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1 The Semi-Full Closure of Pure Type Systems? Gilles Barthe Institutionen for Datavetenskap, Chalmers Tekniska Hogskola, Goteborg, Sweden Departamento de Informatica, Universidade do Minho, Braga, Portugal Abstract. We show that every functional Pure Type System may be extended to a semi-full Pure Type System. Moreover, the extension is conservative and preserves weak normalization. Based on these results, we give a new, conceptually simple type-checking algorithm for functional Pure Type Systems. 1 Introduction Pure Type Systems (PTSs) [1] capture in a unied setting many typed -calculi that form the basis of typed functional languages and type-theory based proofdevelopment systems. One central issue in the theory of PTSs is the problem of type-checking, which consists in deciding whether a judgment M : A is derivable according to the rules of a given PTS S. Although type-checking is undecidable in general, most systems of interest have decidable type-checking. For such systems, the question remains whether it is possible to nd reasonable, sound and complete, algorithms for type-checking. The existence of such algorithms is not obvious and indeed the completeness of the most natural typechecking algorithm, due to R. Pollack [8], remains an open problem. In a nutshell, the problem is caused by the second premise of the abstraction rule, which makes it dicult to prove completeness by induction on the structure of derivations. Nevertheless several authors have proposed type-checking algorithms that are sound and complete for some specic classes of PTSs. In the early 90s, R. Pollack [7, 8] introduced the class of semi-full PTSs informally a PTS is semi-full if it has \enough rules" and gave a sound and complete type-checking algorithm for PTSs in that class. Unfortunately, many PTSs of interest are not semi-full. Later L.S. van Benthem Jutting, J. McKinna and R. Pollack [3, 8] gave an alternative algorithm that is sound and complete for functional PTSs, a large class of PTSs that comprises most of the systems that appear in the literature. In order to check for the second premise of the abstraction rule, their algorithm invokes a complex derivability relation with -application and -conversion, as given by? This is a revised version of the article appearing in the proceedings of MFCS'98. The present version diers from the proceedings version in the proofs of Lemma 4 and Proposition 1, which have been substantially simplied. In addition, the present version corrects the abstraction rule in Figure 2.

2 the application rule M : x:a: B N : A M N : (x:a: B) N and the reduction rule (x: A: B) N! Bfx := Ng. Their algorithm is not fully satisfactory in the sense that it requires to consider an extended framework. More recently, P. Severi [9] has suggested another algorithm that appeals to Pure Type Systems without the -condition (PTSWs). Those are a variant of PTSs in which the abstraction rule is?; x : A ` M : B x:a: M : x:a: B Again PTSWs are used to check for the second premise of the abstraction rule. While Severi's algorithm eliminates the need for considering new reduction relations, it still introduces a new framework. As a result, Severi needs to prove numerous properties for PTSWs before proving the soundness and completeness of the algorithm. Finally there are other algorithms that are concerned with the smaller class of (weakly) injective PTSs [2, 6]. These algorithms are simpler but do not cover all existing systems. For example some of the languages of the Automath family [4] and predicative F! [5] are not weakly injective. The purpose of this paper is to present a new sound and complete typechecking algorithm for functional PTSs. The novelty of our algorithm is to remain within the framework of PTSs. It is an improvement over [3, 8, 9]: our algorithm is conceptually clearer and suppresses the need for introducing new frameworks such as the ones of [3, 8, 9]. In order to dene our algorithm and prove it correct, we show that every functional PTS may be extended conservatively to a semi-full PTS, its semi-full closure. This result makes it possible to check, using Pollack's algorithm for semi-full PTSs, the second clause in the abstraction rule in the semi-full closure of the PTS under consideration. Contents The paper is organized as follows. Section 2 briey reviews the definition of PTSs. Section 3 introduces the semi-full closure of a PTS. Section 4 provides a sound and complete type-checking algorithm for functional PTSs. 2 Pure Type Systems In this section, we present the syntax of PTSs and refer to standard texts, see e.g. [1], for examples and motivations. Denition 1 (Specication). A specication is a triple S = (S; A; R) where S is a set of sorts, A S S is a set of axioms and R S S S is a set of rules. A specication S = (S; A; R) is functional if for every s 1 ; s 2 ; s 0 2 ; s 3; s S, (s 1 ; s 2 ) 2 A ^ (s 1 ; s 0 2) 2 A ) s 2 s 0 2 (s 1 ; s 2 ; s 3 ) 2 R ^ (s 1 ; s 2 ; s 0 3) 2 R ) s 3 s 0 3

