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15
Induction and coinduction in sequent calculus
 Postproceedings of TYPES 2003, number 3085 in LNCS
, 2003
"... Abstract. Proof search has been used to specify a wide range of computation systems. In order to build a framework for reasoning about such specifications, we make use of a sequent calculus involving induction and coinduction. These proof principles are based on a proof theoretic (rather than sett ..."
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Cited by 23 (8 self)
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Abstract. Proof search has been used to specify a wide range of computation systems. In order to build a framework for reasoning about such specifications, we make use of a sequent calculus involving induction and coinduction. These proof principles are based on a proof theoretic (rather than settheoretic) notion of definition [13, 20, 25, 51]. Definitions are akin to (stratified) logic programs, where the left and right rules for defined atoms allow one to view theories as “closed ” or defining fixed points. The use of definitions makes it possible to reason intensionally about syntax, in particular enforcing free equality via unification. We add in a consistent way rules for pre and post fixed points, thus allowing the user to reason inductively and coinductively about properties of computational system making full use of higherorder abstract syntax. Consistency is guaranteed via cutelimination, where we give the first, to our knowledge, cutelimination procedure in the presence of general inductive and coinductive definitions. 1
Representations of stream processors using nested fixed points
 Logical Methods in Computer Science
"... Abstract. We define representations of continuous functions on infinite streams of discrete values, both in the case of discretevalued functions, and in the case of streamvalued functions. We define also an operation on the representations of two continuous functions between streams that yields a ..."
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Cited by 15 (2 self)
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Abstract. We define representations of continuous functions on infinite streams of discrete values, both in the case of discretevalued functions, and in the case of streamvalued functions. We define also an operation on the representations of two continuous functions between streams that yields a representation of their composite. In the case of discretevalued functions, the representatives are wellfounded (finitepath) trees of a certain kind. The underlying idea can be traced back to Brouwer’s justification of barinduction, or to Kreisel and Troelstra’s elimination of choicesequences. In the case of streamvalued functions, the representatives are nonwellfounded trees pieced together in a coinductive fashion from wellfounded trees. The definition requires an alternating fixpoint construction of some ubiquity.
Cyclic proofs of program termination in separation logic. Forthcoming
"... We propose a novel approach to proving the termination of heapmanipulating programs, which combines separation logic with cyclic proof within a Hoarestyle proof system. Judgements in this system express (guaranteed) termination of the program when started from a given line in the program and in a s ..."
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Cited by 12 (3 self)
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We propose a novel approach to proving the termination of heapmanipulating programs, which combines separation logic with cyclic proof within a Hoarestyle proof system. Judgements in this system express (guaranteed) termination of the program when started from a given line in the program and in a state satisfying a given precondition, which is expressed as a formula of separation logic. The proof rules of our system are of two types: logical rules that operate on preconditions; and symbolic execution rules that capture the effect of executing program commands. Our logical preconditions employ inductively defined predicates to describe heap properties, and proofs in our system are cyclic proofs: cyclic derivations in which some inductive predicate is unfolded infinitely often along every infinite path, thus allowing us to discard all infinite paths in the proof by an infinite descent argument. Moreover, the use of this soundness condition enables us to avoid the explicit construction and use of ranking functions for termination. We also give a completeness result for our system, which is relative in that it relies upon completeness of a proof system for logical implications in separation logic. We give examples illustrating our approach, including one example for which the corresponding ranking function is nonobvious: termination of the classical algorithm for inplace reversal of a (possibly cyclic) linked list.
On the proof theory of modal mucalculus
 Studia Logica
, 2008
"... We study the prooftheoretic relationship between two deductive systems for the modal mucalculus. First we recall an infinitary system which contains an omega rule allowing to derive the truth of a greatest fixed point from the truth of each of its (infinitely many) approximations. Then we recall a ..."
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Cited by 7 (2 self)
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We study the prooftheoretic relationship between two deductive systems for the modal mucalculus. First we recall an infinitary system which contains an omega rule allowing to derive the truth of a greatest fixed point from the truth of each of its (infinitely many) approximations. Then we recall a second infinitary calculus which is based on nonwellfounded trees. In this system proofs are finitely branching but may contain infinite branches as long as some greatest fixed point is unfolded infinitely often along every branch. The main contribution of our paper is a translation from proofs in the first system to proofs in the second system. Completeness of the second system then follows from completeness of the first, and a new proof of the finite model property also follows as corollary. 1
Formalised inductive reasoning in the logic of bunched implications
 In SAS14, volume 4634 of LNCS
, 2007
"... Abstract. We present a framework for inductive definitions in the logic of bunched implications, BI, and formulate two sequent calculus proof systems for inductive reasoning in this framework. The first proof system adopts a traditional approach to inductive proof, extending the usual sequent calcul ..."
