^{4, 21}

How the rest of the grammar is handled varies across approaches. In CCG-style approach, inducing lexical rules is an important part of learning. In Zettlemoyer and Collins,³⁷ a procedure called GENLEX is used to generate candidate lexical rules from a utterancelogical form pair (x, z). A more generic induction algorithm based on higher-order unification does not require any initial grammar.¹⁹ Wong and Mooney³³ use machine translation ideas to induce a synchronous grammar (which can also be used to generate utterances from logical forms). However, all these lexicon induction methods require annotated logical forms z. In approaches that learn from denotations y,^4,21 an initial crude grammar is used to generate candidate logical forms, and rest of the work is done by the features.

As we discussed earlier, parameter estimation can be performed by SGD on the log-likelihood; similar objectives based on max-margin are also possible.²² It can be helpful to also add an L₁ regularization term λ ||θ||₁, which encourages feature weights to be zero, which produces a more compact model that generalizes better.⁵ In addition, one can use AdaGrad,¹⁴ which maintains a separate step size for each feature. This can improve stability and convergence.

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Datasets and Results

In a strong sense, datasets are the main driver of progress for statistical approaches. We will now survey some of the existing datasets, describe their properties, and discuss the state of the art.

The Geo880 dataset³⁶ drove nearly a decade of semantic parsing research. This dataset consists of 880 questions and database queries about U.S. geography (for example, "what is the highest point in the largest state?"). The utterances are compositional, but the language is clean and the domain is limited. On this dataset, learning from logical forms¹⁸ and answers²¹ both achieve around 90% accuracy.

The ATIS-3 dataset³⁸ consists of 5418 utterances paired with logical forms (for example, "show me information on american airlines from fort worth texas to philadelphia"). The utterances contain more disfluencies and flexible word order compared with Geo880, but they are logically simpler. As a result, slot filling methods have been a successful paradigm in the spoken language understanding community for this domain since the 1990s. The best reported result is based on semantic parsing and obtains 84.6% accuracy.³⁸

The Regexp824 dataset¹⁷ consists of 824 natural language and regular expression pairs (for example, "three letter word starting with 'X'"). The main challenge here is that there are many logically equivalent regular expressions, some aligning better to the natural language than others. Kushman and Barzilay¹⁷ uses semantic unification to test for logical form equivalence and obtains 65.6% accuracy.

The Free917 dataset¹¹ consists of 917 examples of question-logical form pairs that can be answered via Freebase, for example, "how many works did mozart dedicate to joseph haydn?" The questions are logically less complex than those in the semantic parsing datasets earlier, but introduces the new challenge of scaling up to many more predicates (but is in practice manageable by assuming perfect named entity resolution and leveraging the strong type constraints in Freebase). The state-of-the-art accuracy is 68% accuracy.¹⁸

WebQuestions⁴ is another dataset on Freebase consisting of 5810 question–answer pairs (no logical forms) such as "what do australians call their money?" Like Free917, the questions are not very compositional, but unlike Free917, they are real questions asked by people on the Web independent from Freebase, so they are more realistic and more varied. Because the answers are required to come from a single Freebase page, a noticeable fraction of the answers are imperfect. The current state of the art is 52.5%.³⁵

The goal of WikiTableQuestions²⁶ is to extend question answering beyond Freebase to HTML tables on Wikipedia, which are semi-structured. The dataset consists of 22033 question-table-answer triples (for example, "how many runners took 2 minutes at the most to run 1500 meters?"), where each question can be answered by aggregating information across the table. At test time, we are given new tables, so methods must learn how to generalize to new predicates. The result on this new dataset is 37.1%.

Wang et al.³⁰ proposed a new recipe for quickly using crowdsourcing to generate new compositional semantic parsing datasets consisting of question–logical form pairs. Using this recipe, they created eight new datasets in small domains consisting of 12,602 total question-answer pairs, and achieved an average accuracy across datasets of 58.8%.

Chen and Mooney¹² introduced a dataset of 706 navigation instructions (for example, "facing the lamp go until you reach a chair") in a simple grid world. Each instruction sequence contains multiple sentences with various imperative and context-dependent constructions not found in previous datasets. Artzi and Zettlemoyer² obtained 65.3% sentence-level accuracy.

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Discussion

We have presented a semantic parsing framework for the problem of natural language understanding. Going forward, the two big questions are: How to represent the semantics of language and what supervision to use to learn the semantics from.

Alternative semantic representations. One of the main difficulties with semantic parsing is the divergence between the structure of the natural language and the logical forms—purely compositional semantics will not work. This has led to some efforts to introduce an intermediate layer between utterances and logical forms. One idea is to use general paraphrasing models to map input utterances to the "canonical utterances" of logical forms.^5,30 This reduces semantic parsing to a text-only problem for which there is much more data and resources.

One could also use domain-general logical forms that capture the basic predicate-argument structures of sentences.¹⁸ Abstract meaning representation (AMR)³ is one popular representation backed by an extension linguistic annotation effort. Multiple AMR parsers have been developed, including one based on CCG.¹ While these representations offer broad coverage, solving downstream understanding tasks still requires additional work (for example, inferring that "population of X" is synonymous with "number of people living in X"). In contrast, executable semantic parsing operates on the full pipeline from utterance to task output, but compromises on domain-generality. This is partially inevitable, as any understanding task requires some domain-specific knowledge or grounding. Designing the best general representation that supports many downstream tasks remains an open challenge.

Alternative supervision. Much of the progress in semantic parsing has been due to being able to learn from weaker supervision. In the framework we presented, this supervision is the desired actions y (for example, answers to questions). One can use a large corpus of text to exploit even weaker supervision.^16,27 More generally, one can think about language interpretation in a reinforcement learning setting,⁹ where an agent presented with an utterance in some context performs some action, and receives a corresponding reward signal. This framework highlights the importance of context-dependence in language interpretation.^2,39

Due to their empirical success, there has been a recent surge of interest in using recurrent neural networks and their extensions for solving NLP tasks such as machine translation and question answering.^31,35 These methods share the same spirit of end-to-end utterance-to-behavior learning as executable semantic parsing, but they do not explicitly separate parsing from execution. This makes them architecturally simpler than semantic parsers, but they are more data hungry and it is unclear whether they can learn to perform complex logical reasoning in a generalizable way. Nonetheless, it is quite likely that these methods will play an important role in the story of language understanding.

Outlook. This is an exciting time for semantic parsing and natural language understanding. As natural language interfaces (for example, Siri) become more ubiquitous, the demand for deep understanding will continue to grow. At the same time, there is a fresh wave of ambition pushing the limits of what can be machine learnable. The confluence of these two factors will likely generate both end-user impact as well as new insights into the nature of language and learning.

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