3 Every specication S yields a PTS S as specied below. Throughout this section, S = (S; A; R) is a xed specication. Denition 2 (Pure Type System). 1. The set T of pseudo-terms is given by the abstract syntax T = V j S j T T j V : T :T j V : T :T where V is a xed countably innite set of variables. 2. -reduction! is dened as the compatible closure of the contraction (x:a: M) N! Mfx := Ng where f := g is the standard substitution operator. The reexive-transitive and reexive-symmetric-transitive closures of! are denoted by! and = respectively. 3. A pseudo-context is a nite ordered list x 1 : A 1 ; : : : ; x n : A n where x 1 ; : : : ; x n 2 V and A 1 ; : : : ; A n 2 T. The empty context is denoted by hi and the set of pseudo-contexts is denoted by G. If? 2 G, we let dom(? ) = fx j 9t 2 T : x : t 2? g. 4. A judgment is a triple M : A where? 2 G and M; A 2 T. The rules of Pure Type Systems are given in Figure 1. If M : A is derivable according to those rules, then? and M are legal. 5. S = (E; G;! ; `) is the Pure Type System (PTS) induced by S. Some of the results of this paper are concerned with normalization. Denition 3. We write S j= WN() and S j= SN() respectively if every legal term in S is -weakly normalizing and -strongly normalizing respectively. We conclude this section with a list of properties of PTSs. Lemma 1 (Closure properties). 1. Substitution. If?; x : A; ` B : C and a : A, then 1?; fx := ag ` Afx := ag : Bfx := ag. 2. Correctness of Types. If A : B then either B 2 S or there exists s 2 S such that B : s. 3. Correctness of Contexts. If?; x : C; ` A : B then there exists s 2 S such that C : s. 4. Subject Reduction. If M : A and M! N then N : A. 5. Predicate Reduction. If M : A and A! A 0 then M : A 0. Lemma 2 (Uniqueness of Types). Assume S is functional. M : A ^ M : A 0 ) A = A 0 1 Substitution is extended from pseudo-terms to pseudo-contexts in the usual way.

4 (axiom) hi ` s 1 : s 2 if (s 1; s 2) 2 A (start) (weakening) A : s?; x : A ` x : A A : B C : s?; x : C ` A : B and A 2 V [ S (product) (application) (abstraction) (conversion) A : s 1?; x : A ` B : s 2 (x: A: B) : s 3 if (s 1; s 2; s 3) 2 R F : (x: A: B) a : A F a : Bfx := ag?; x : A ` b : B (x: A: B) : s x:a: b : x: A: B A : B B 0 : s A : B 0 if B = B 0 Fig. 1. Rules for Pure Type Systems 3 The semi-full closure of a specication Semi-fullness is a technical condition ensuring that a PTS \has enough rules". This is to be contrasted with negative notions such as functionality or injectivity which ensure that a PTS \does not have too many rules". Because of the nature of semi-fullness, every PTS may be extended to a semi-full one while a non-functional or non-injective PTS may not be extended to a functional or an injective one. In fact, there are several ways to extend a PTS into a semi-full one. The next denition suggests two possibilities: the stratied closure, which is layered so as to facilitate reasoning, and the compact closure, which is more suited for type-checking purposes. Denition 4 (Semi-full, semi-full closure). Let S = (S; A; R) be a specication. Dene O = fs 2 S j 9s 0 ; s 00 2 S: (s; s 0 ; s 00 ) 2 Rg P = f(s 1 ; s 2 ) 2 O S j 8s 2 S: (s 1 ; s 2 ; s) 62 Rg 1. S is semi-full if P = ;. 2. The semi-full closure of S is the specication S = (S ; A; R ) where S = S [ fg and R = R [ f(s 1 ; s 2 ; ) j (s 1 ; s 2 ) 2 Pg [ f(s; ; ) j s 2 Og

