Inductive Programming: A New Approach16 May 2018
In the context of a job interview, I was asked to write a small paper outlining a research direction to tackle a problem in the field of Inductive Programming — essentially how to generate programs from examples. Here it is reproduced for your reading pleasure.
Devise an approach to automatically generate programs which solve programming challenges, the like of which are used in hiring interviews, or listed on programming challenge websites (e.g. Project Euler).
In particular, to generate a program we can use (1) a natural-language textual description of the problem to solve (including descriptions of the input and output), (2) a set of input/output (I/O) examples and (3) a piece of skeleton code for the solution.
State of the Art
The area of program generation from examples is known as Inductive Programming or Inductive Program Synthesis. We also consider other forms of Program Synthesis, where a higher-level (formal) specification of the problem may be given.
The introductory paper by Gulwani et al.  is rather useful to get a sense of what the field is about. It further points to three surveys of the field [2, 3, 4]. The survey by Kitzelmann  is particularly useful, as it divides the field into three tried-and-true approaches: analytical-functional, inductive logic programming and generate-and-test. Another survey by Gulwani et al. , has its own classification, which largely overlaps with that of Kitzelmann. In particular, it introduces the constraint-solving approach, which includes inductive logic programming.
Let's note that classification is sometimes tricky, as many approaches accross categories use elements of search, constraint-solving and statistics.
Below, I offer a brief summary of the advantages, drawbacks and results of different approaches as I understood them. Beware that this is written after a cursory literature review, by someone who is unexperienced in the field. It may be wrong in some signifiant regard.
The analytical-functional approach works well with "symbolic" problems where the structure of the input and output is significant: e.g. simple list processing problem such as the reverse function. The state of the art in this approach is Igor2 , a system that mixes the analytical-functional approach with search principles (cf. generate-and-test). Igor2 is able to induce the insertion sort and quicksort algorithms, albeit slowly for the later (~63s on the test setup versus ~0.2s for insertion sort).
The constraint-solving approach usually (but not always) makes use of a SAT or SMT solver. A representative example is Rosette . It is able to generate relatively specialised programs, such as specifications for an HTML scraper. This approach does however struggle with complex loops and recursion.
Inductive Logic Programming (ILP) is a form of constraint-solving. ILP techniques are based on first-order predicate logic and typically generate Prolog programs. They use some form of solving in order to produce Prolog rules that enables the derivation of a set of facts (~ the output). From what I could gather, they're generally not more powerful than Igor2.
The generate-and-test approach is based on the idea of generating many candidate programs and testing them against a specification or a set of I/O pairs. Typically, a fitness function is used to guide the search, and the algorithms are genetic in nature: promising candidate solutions are mutated and crossed to generated new candidates. The approach is in principle very general, but slow in practice.
Gulwani et al.  call this the stochastic search approach and offer the following definition:
The stochastic synthesis approaches learn a distribution over the space of programs in the hypothesis space that is conditioned on the specification, and then sample programs from the distribution to learn a consistent program. The goal of these techniques is to use the learnt distribution to guide the search of programs that are more likely to lead to programs that conform with the specification.
A major representative of this approach is ADATE , which is able to induce programs that sort lists, insert nodes in a binary search tree, or generate all permutations of a list. However, the generation process takes at least hours and up to 9 days.
Another idea is to use a set of higher-order functions (map, fold, etc...) as primitives. The assumption is that most programs can be more tersely expressed using these, leading to a reduction in search space that makes it practical to enumerate all possible programs. This approach is notably implemented in MagicHaskeller . In practice, the results are very limited (the examples given in the paper are nth, map and length).
A third notable example is SyGuS (for Syntax-Guided Synthesis) . SyGuS uses SMT solving to check the conformance of a candidate solution to a specification. In case of failure, it outputs a counter-example. Correctness on previously encountered counter-examples is then used to derive the fitness function that guides the search. Another key idea is the ability to supply a syntactic restriction on the program search space. For instance, if we were searching for a matrix-multplication routine for 2x2 matrices, we could submit the syntactic restriction that candidate solutions must use only 7 multiplication operations.
