Mechanized Metatheory Revisited: An Extended Abstract (Invited Paper)

LIPICS - Leibniz International Proceedings in Informatics, Oct 2018

Proof assistants and the programming languages that implement them need to deal with a range of linguistic expressions that involve bindings. Since most mature proof assistants do not have built-in methods to treat this aspect of syntax, many of them have been extended with various packages and libraries that allow them to encode bindings using, for example, de Bruijn numerals and nominal logic features. I put forward the argument that bindings are such an intimate aspect of the structure of expressions that they should be accounted for directly in the underlying programming language support for proof assistants and not added later using packages and libraries. One possible approach to designing programming languages and proof assistants that directly supports such an approach to bindings in syntax is presented. The roots of such an approach can be found in the mobility of binders between term-level bindings, formula-level bindings (quantifiers), and proof-level bindings (eigenvariables). In particular, by combining Church's approach to terms and formulas (found in his Simple Theory of Types) and Gentzen's approach to sequent calculus proofs, we can learn how bindings can declaratively interact with the full range of logical connectives and quantifiers. I will also illustrate how that framework provides an intimate and semantically clean treatment of computation and reasoning with syntax containing bindings. Some implemented systems, which support this intimate and built-in treatment of bindings, will be briefly described.

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Mechanized Metatheory Revisited: An Extended Abstract (Invited Paper)

T Y P E S Mechanized Metatheory Revisited: An Extended Abstract Dale Miller Inria ?cole Polytechnique Palaiseau France https://orcid.org/ Category Invited Paper Proof assistants and the programming languages that implement them need to deal with a range of linguistic expressions that involve bindings. Since most mature proof assistants do not have built-in methods to treat this aspect of syntax, many of them have been extended with various packages and libraries that allow them to encode bindings using, for example, de Bruijn numerals and nominal logic features. I put forward the argument that bindings are such an intimate aspect of the structure of expressions that they should be accounted for directly in the underlying programming language support for proof assistants and not added later using packages and libraries. One possible approach to designing programming languages and proof assistants that directly supports such an approach to bindings in syntax is presented. The roots of such an approach can be found in the mobility of binders between term-level bindings, formula-level bindings (quantifiers), and proof-level bindings (eigenvariables). In particular, by combining Church's approach to terms and formulas (found in his Simple Theory of Types) and Gentzen's approach to sequent calculus proofs, we can learn how bindings can declaratively interact with the full range of logical connectives and quantifiers. I will also illustrate how that framework provides an intimate and semantically clean treatment of computation and reasoning with syntax containing bindings. Some implemented systems, which support this intimate and built-in treatment of bindings, will be briefly described. 2012 ACM Subject Classification Theory of computation ? Proof theory, Theory of computation ? Automated reasoning, Theory of computation ? Higher order logic Related Version A full version of this paper, titled ?Mechanized metatheory revisited?, is available at http://www.lix.polytechnique.fr/Labo/Dale.Miller/papers/pubs.html. Funding This work was funded by the ERC Advanced Grant ProofCert. Foreword This extended abstract is a non-technical look at the mechanization of formalized metatheory. While this paper may be provocative at times, I mainly intend to shine light on a slice of literature that is developing a coherent and maturing approach to mechanizing metatheory. and phrases mechanized metatheory; mobility of binders; lambda-tree syntax; higherorder abstract syntax - looked at existing practices and systems and urged the developers of proof assistants to make improvements to existing systems. Our conclusion from these experiments is that the relevant technology has developed almost to the point where it can be widely used by language researchers. We seek to push it over the threshold, making the use of proof tools common practice in programming language research ? mechanized metatheory for the masses. [5] In fact, a number of research teams have used proof assistants to formally prove significant properties of programming language related systems. Such properties include type preservation, determinancy