5 The next result provides an alternative characterization of semi-fullness; in fact it corresponds to Pollack's original denition of semi-fullness. Lemma 3. Let S = (S; A; R) be a specication. 1. S is semi-full if for every s S and (s 1; s 2 ; s 3 ) 2 R, there exists s S such that (s 1 ; s 0 2 ; s0 3) 2 R. 2. S is semi-full. 3. If S is functional so is S. We conclude this section with two basic observations, which are fundamental for type-checking purposes. Below we use ` to denote derivability in S, where S is any given specication. Lemma 4. Let S = (S; A; R) be a functional specication. If x:a: B : s and?; x : A ` B : s 2 and s 2 S then x:a: B : s. Proof. By Correctness of sorts, there exists s 2 S such that A : s 1. A fortiori A : s 1 and?; x : A ` B : s 2. Moreover, we know (from generation on x:a:b : s that there exists (s 0 1 ; s0 2 ; s) 2 such that? R ` A : s 0 1 and?; x : A ` B : s 0 2. Since s 2 S, one concludes (s 1 ; s 2 ; s 3 ) 2 R. By (product), it follows x:a: B : s. Proposition 1. Let S be a functional specication. Proof. Dene : T! T as follows: S j= WN() ) S j= WN() (s) = s (x) = x (x:a: B) = x:(a): (B) (x:a: M) = x:(a): (M) ((P ))fx := (N)g (M N) = (M) (N) s 2 S x 2 V if M x:a: P otherwise One can show by induction on the structure of derivations that M : A ) (? ) ` (M) : (A) provided A 6= and (? ) is legal in S. As M = (M), it follows that M 2 WN(). To conclude, rst observe that for every M : A with A 6=, there exists? 0, M 0 and A 0 such that (? 0 ) is legal in S,? 0 ` M 0 : A 0 and M 0 2 WN() ) M 2 WN(). The key observation is that y cannot be bound because there is no rule of the form (; s; s 0 ). Hence one can safely replace subterms of the form y P by variables, say x y P, in such a way that x y P = x whenever y0 P 0 y P = y 0 P 0. By doing so, one obtains the desired judgement. Finally, we are left to treat the case where M : A with A =. One can proceed by induction on the structure of terms.

6 4 Type-checking In this section, we exploit the decidability of type-checking for semi-full normalizing PTSs [3, 8] to establish the decidability of type-checking for functional normalizing PTSs. (axiom) hi ` s 1 : s 2 if (s 1; s 2) 2 A (start)?; x : A ` A :! wh s x : A (weakening) (product) A : B?; x : C ` C :! wh s A : B and A 2 V [ S A :! wh s 1?; x : A ` B :! wh s 2 (x: A: B) : s 3 if (s 1; s 2; s 3) 2 R (application)? ` F :! wh (x: A 0 : B) F a : Bfx := ag a : A if A = A 0 (abstraction)?; x : A ` b : B A : s B 2 S ) B 2 S x:a: b : x: A: B if s 2 O Fig. 2. Syntax-directed rules for semi-full closures An important step towards decidability of type-checking is to provide a syntax-directed presentation of the rules of PTSs. In a nutshell, a set of rules is syntax-directed if the premises of a rule are determined up to inessential details by its conclusion. The next denition provides such a set of rules. It uses an auxiliary relation ` which instantiates the derivability relation ` of [3, 8] to S. Denition 5 (Syntax-directed Rules). 1. Weak-head reduction! wh is dened as the closure 2 of the contraction (x : A: P ) Q R 1 : : : R n! wh P fx := Qg R 1 : : : R n 2. A sort s is a typed sort, written s 2 S, if 9s 0 2 S: (s; s 0 ) 2 A. 3. The derivability relation M : A is given by the rules of Figure 2 where we write wh A if 9A 0 2 T : ^ A A0 0! wh A. 2 We insist on the closure not being compatible so weak-head reduction diers from -reduction by applying only at the top-level.