SyGuS is evaluated using bit-twiddling problems (transformations of bit vectors). This is a quite limited set of problems, that has the advantage to fit the nature of SMT solving (which is a generalization of SAT solving — based on the boolean satisfiability problem). The generated programs are relatively simple, and the results are rather inconclusive beyond this class of problems.
Finally, the generate-and-test approach also includes techniques based on neural networks. Gulwani et al. :
The proposed approaches can be divided into two categories: i) program induction, and ii) program synthesis. The neural architectures that perform program induction learn a network that is capable of replicating the behavior of the desired program, whereas the neural systems that perform program synthesis return an interpretable program that matches the desired specification behavior.
As an example of program synthesis, DeepCoder  uses a deep learning algorithm to learn features from I/O pairs. The result is a way to map any set of I/O pairs to a probability distribution. The distribution indicates how likely it is that a programming language component will be present in a program that turns input into output.
The results are quite limited in applicability. In particular, DeepCoder works with an ad hoc programming language that is very high-level and very limited. The language processes vectors and only has a very small number of built-in functions. The approach is also very limited in the program size: The evaluation is done on programming problems whose solution is a 5-statements program.
However, the technique can still be useful in practical cases, with some relaxations. Microsoft Excel uses FlashFill , a technique that is roughly similar to that of DeepCoder, but relies on a manually crafted set of heuristics (clues) in order to derive the feature set. Flashfill is used to infer Excel formulas matching the user intent. Because these formulas do not feature loops or recursion, and because they draw on a set of primitive functions, the expressivity is roughly on par with constraint-solving approaches like Rosette , although the success rate is much better, thanks to clues. FlashFill does not feature neural networks, but a version that does replace manual clues with neural networks has been devised . It is able to pass 38% of the original FlashFill benchmarks. One major limitation regards the size of the generated programs, currently limited to 13 (presumably, 13 functions).
My first thoughts upon hearing the problem was that it was impossible. That's too strong a claim, as there are no obvious theoretical impossibilities. Since then, I've somewhat revised my opinion. I still think the problem is incredibly ambitious and hard, but there might be ways to do much better than the state of the art, if one stumbles upon a breakthrough technique.
As seen in the small literature review above, there is quite a bit of work on program synthesis, and the results fall quite short of the mark as I understand it — which is to tackle even relatively challenging programming tasks.
If we consider a more modest objective, then arguably current systems might already be good enough. Igor2  being able to infer a quicksort in 60 seconds is impressive. The question is then: What goal do we want to reach? What is the benchmark for success?
It is doubtful that incremental improvements over existing techniques can lead very far. Popular techniques have been combined to some success (again, Igor2 is a case in point). This means that if we want to solve the problem, we need to try something new. But that is, of course, a much bigger research risk.
Divide And Conquer
Clearly, the area of inductive programming is active and already well-researched. I don't think I can come up with a worthy research direction without spending a few weeks in the literature and talking with people well acquainted with the topic.
But what about dividing the problem into parts and trying to solve each part individually? That might be easier. Let's cheat!
My suggestion is that, instead of working from a natural language description, we work from a formal, unambiguous, specification. We would still use examples as well. My idea for this part of the solution looks roughly like a compiler, which is maybe not a surprise given my background in language engineering.
Then, as a second step, we could try to derive a formal specification from the natural-language specification. It is probably impossible to do this very well — given the ambiguity of natural language — but maybe we can still extract a partial specification, or extract many candidate specifications, over which we can conduct a search.
A few words on one of the big problems facing us: ambiguity. It occurs at multiple levels in the problem.
First, inductive programming based on examples is inherently ambiguous, given that there are infinitely many semantically distinct programs that can match the I/O pairs.