of evaluation, and the correctness of an OS microkernel and of various compilers: see, for example, [41, 42, 44, 59]. As noted in [5], the poor support for binders in syntax was one problem that held back proof assistants from achieving even more widespread use by programming language researchers and practitioners. In recent years, a number of enhancements to programming languages and to proof assistants have been developed for treating bindings. These go by names such as locally nameless [12, 76], nominal reasoning [3, 14, 69, 83], and parametric higher-order abstract syntax [15]. Some of these approaches involve extending underlying programming language implementations while the others do not extend the proof assistant or programming language but provide various packages, libraries, and/or abstract datatypes that attempt to orchestrate various issues surrounding the syntax of bindings. In the end, nothing canonical seems to have arisen: see [4, 68] for detailed comparisons. 2 An analogy: concurrency theory While extending mature proof assistants (such as Coq, HOL, and Isabelle) with facilities to handle bindings is clearly possible, it seems desirable to consider directly the computational principles surrounding the treatment of binding in syntax independent of a given programming language. Developments in programming design has, of course, run into similar situations where there was a choice to be made between accounting for features by extending existing programming languages or by the development of new programming languages. Consider, for example, the following analogous (but more momentous) situation. Historically speaking, the first high-level, mature, and expressive programming languages to be developed were based on sequential computation. When those languages were forced to deal with concurrency, parallelism, and distributed computing, they were augmented with, say, thread packages and remote procedure calls. Earlier pioneers of computer programming languages and systems ? e.g., Dijkstra, Hoare, Milner ? saw concurrency and communications not as incremental improvements to existing imperative languages but as a new paradigm deserving a separate study. The concurrency paradigm required a fresh and direct examination and in this respect, we have seen a great number of concurrency frameworks appear: e.g., Petri nets, CSP, CCS, IO-automata, and the ?-calculus. Given the theoretical results and understanding that have flowed from work on these and related calculi, it has been possible to find ways for conventional programming languages to make accommodations within the concurrency and distributed computing settings. Such understanding and accommodations were not likely to flow from clever packages added to programming languages: new programming principles from the theory of concurrency and distributed computing were needed. Before directly addressing some of the computational principles behind bindings in syntax, it seems prudent to critically examine the conventional design of a wide range of proof assistants. (The following section updates a similar argument found in [52].) Dropping mathematics as an intermediate Almost all ambitious theorem provers in use today follow the following two step approach to reasoning about computation. Step 1: Implement mathematics. This step is achieved by picking a general, well understood formal system. Common choices are first-order logic, set theory, higher-order logic [16, 36], or some foundation for constructive mathematics, such as Martin-L?f type theory [18, 19, 45]. Step 2: Reduce reasoning about computation to mathematics. Computation is generally encoded via some model theoretic semantics (such as denotational semantics) or as an inductive definition over an operational semantics. A key methodological element of this proposal is that we shall drop mathematics as an intermediate and attempt to find more direct and intimate connections between computation, reasoning, and logic. The main problem with having mathematics in the middle seems to be that many aspects of computation are rather ?intensional? but a mathematical treatment requires an extensional encoding. The notion of algorithm is an example of this kind of distinction: there are many algorithms that can compute the same function (say, the function that sorts lists). In a purely extensional treatment, it is functions that are represented directly and algorithm descriptions that are secondary. If an intensional default can be managed instead, then function values are secondary (usually captured via the specification of evaluators or interpreters). For a more explicit example, consider whether or not the formula ?wi. ?x.x 6= ?x.w is a theorem. In a setting where ?-abstractions denote functions (the usual extensional treatment), we have not provided enough information to answer this question: in particular, this formula is true if and only if the domain type i is not a singleton. If, however, we are in a setting where ?