7 4. The derivability relation nat M : A is given by the rules of Figure 3 where we write nat M :! wh A if 9A 0 2 T : nat M : A 0 ^ A 0! wh A. The soundness and completeness of ` Proposition 2 ([3]). For every specication S, over ` is already known. 1. Soundness: M : A ) M : A. 2. Completeness: M : A ) 9A 0 2 T : M : A0 ^ A = A 0 (axiom) hi `nat s 1 : s 2 if (s 1; s 2) 2 A (start) (weakening) nat A :! wh s?; x : A `nat x : A nat A : B nat C :! wh s?; x : C `nat A : B and A 2 V [ S (product) nat A :! wh s 1?; x : A `nat B :! wh s 2 nat (x: A: B) : s 3 if (s 1; s 2; s 3) 2 R (application) nat F :! wh (x: A 0 : B) nat a : A nat F a : Bfx := ag (abstraction)?; x : A `nat b : B nat x:a: b : x: A: B x: A: B : s if A = A 0 if s 2 S Fig. 3. Syntax-directed rules for functional Pure Type Systems Using the above proposition, we conclude that `nat is sound and complete with respect to `. Theorem 1. If S is functional, then 1. Soundness: nat M : A ) M : A. 2. Completeness: M : A ) 9A 0 2 T : nat M : A 0 ^ A = A 0 Proof. Soundness is proved by induction on the derivations, using Lemma 4 in the (abstraction) rule. Completeness is also proved by induction on the structure of derivations, using soundness. The proofs are routine. Corollary 1 (Decidability of type-checking). If S = (S; A; R) is a functional specication, then type-checking is decidable provided S, A and R are recursive and S j= WN(). Proof. We need to prove that all side-conditions are decidable. Weak normalization is needed in order to decide -convertibility. Details are omitted.

8 Acknowledgments The author is supported by a European TMR Fellowship. References 1. H. Barendregt. Lambda calculi with types. In S. Abramsky, D. Gabbay, and T. Maibaum, editors, Handbook of Logic in Computer Science, pages 117{309. Oxford Science Publications, Volume G. Barthe. Type checking injective pure type systems. Manuscript, L.S. van Benthem Jutting, J. McKinna, and R. Pollack. Checking algorithms for pure type systems. In H. Barendregt and T. Nipkow, editors, Proceedings of TYPES'93, volume 806 of Lecture Notes in Computer Science, pages 19{61. Springer-Verlag, R. Nederpelt, H. Geuvers, and R. de Vrijer, editors. Selected papers on Automath, volume 133 of Studies in Logic and the Foundations of Mathematics. North-Holland, Amsterdam, S. Peyton Jones and E. Meijer. Henk: a typed intermediate language. Proceedings of the ACM Workshop on Types in Compilation, E. Poll. A typechecker for bijective pure type systems. Technical Report CSN93/22, Technical University of Eindhoven, June R. Pollack. Typechecking in pure type systems. In B. Nordstrom, editor, Informal proceedings of Logical Frameworks'92, pages 271{288, R. Pollack. The Theory of LEGO: A Proof Checker for the Extended Calculus of Constructions. PhD thesis, University of Edinburgh, P. Severi. Normalisation in lambda calculus and its relation to type inference. PhD thesis, Technical University of Eindhoven, 1996.

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