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| In [[logic]], a '''logical framework''' provides a means to define (or present) a logic as a signature in a higher-order [[type theory]] in such a way that [[provability]] of a formula in the original logic reduces to a [[type inhabitation]] problem in the framework type theory.<ref name="Jacobs2001">{{cite book|author=Bart Jacobs|title=Categorical Logic and Type Theory|year=2001|publisher=Elsevier|isbn=978-0-444-50853-9|page=598}}</ref><ref name="Gabbay1994">{{cite book|editor=Dov M. Gabbay|title=What is a logical system?|url=http://books.google.com/books?id=XqCu4XjHrIQC&pg=PA382|year=1994|publisher=Clarendon Press|isbn=978-0-19-853859-2|page=382}}</ref> This approach has been used successfully for (interactive) [[automated theorem proving]]. The first logical framework was [[Automath]], however the name of the idea comes from the more widely known Edinburgh Logical Framework, '''LF'''. Several more recent proof tools like [[Isabelle (theorem prover)|Isabelle]] are based on this idea.<ref name="Jacobs2001"/> Unlike a direct embedding, the logical framework approach allows many logics to be embedded in the same type system.<ref name="BoveBarbosa2009">{{cite book|author1=Ana Bove|author2=Luis Soares Barbosa|author3=Alberto Pardo|title=Language Engineering and Rigourous (sic) Software Development: International LerNet ALFA Summer School 2008, Piriapolis, Uruguay, February 24 - March 1, 2008, Revised, Selected Papers|url=http://books.google.com/books?id=YOqHiA5MYpEC&pg=PA48|year=2009|publisher=Springer|isbn=978-3-642-03152-6|pages=48}}</ref>
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| A logical framework is based on a general treatment of syntax, rules and proofs by means of a [[dependent type theory|dependently typed lambda calculus]]. Syntax is treated in a style similar to, but more general than [[Per Martin-Löf]]'s system of arities.
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| To describe a logical framework, one must provide the following:
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| # A characterization of the class of object-logics to be represented;
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| # An appropriate meta-language;
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| # A characterization of the mechanism by which object-logics are represented.
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| This is summarized by:
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| ''‘Framework = Language + Representation’''.
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| == LF ==
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| In the case of the '''LF logical framework''', the meta-language is the [[λΠ-calculus|<math>\lambda\Pi</math>-calculus]]. This is a system of first-order dependent function types which are related by the [[propositions as types principle]] to first-order minimal logic. The key features of the <math>\lambda\Pi</math>-calculus are that it consists of entities of three levels: objects, types and families of types. It is [[Impredicativity | predicative]], all well-typed terms are [[strongly normalizing]] and [[Church-Rosser]] and the property of being well-typed is [[Decidability (logic)|decidable]]. However, [[type inference]] is [[Decidability (logic)|undecidable]].
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| A logic is represented in the '''LF logical framework''' by the judgements-as-types representation mechanism. This is inspired by [[Per Martin-Löf]]'s development of [[Immanuel Kant|Kant]]'s notion of [[judgement (mathematical logic)|judgement]], in the 1983 Siena Lectures. The two higher-order judgements, the hypothetical <math>J\vdash K</math> and the general, <math>\Lambda x\in J. K(x)</math>, correspond to the ordinary and dependent function space, respectively. The methodology of judgements-as-types is that judgements are represented as the types of their proofs. A [[logical system]] <math>{\mathcal L}</math> is represented by its signature which assigns kinds and types to a finite set of constants that represents its syntax, its judgements and its rule schemes. An object-logic's rules and proofs are seen as primitive proofs of hypothetico-general judgements <math>\Lambda x\in C. J(x)\vdash K</math>.
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| An implementation of the LF logical framework is provided by the [[Twelf]] system at [[Carnegie Mellon University]]. Twelf includes
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| :* a logic programming engine
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| :* meta-theoretic reasoning about logic programs (termination, coverage, etc.)