-abstractions denote syntactic expressions, then it is sensible for this formula to be provable since no (capture avoiding) substitution of an expression of type i for the w in ?x.w can yield ?x.x. For a more significant example, consider the problem of formalizing the metatheory of bisimulation-up-to [56, 72] for the ?-calculus [57]. Such a metatheory can be used to allow people working in concurrent systems to write hopefully small certificates (actual bisimulations-up-to) in order to guarantee that bisimulation holds (usually witnessed directly by only infinite sets of pairs of processes). In order to employ the Coq theorem prover, for example, to attack such metatheory, Coq would probably need to be extended with packages in two directions. First, a package that provides flexible methods for doing coinduction following, say, the Knaster-Tarski fixed point theorems, would be necessary. Indeed, such a package has been implemented and used to prove various metatheorems surrounding bisimulation-up-to (including the subtle metatheory surrounding weak bisimulation) [11, 70, 71]. Second, a package for the treatment of bindings and names that are used to describe the operational semantics of the ?-calculus would need to be added. Such packages exist (for example, see [6]) and, when combined with treatments of coinduction, may allow one to make progress on the metatheory of the ?-calculus. Recently, the Hybrid systems [27] has shown a different way to incorporate both induction, coinduction, and binding into a Coq (and Isabelle) implementation. Such an approach could be seen as one way to implement this metatheory task on top of an established formalization of mathematics. There is another approach that seeks to return to the most basic elements of logic by reconsidering the notion of terms (allowing them to have binders as primitive features) and the notion of logical inference rules so that coinduction can be seen as, say, the de Morgan (and proof theoretic) dual to induction. In that approach, proof theory principles can be identified in that enriched logic with least and greatest fixed points [7, 47, 58] and with a treatment of bindings [81, 32]. Such a logic has been given a model-checking-style implementation [9] and is the basis of the Abella theorem prover [8, 31]. Using such implementations, the ?-calculus has been implemented, formalized, and analyzed in some detail [80, 79] including some of the metatheory of bisimulation-up-to for the ?-calculus [13]. I will now present some foundational principles in the treatment of bindings that are important to accommodate directly, even if we cannot immediately see how those principles might fit into existing mature programming languages and proof assistants. 4 How abstract is your syntax? Two of the earliest formal treatments of the syntax of logical expressions were given by G?del [35] and Church [16] and, in both of these cases, their formalization involved viewing formulas as strings of characters. Clearly, such a view of logical expressions contains too much information that is not semantically meaningful (e.g., white space, infix/prefix distinctions, parenthesis) and does not contain explicitly semantically relevant information (e.g., the function-argument relationship). For this reason, those working with syntactic expressions generally parse such expressions into parse trees: such trees discard much that is meaningless (e.g., the infix/prefix distinction) and records directly more meaningful information (e.g., the child relation denotes the function-argument relation). One form of ?concrete nonsense? generally remains in parse trees since they traditionally contain the names of bound variables. One way to get rid of bound variable names is to use de Bruijn?s nameless dummy technique [21] in which (non-binding) occurrences of variables are replaced by positive integers that count the number of bindings above the variable occurrence through which one must move in order to find the correct binding site for that variable. While such an encoding makes the check for ?-conversion easy, it can greatly complicate other operations that one might want to do on syntax, such as substitution, matching, and unification. While all such operations can be supported and implemented using the nameless dummy encoding [21, 43, 61], the complex operations on indexes that are needed to support those operations clearly suggests that they are best dealt within the implementation of a framework and not in the framework itself. The following four principles about the treatment of bindings in syntax will guide our further discussions. Principle 1: The names of bound variables should be treated in the same way we treat white space: they are artifacts of how we write expressions and they have no semantic content. Of course, the name of variables are important for parsing and printing expressions (just as is white space) but such names should not be part of the meaning of an expression. This first principle simply repeats what we stated earlier. The second principle is a bit more concrete. Principle 2: There is ?one binder to ring them all.?1 With this principle, we are adopting Church?s approach [16] to binding in logic, namely, that one has only ?