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Cited by 4 (4 self)
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Abstract. We present a framework for inductive definitions in the logic of bunched implications, BI, and formulate two sequent calculus proof systems for inductive reasoning in this framework. The first proof system adopts a traditional approach to inductive proof, extending the usual sequent calculus for predicate BI with explicit induction rules for the inductively defined predicates. The second system allows an alternative mode of reasoning with inductive definitions by cyclic proof. In this system, the induction rules are replaced by simple casesplit rules, and the proof structures are cyclic graphs formed by identifying some sequent occurrences in a derivation tree. Because such proof structures are not sound in general, we demand that cyclic proofs must additionally satisfy a global trace condition that ensures soundness. We illustrate our inductive definition framework and proof systems with simple examples which indicate that, in our setting, cyclic proof may enjoy certain advantages over the traditional induction approach. 1
The Recursion Scheme from the Cofree Recursive Comonad
"... We instantiate the general comonadbased construction of recursion schemes for the initial algebra of a functor F to the cofree recursive comonad on F. Differently from the scheme based on the cofree comonad on F in a similar fashion, this scheme allows not only recursive calls on elements structura ..."
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Cited by 2 (1 self)
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We instantiate the general comonadbased construction of recursion schemes for the initial algebra of a functor F to the cofree recursive comonad on F. Differently from the scheme based on the cofree comonad on F in a similar fashion, this scheme allows not only recursive calls on elements structurally smaller than the given argument, but also subsidiary recursions. We develop a Mendler formulation of the scheme via a generalized Yoneda lemma for initial algebras involving strong dinaturality and hint a relation to circular proofs à la Cockett, Santocanale.
Refinement types and computational duality
 In: ACM SIGPLANSIGACT Workshop on Programming Languages Meets Program Verification
, 2009
"... One lesson learned painfully over the past twenty years is the perilous interaction of Currystyle typing with evaluation order and sideeffects. This led eventually to the value restriction on polymorphism in ML, as well as, more recently, to similar artifacts in type systems for ML with intersecti ..."
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Cited by 2 (1 self)
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One lesson learned painfully over the past twenty years is the perilous interaction of Currystyle typing with evaluation order and sideeffects. This led eventually to the value restriction on polymorphism in ML, as well as, more recently, to similar artifacts in type systems for ML with intersection and union refinement types. For example, some of the traditional subtyping laws for unions and intersections are unsound in the presence of effects, while unionelimination requires an evaluation context restriction in addition to the value restriction on intersectionintroduction. Our aim is to show that rather than being ad hoc artifacts, phenomena such as the value and evaluation context restrictions arise naturally in type systems for effectful languages, out of principles of duality. Beginning with a review of recent work on the CurryHoward interpretation of focusing proofs as patternmatching programs,
Cut elimination for a logic with induction and coinduction
 JOURNAL OF APPLIED LOGIC
, 2012
"... ..."
A Unified View of Induction Reasoning for FirstOrder Logic
"... Induction is a powerful proof technique adapted to reason on sets with an unbounded number of elements. In a firstorder setting, two different methods are distinguished: the conventional induction, based on explicit induction schemas, and the implicit induction, based on reductive procedures. We pr ..."
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Cited by 1 (0 self)
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Induction is a powerful proof technique adapted to reason on sets with an unbounded number of elements. In a firstorder setting, two different methods are distinguished: the conventional induction, based on explicit induction schemas, and the implicit induction, based on reductive procedures. We propose a new cyclebased induction method that keeps their best features, i.e., performs local and nonreductive reasoning, and naturally fits for mutual and lazy induction. The heart of the method is a proof strategy that identifies in the proof script the subset of formulas contributing to validate the application of induction hypotheses. The conventional and implicit induction are particular cases of our method. 1
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"... A linear approach to the prooftheory of least and greatest fixed points Présentée et soutenue publiquement par ..."
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A linear approach to the prooftheory of least and greatest fixed points Présentée et soutenue publiquement par