In fact, a simple program mapping example inputs to outputs is a solution. Of course, that's typical overfitting. We'd like a solution that, for instance, can generate a program matching all I/O pairs when only given any set containing half of the pairs as input. Some similar notion is probably defined in the literature, but I am not familiar with it.
Still, this leads to an interesting question: What are the properties of the program that we want to generate, given that there are many that fit the bill? Probably we want something approaching the simplest solution. And probably, we can approximate this by the program size.
Second, natural language is naturally ambiguous. From what I know (and looked up) on Natural Language Processing (NLP) , it's already not trivial to acquire an unambiguous parse tree from an English text. Moreover, within that parse tree, references (e.g., pronouns) are quintessentially ambiguous. Humans usually use the available context to disambiguate, although even that is not always enough. Should we even assume that such a parse tree is available, we are faced with the daunting task of assigning meaning to the words that make up the tree. This probably requires some kind of semantic network that relates the words to one another and to programming language concepts. Creating such a network by hand is possible but unpleasant and very time-consuming. Generating it from existing text is probably as difficult as the problem we're trying to solve in the first place.
So a precise interpretation of the natural-language description is off. We could, instead, try to mine it for hints about the specification. It's not difficult to imagine we could employ machine learning techniques to extract some meaning from the description. For instance, a correlation between the presence of the word "sorted" and the presence of a sort component in the solution.
This is similar to what DeepCoder  does, but with the I/O examples. It uses deep learning to learn to correlate some features in the examples with programming language constructs. This correlation (which takes the form of a probability distribution for various programming language constructs) is then used to guide a local search on the space of programs.
And indeed, we could also add the natural-language description as a second source of heuristics to guide the local search. My feeling is that this is not anywhere near enough to make significant progress on the problem.
There is another approach, which is to translate between the natural and the formal language, possibly reusing existing machine learning techniques*. This is what I suggested as a second step in the previous section. The result is still ambiguous, but much more readily exploitable assuming we manage to make good on the first problem — generating code based on the I/O examples and a formal specification.
There is a vast literature on the topic, including many papers related to Google Translate  — arguably the state of the art solution in machine translation.
From Specification to Program
My proposed first step in the research would be to devise a way to turn a formal specification into a program.
It's actually very easy to come up with a terrible solution to this problem: generate a program that enumerates all possible outputs, then filter using the specification. This solution is incredibly inefficient, but maybe it can serves as the basis for something better.
First off, we can probably use part of the specification to constrain the generated outputs. The outputs must probably conform to a precise shape, which may also be related to the input (e.g. one line of output per line of input).
Some parts of the specification might be naturally ammenable to be translated
into a constructive algorithm — as opposed to simply filtering out bad
solutions. For instance, the assertion that
y == f(x) where
y is a part of
the output and
x a part of the input could be translated immediately as
f(x), assuming it is the only thing that constrains
I also expect that, starting from our initial program, we can backward-propagated some of the constraints to the enumeration logic itself. Some of the constraints could be translated into assertions used to prune the search tree, or invariants to be maintained. Static analysis and optimization techniques from compiler engineering should be relevant here.
The big question is how far we can push these efforts. Can we generate enough stub code and optimize away enough of the search that we end up with an program that does little enough guessing?
If we can't, all is not lost. We can still use this as a correct-by-construction candidate solution, and use it as a starting point for a local search in the space of programs. Within that search, we could exploit knowledge about the specification to make smart guesses as to what to change. We can use our I/O pairs to validate that the mutated program is still considered correct, as well as fresh inputs, whose corresponding output must satisfy the specification. The goal of the search is to derive a program that is simpler and more efficient.
As a side note, it is not possible to check candidate programs against the specification directly, as per Rice's Theorem: "all non-trivial semantic properties of programs are undecidable". It would probably be worthwhile to check a computability theory handbook to see if there are no other impossibility results that could orient our efforts.