-abstraction and all other bindings are encoded using that binder. For example, the universally quantified expression (?x. B x) is actually broken into the expression 1 A scrambling of J. R. R. Tolkien?s ?One Ring to rule them all, ... and in the darkness bind them.? (?(?x. B x)), where ? is treated as a constant of higher-type. Note that this latter expression is ?-equivalent to (? B) and universal instantiation of that quantified expression is simply the result of using ?-normalization on the expression (B t). In this way, many details about quantifiers can be reduced to details about ?-terms. Principle 3: There is no such thing as a free variable. This principle is taken from Alan Perlis?s epigram 47 [63]. By accepting this principle, we recognize that bindings are never dropped to reveal a free variable: instead, we will ask for bindings to move. This possibility suggests the main novelty in this list of principles. Principle 4: Bindings have mobility and the equality theory of expressions must support such mobility [51, 53]. Since the other principles are most likely familiar to the reader, I will now describe this last principle in more detail. 5 Mobility of bindings Since typing rules are a common operation in metatheory, I illustrate the notion of binding mobility in that setting. In order to specify untyped ?-terms (to which one might attribute a simple type via an inference), we introduce a (syntactic) type tm and two constants abs : (tm ? tm) ? tm and app : tm ? tm ? tm. Untyped ?-terms are encoded as terms of type tm using the translation define as dxe = x, d?x.te = (abs (?x.dte)), and d(t s)e = (app dte dse). The first clause here indicates that bound variables in untyped ?-terms are mapped to bound variables in the encoding. For example, the untyped ?-term ?w.ww is encoded as (abs ?w. app w w). This translation has the property that it maps bijectively ?-equivalence classes of untyped ?-terms to ???-equivalence classes of simply typed ?-terms of type tm. In order to satisfy Principle 3 above, we shall describe a Gentzen-style sequent as a triple ? : ? ` B where B is the succedent (a formula), ? is the antecedent (a multiset of formulas), and ? a signature, that is, a list of variables that are formally bound over the scope of the sequent. Thus all free variables in the formulas in ? ? {B} are bound by ?. Gentzen referred to the variables in ? as eigenvariables (although he did not consider them as binders over sequents). The following inference rule is a familiar rule. ? : ?, typeof x (int ? int) ` C ? : ?, ?? (typeof x (? ? ? )) ` C ?L This rule states (when reading it from conclusions to premise) that if the symbol x can be attributed the type ? ? ? for all instances of ? , then it can be assumed to have the type int ? int. Thus, bindings can be instantiated (the ?? is removed by instantiation). On the other hand, consider the following inferences. ?, x : ?, typeof dxe ? ` typeof dBe ? ? : ? ` ?x(typeof dxe ? ? typeof dBe ? 0) ?R ? : ? ` typeof d?x.Be (? ? ? 0) These inferences illustrate how bindings can, instead, move during the construction of a proof. In this case, the term-level binding for x in the lower sequent can be seen as moving to the formula-level binding for x in the middle sequent and then to the proof-level binding (as an eigenvariable) for x in the upper sequent. Thus, a binding is not lost or converted to a ?free variable?: it simply moves. The mobility of bindings needs to be supported by the equality theory of expressions. Clearly, equality already includes ?-conversion by Property 1. We also need a small amount of ?-conversion. If we rewrite this last inference rule using the definition of the d?e translation, we have the inference figure. ?, x : ?, typeof x ? ` typeof (Bx) ? 0 ? : ? ` ?x(typeof x ? ? typeof (Bx) ? 0) ?R ? : ? ` typeof (abs B) (? ? ? 0) Note that here B is a variable of arrow type tm ? tm and that instances of these inference figures will create an instance of (B x) that may be a ?-redex. As I now argue, that ?-redex has a limited form. First, observe that B is a schema variable that is implicitly universally quantified around this inference rule: if one formalizes this approach to type inference in, say, ?Prolog, one would write a specification similar to the formula ?B?? ?? 0[?x(typeof x ? ? typeof (Bx) ? 0) ? typeof (abs B) (? ? ? 0)]. Second, any closed instance of (B x) that is a ?-redex is such that the argument x is not free in the instance of B: this is enforced by the nature of (quantificational) logic since the scope of B is outside the scope of x. Thus, the only form of ?