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| :* an inductive [[meta-logic]]al theorem prover
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| ==References==
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| {{reflist}}
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| ==See also==
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| * [[Grammatical Framework]]
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| ==Further reading==
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| * {{cite book|editor=Helmut Schwichtenberg, Ralf Steinbrüggen|title=Proof and system-reliability|chapter=Logical frameworks—a brief introduction|year=2002|publisher=Springer|isbn=978-1-4020-0608-1|author=[[Frank Pfenning]]|url=http://www.cs.cmu.edu/~fp/papers/mdorf01.pdf}}
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| *[[Robert Harper (computer scientist)|Robert Harper]], Furio Honsell and [[Gordon Plotkin]]. ''A Framework For Defining Logics''. Journal of the Association for Computing Machinery, 40(1):143-184, 1993
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| *Arnon Avron, Furio Honsell, Ian Mason and Randy Pollack. ''Using typed lambda calculus to implement formal systems on a machine''. Journal of Automated Reasoning, 9:309-354, 1992.
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| *Robert Harper. ''An Equational Formulation of LF''. Technical Report, University of Edinburgh, 1988. LFCS report ECS-LFCS-88-67.
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| *Robert Harper, Donald Sannella and Andrzej Tarlecki. ''Structured Theory Presentations and Logic Representations''. Annals of Pure and Applied Logic, 67(1-3):113-160, 1994.
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| *Samin Ishtiaq and David Pym. ''A Relevant Analysis of Natural Deduction''. Journal of Logic and Computation 8, 809-838, 1998.
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| * Samin Ishtiaq and David Pym. ''Kripke Resource Models of a Dependently-typed, Bunched <math>\lambda</math>-calculus''. Journal of Logic and Computation 12(6), 1061-1104, 2002.
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| * Per Martin-Löf. "[http://www.hf.uio.no/filosofi/njpl/vol1no1/meaning/meaning.html On the Meanings of the Logical Constants and the Justifications of the Logical Laws]." "[[Nordic Journal of Philosophical Logic]]", 1(1): 11-60, 1996.
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| * Bengt Nordström, Kent Petersson, and Jan M. Smith. ''Programming in Martin-Löf's Type Theory''. Oxford University Press, 1990. (The book is out of print, but [http://www.cs.chalmers.se/Cs/Research/Logic/book/ a free version] has been made available.)
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| *David Pym. ''A Note on the Proof Theory of the <math>\lambda\Pi</math>-calculus''. Studia Logica 54: 199-230, 1995.
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| *David Pym and Lincoln Wallen. ''Proof-search in the <math>\lambda\Pi</math>-calculus''. In: G. Huet and G. Plotkin (eds), Logical Frameworks, Cambridge University Press, 1991.
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| *Didier Galmiche and David Pym. ''Proof-search in type-theoretic languages:an introduction''. Theoretical Computer Science 232 (2000) 5-53.
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| *Philippa Gardner. ''Representing Logics in Type Theory''. Technical Report, University of Edinburgh, 1992. LFCS report ECS-LFCS-92-227.
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| *Gilles Dowek. ''The undecidability of typability in the lambda-pi-calculus''. In M. Bezem, J.F. Groote (Eds.), Typed Lambda Calculi and Applications. Volume 664 of ''Lecture Notes in Computer Science'', 139-145, 1993.
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| *David Pym. ''Proofs, Search and Computation in General Logic''. Ph.D. thesis, University of Edinburgh, 1990.
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| *David Pym. ''A Unification Algorithm for the <math>\lambda\Pi</math>-calculus.'' Int. J. of Foundations of Computer Science 3(3), 333-378, 1992.
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| == External links ==
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| * [http://www.cs.cmu.edu/~fp/lfs-impl.html Specific Logical Frameworks and Implementations] (a list maintained by [[Frank Pfenning]])
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| [[Category:Logic in computer science]]
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| [[Category:Type theory]]
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| [[Category:Proof assistants]]
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| [[Category:Dependently typed programming]]
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