For the search in program-space, we can use the whole bag of tricks from the generate-and-test approach, notably fitness functions and genetic algorithms. But other, more specific methods could also be tried: using higher-order functions as building blocks, syntactic restrictions, ...
Other classical AI techniques can be handy. I'm thinking notably of simulated annealing, where we first increase entropy by allowing a certain amount of noise. In our case, we allow the initially-correct program to get worse, because we want to move away from that solution towards a shorter/more efficient/cleaner one; and then we progressively cool things down, trying to zero in on new correct solution.
Defining The Specification Language
A key concern in the proposition made above is the shape of the specification language. According to me, two concerns are in tension.
First, the language should as broad and expressive as possible. The goal is to make it easy to write the specifications — otherwise, they won't get written at all. This is also beneficial because it makes extracting formal specifications from a natural-language document easier, as we may have constructs that better reflect the natural-language explanation.
Second, the language should be a good basis for the kind of code generation and constraint propagation proposed in the previous section.
There is some inherent contradiction between these two goals: if the specification language is bigger, it will neccessarily includes multiple many ways to specify the same constraint, some of which will be less amenable to code generation or constraint propagation. It would be interesting to study this in order to make an informed decision on what to include in the specification language.
An interesting source of inspiration is the Z Notation , which mixes set theory, lambda calculus and first-order predicate logic.
Fuzzy Specifications & Natural Language
The second part of my solution calls for a way to acquire a formal specification from the natural-language specification and/or the examples.
From examples, a partial specification can be obtained by looking for relationships between input and output that are consistent accross all I/O pairs. If the example set is too small, overfitting could be a danger.
It's not quite clear to me how to mine the relationships from I/O pairs. An idea is to train a neural network as an oracle that indicates whether a new I/O pair is valid, that is to say, consistent with the training set of legitimate I/O pairs, a proxy for satisfying the specification. We could then train a second neural network to derive formal relationships based on the oracle itself. Such a two-part architecture is featured in Neural FlashFill . The idea is very out there, however.
The other (potentially complementary) option is to use the natural-language specification to guess at the formal specification. As discussed before, "traditional" (i.e., non-statistical) NLP is probably too arduous. However, given a big enough corpus of specifications available both in natural-language and formal versions, we can statistically learn a way to translate between both. This would work similarly to translation between two natural-languages (e.g., what Google Translate does). It might be possible to retool these efforts towards our goal. A potential problem here is to find or produce this "big enough" corpus of natural and formal specifications.
In any case, we must be ready to deal with mistakes in the generated specification, and our proposition so far assumed that the specification was correct. Fortunately, there is something very easy and effective we can do: simply run the generated specification against our I/O examples, and see if the specification is satisfied. If not, it cannot possibly be correct, and the invalid parts can be discarded.
Furthermore, we can generate many candidate solutions — the translation process being inherently statistical — and perform a search on those.
Even incorrect specifications have value, as we can perform a local search that mutates the specifications to try to find adjacent correct specifications. Here too, fitness functions and genetic algorithms could be helpful.
Given that (1) current inductive programming techniques results are underwhelming with respect to our goal and (2) non-statistical NLP techniques can hardly help us, I propose a two-pronged approach to tackle our challenge.
First, devise a way to generate programs based on a correct formal specification of the problem. For this, I suggest using techniques from statical analysis and compiler optimizations. We would tackle the generation process both forward, by constructively generating code from amenable specification fragments; and backwards, by propagation constraint into our code — initially a simple enumeration routine.
Second, manage to extract a formal specification from the natural-language specificaton of the problem and from the initial set of I/O examples. For this, I propose to exploit statistical NLP techniques to "translate" between English and our formal specification language.
The whole proposal is of course a rather wild shot in the dark. But it is different enough from previously-proposed approaches to have a chance — with some luck — to yield some low-hanging fruits, or maybe even — with luck and diligence — to go further than existing approaches.
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https://research.google.com/pubs/MachineTranslation.html Consulted on the 20th April 2018.
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