-conversion that is needed to support this notion of binding mobility is the so-called ?0-conversion rule [50]: (?x.t)x = t or equivalently (in the presence of ?-conversion) (?y.t)x = t[x/y], provided that x is not free in ?y.t. Given that ?0-conversion is such a simple operation, it is not surprising that higher-order pattern unification, which simplifies higher-order unification to a setting only needing ?, ?0, and ? conversion, is decidable and unitary [50]. For this reason, matching and unification can be used to help account for the mobility of binding. Note also that there is an elegant symmetry provided by binding and ?0-reduction: if t is a term over the signature ? ? {x} then ?x.t is a term over the signature ? and, conversely, if ?x.s is a term over the signature ? then the ?0-reduction of ((?x.s) y) is a term over the signature ? ? {y}. To illustrate how ?0-conversion supports the mobility of binders, consider how one specifies the following rewriting rule: given a conjunction of universally quantified formulas, rewrite it to be the universal quantification of the conjunction of formulas. In this setting, we would write something like: (?(?x.A x)) ? (?(?x.B x)) 7? (?(?x.(A x ? B x))). To rewrite an expression such as (??z(p z z)) ? (??z(q a z)) (where p, q, and a are constants) we first need to use ?0-expansion to get the expression (??z((?w.(p w w))z)) ? (??z((?w.(q a w))z)) At this point, the pattern variables A and B in the rewriting rule can now be instantiated by the closed terms ?w.(p w w) and ?w.(q a w), respectively, which yields the expression (?(?x.((?w.(p w w)) x ? (?w.(q a w)) x))). Finally, a ?0-contraction yields the expected expression (?(?x.(p x x) ? (q a x))). Note that at no time did a bound variable become unbound. Since pattern unification incorporates ?0-conversion, such rewriting can be accommodated simply by calls to such unification. The analysis of these four principles above do not imply that full ?-conversion is needed to support them. Clearly, full ?-conversion will implement ?0-conversion and several systems (which we shall speak about more below) that support ?-tree syntax do, in fact, implement ?-conversion. Systems that only implement ?0-conversion have only been described in print. For example, the L? logic programming language of [50] was restricted so that proof search could be complete while only needing to do ?0-conversion. The ?I -calculus (the ?-calculus with internal mobility [74]) can also be seen as a setting where only ?0-conversion is needed [53]. 6 Logic programming provides a framework As the discussion above suggests, quantificational logic using the proof-search model of computation can capture all four principles listed in the previous section. While it might be possible to account for these principles also in, say, a functional programming language (a half-hearted attempt at such a design was made in [49]), the logic programming paradigm supplies an appropriate framework for satisfying all these properties. Such a framework is available using the higher-order hereditary Harrop [54] subset of an intuitionistic variant of Church?s Simple Theory of Types [16]: ?Prolog [53] is a logic programming language based on that logic and implemented by the Teyjus compiler [73] and the ELPI interpreter [24]. The use of logic programming principles in proof assistants pushes against usual practice: since the first LCF prover [37], many (most?) proof assistants have had intimate ties to functional programming. For example, such theorem provers are often implemented using functional programming languages: in fact, the notion of LCF tactics and tacticals was originally designed and illustrated using functional programming principles [37]. Also, such provers frequently view proofs constructively and can output the computational content of proofs as functional programs [ 10 ]. I argue here that a framework based on logic programming principles might be more appropriate for mechanizing metatheory than one based on functional programming principles. Note that the arguments below do not lead to the conclusion that first-order logic programming languages, such as Prolog, are appropriate for metalevel reasoning: direct support for ?abstractions and quantifiers (as well as hypothetical reasoning) are critical and are not supported in first-order logic programming languages. Also, I shall focus on the specification of mechanized metatheory tasks and not on their implementation: it is completely possible that logic programming principles are used in specifications while a functional programming language is used to implement that specification language (for example, Teyjus and Abella are both implemented in OCaml). 6.1 Expressions versus values In logic programming, (closed) terms denote themselves and only themselves (in the sense of free algebra). It often surprises people that in Prolog, the goal ?- 3 = 1 + 2 fails, but the expression that is the numeral 3 and the expression 1 + 2 are, of course, different expressions. The fact that they have the same value is a secondary calculation (performed in Prolog using the is predicate). Functional programming, however, fundamentally links expressions and values: the value of an expression is the result of applying some evaluation strategy (e.g., call-by-value) to an expression. Thus the value of both 3 and 1 + 2 is 3 and these two expressions are, in fact, equated. Of course, one can easily write datatypes in functional programming languages that denote only expressions: datatypes for parse trees are such an example. However, the global notion that expressions denote values is particularly problematic when expressions denote ?-abstractions. The value of such expressions in functional programming is trivial and immediate: such values simply denote a function (a closure). In the logic programming setting, however, an expression that is a ?-abstraction is just another expression: following the principles stated in Section 4, equality of two such expressions needs to be based on the rather simple set of conversion rules ?, ?0, and ?. The ?-abstraction-as-expression aspect of logic programming is one of that paradigm?s major advantages for the mechanization of metatheory. 6.2 Syntactic types Given the central role of expressions (and not values), types in logic programming are better thought of as denoting syntactic categories. That is, such syntactic types are useful for distinguishing, say, encodings of types from terms from formula from proofs or program expressions from commands from evaluation contexts. For example, the typeof specification in Section 5 is a binary relation between the syntactic categories tm (for untyped ?-terms) and, say, ty (for simple type expression). The logical specification of the typeof predicate might attribute integer type or list type to different expressions via clauses such as ?T : tm ?L : tm ?? : ty [typeof T ? ? typeof L (list ? ) ? typeof (T :: L) (list ? )]. Given our discussion above, it seems natural to propose that if ? and ? 0 are both syntactic categories, then ? ? ? 0 is a new syntactic category that describes objects of category ? 0 with a variable of category ? abstracted. For example, if o denotes the category of formulas (a la [16]) and tm denotes the category of terms, then tm ? o denotes the type of term-level abstractions over formulas. As we have been taught by Church, the quantifiers ? and ? can then be seen as constructors that take expressions of syntactic category tm ? o to formulas: that is, these quantifiers are given the syntactic category (tm ? o) ? o. 6.3 Substitution lemmas for free Consider an attempt to prove the sequent ? : ? ` typeof (abs R) (? ? ? 0) where the assumptions (the theory) contains only one rule for proving such a statement, such as the clause used in the discussion of Section 5. Since the introduction rules for ? and ? are invertible, the sequent above is provable if and only if the sequent ?, x : ?, typeof x ? ` typeof (R x) ? 0 is provable. Given that we are committed to using a proper logic (such as higher-order intuitionistic logic), it is the case that modus ponens is valid and that instantiating an eigenvariable in a provable sequent yields a provable sequent. In this case, the sequent ? : ?, typeof N ? ` typeof (R N ) ? 0 must be provable (for N a term of syntactic type tm all of whose free variables are in ?). Thus, we have just shown, using nothing more than rather minimal assumptions about the specification of typeof (and formal properties of logic) that if ? : ? ` typeof (abs B) (? ? ? 0) and ? : ? ` typeof N ? then ? : ? ` typeof (B N ) ? 0. (Of course, instances of the term (B N ) are ?-redexes and the reduction of such redexes result in the substitution of N into the bound variable of the term that instantiates B.) Such lemmas about substitutions are common and often difficult to prove [85]: in this setting, this lemma is essentially an immediate consequent of using logic and logic programming principles [8, 46]. In this way, Gentzen?s cut-elimination theorem (the formal justification of modus ponens) can be seen as the mother of all substitution lemmas. The Abella theorem prover?s implementation of the two-level logic approach to reasoning about computation [33, 48] makes it possible to employ the cut-elimination theorem in exactly the style illustrated above. 6.4 Dominance of relational specifications Another reason that logic programming can make a good choice for metatheoretic reasoning systems is that logic programming is based on relations (not functions) and that metatheoretic specifications are often dominated by relations. For example, the typing judgment describe in the Section 5 is a relation. Similarly, both small step (SOS) and big step (natural semantics) approaches to operational semantics describe evaluation, for example, as a relation. Occasionally, specified relations ? typing or evaluation ? describe a partial function but that is generally a result proved about the relation. A few logic programming-based systems have been used to illustrate how typing and operational semantic specifications can be animated. The core engine of the Centaur project, called Typol, used Prolog to animate metatheoretic specifications [17] and ?Prolog has been used to provide convincing and elegant specifications of typing and operational semantics for expressions involving bindings [2, 53]. 6.5 Dependent typing The typing that has been motivated above is rather simple: one takes the notions of syntactic types as syntactic category ? e.g., programs, formulas, types, terms, etc ? and adds the arrow type constructor to denote abstractions of one syntactic type over another one. Since typing is, of course, an open-ended concept, it is completely possible to consider any number of ways to refine types. For example, instead of saying that a given expression denotes a term (that is, the expression has the syntactic type for terms), one could instead say that such an expression denotes, for example, a function from integers to integers. For example, the typing judgment t : tm (?t denotes a term?) can be refined to t : tm (int ? int) (?t denotes a term of type int ? int). Such richer types are supported (and generalized) by the dependent type paradigm [20, 38] and given a logic programming implementation in, for example, Twelf [64, 66]. Most dependently typed ?-calculi come with a fixed notion of typing and with a fixed notion of proof (natural deduction proofs encoded as typed ?-terms). The reliance described here on logical connectives and relations is expressive enough to specify dependently typed frameworks [26, 77, 78] but it is not committed to only that notion of typing and proof. 7 ?-tree syntax The term higher-order abstract syntax (HOAS) was originally defined as an approach to syntax that used ?a simply typed ?-calculus enriched with products and polymorphism? [65]. A subsequent paper identified HOAS as a technique ?whereby variables of an object language are mapped to variables in the meta-language? [66]. The term HOAS is problematic for a number of reasons. First, it seems that few, if any, researchers use this term in a setting that includes products and polymorphism (although simple and dependently typed ?-calculus are often used). Second, since the metalanguage (often the programming language) can vary a great deal, the resulting notion of HOAS can vary similarly, including the case where HOAS is a representation of syntax that incorporates function spaces on expressions [22, 39]. Third, the adjective higher-order seems inappropriate here: in particular, the equality (and unification) of terms discussed in Section 5 is completely valid without reference to typing. If there are no types, what exactly is ?higher-order?? For these reasons, the term ??-tree syntax? [8, 53], with its obvious parallel to the term ?parse tree syntax,? has been introduced as a more appropriate term for the approach to syntactic representation described here. While ?-tree syntax can be seen as a kind of HOAS (using the broad definition of HOAS given in [66]), there is little connections between ?-tree syntax and the problematic aspects of HOAS that arise when the latter uses function spaces to encode abstractions. For example, there are frequent claims that structural induction and structural recursive definitions are either difficult, impossible, or semantically problematic for HOAS: see, for example, [29, 39, 40]. When we consider specifically ?-tree syntax, however, induction (and coinduction) and structural recursion in the ?-tree setting have been given proof theoretic treatments and implementations. 8 Reasoning with ?-tree syntax Proof search (logic programming) style implementations of specifications can provide simple forms of metatheory reasoning. For example, given the specification of typing, both type checking and type inference are possible to automate using unification and backtracking search. Similarly, a specification of, say, big step evaluation can be used to provide a symbolic evaluator for at least simple expressions [17]. There is, however, much more to mechanizing metatheory than performing unification and doing logic programming-style search. One must also deal with negations (difficult for straightforward logic programming engines): for example, one wants to prove that certain terms do not have simple types: for example, ` ? ?? : ty. typeof (abs ?x (app x x)) ?. Proving that a certain relation actually describes a (partial or total) function has proved to be an important kind of metatheorem to prove: the Twelf system [66] is able to automatically prove many of the simpler forms of such metatheorems. Additionally, one should also deal with induction and coinduction and be able to reason directly about, say, bisimulation of ?-calculus expressions as well as confluence of ?-conversion. In recent years, several researchers have developed two extensions to logic and proof theory that have made it possible to reason in rich and natural ways about expressions containing bindings. One of these extensions involved a proof theory for least and greatest fixed points: results from [47, 82] have made it possible to build automated and interactive inductive and coinductive theorem provers in a simple, relational setting. Another extension [32, 55] introduced the ?-quantifier which allows logic to reason in a rich and natural way with bindings: in terms of mobility of bindings, the ?-quantifier provides an additional formula-level and proof-level binder, thereby enriching the expressiveness of quantificational logic. Given these developments in proof theory, it has been possible to build both an interactive theorem prover, called Abella [8, 30], and an automatic theorem prover, called Bedwyr [9], that unfolds fixed points in a style similar to a model checker. These systems have successfully been able to prove a range of metatheoretic properties about the ?-calculus and the ?-calculus [ 1, 8, 81 ]. The directness and naturalness of the encoding for the ?-calculus bisimulation is evident in the fact that simply adding the excluded middle on name equality changes the interpretation of that one definition from open bisimulation to late bisimulation [81]. Besides the Abella, Bedwyr, and Twelf system mentioned above, there are a number of other implemented systems that support some or all aspects of ?-tree syntax: these include Beluga [67], Hybrid [27], Isabelle [62], Minlog [75], and Teyjus [60]. See [28] for a survey and comparison of several of these systems. The shift from conventional proof assistants based on functional programming principles to assistants based on logic programming principles does disrupt a number of aspects of proof assistants. For example, when computations are naturally considered as functional, it seems that there is a lost of expressiveness and effectiveness if one must write those specifications using relations. Recent work shows, however, that when a relation actually encodes a function, it is possible to use the proof search framework to actually compute that function [34]. A popular feature of many proof assistants is the use of tactics and tacticals, which have been implemented using functional programs since their introduction [37]. There are good arguments, however, that those operators can be given elegant and natural implementations using (higher-order) logic programs [23, 25, 53]. The disruptions that result from such a shift seem well worth exploring. 9 Conclusions I have argued that parsing concrete syntax into parse trees does not yield a sufficiently abstract representation of expressions: the treatment of bindings should be made more abstract. I have also described and motivated the ?-tree syntax approach to such a more abstract framework. For a programming language or proof assistant to support this level of abstraction in syntax, equality of syntax must be based on ? and ?0 (at least) and must allow for the mobility of binders from within terms to within formulas (i.e., quantifiers) to within proofs (i.e., eigenvariables). I have also argued that the logic programming paradigm ? broadly interpreted ? provides an elegant and high-level framework for specifying both computation and deduction involving syntax containing bindings. This framework is offered up as an alternative to the more conventional approaches to mechanizing metatheory using formalizations based on more conventional mathematical concepts. While the POPLmark challenge was based on the assumption that increments to existing provers will solve the problems surrounding the mechanization of metatheory, I have argued and illustrated here that we need to make a significant shift in the underlying paradigm that has been built into today?s most mature proof assistants. 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 Brian Aydemir, Aaron Bohannon, and Stephanie Weirich. Nominal reasoning techniques in Coq. In International Workshop on Logical Frameworks and Meta-Languages:Theory and Practice (LFMTP), pages 69?77, Seattle, WA, USA, 2006. Brian Aydemir, Stephan A. Zdancewic, and Stephanie Weirich. Abstracting syntax. Technical Report MS-CIS-09-06, University of Pennsylvania, 2009. Brian E. Aydemir, Aaron Bohannon, Matthew Fairbairn, J. Nathan Foster, Benjamin C. Pierce, Peter Sewell, Dimitrios Vytiniotis, Geoffrey Washburn, Stephanie Weirich, and Steve Zdancewic. Mechanized metatheory for the masses: The POPLmark challenge. 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Dale Miller. Mechanized Metatheory Revisited: An Extended Abstract (Invited Paper), LIPICS - Leibniz International Proceedings in Informatics, 2018, 1:1-1:16, DOI: 10.4230/LIPIcs.TYPES.2016.1