A unified display proof theory for bunched logic · 2010-05-09 · • Structures are constructed...
Transcript of A unified display proof theory for bunched logic · 2010-05-09 · • Structures are constructed...
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A unified display proof theory
for bunched logic
James Brotherston
Imperial College London
MFPS 2010University of Ottawa, 9 May 2010
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Substructural logics: an overview
Substructural logics restrict the structural principles of ordinaryclassical logic (weakening, contraction, associativity, exchange. . . ).
Examples:
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Substructural logics: an overview
Substructural logics restrict the structural principles of ordinaryclassical logic (weakening, contraction, associativity, exchange. . . ).
Examples:
• Lambek calculus totally rejects weakening and contraction(commutativity and associativity are optional too);
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Substructural logics: an overview
Substructural logics restrict the structural principles of ordinaryclassical logic (weakening, contraction, associativity, exchange. . . ).
Examples:
• Lambek calculus totally rejects weakening and contraction(commutativity and associativity are optional too);
• Linear logic permits weakening and contraction only forformulas prefixed with “exponential” modalities;
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Substructural logics: an overview
Substructural logics restrict the structural principles of ordinaryclassical logic (weakening, contraction, associativity, exchange. . . ).
Examples:
• Lambek calculus totally rejects weakening and contraction(commutativity and associativity are optional too);
• Linear logic permits weakening and contraction only forformulas prefixed with “exponential” modalities;
• Relevant logic replaces some of the standard ‘additive’connectives, which obey weakening and contraction, with‘multiplicative’ variants which do not;
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Substructural logics: an overview
Substructural logics restrict the structural principles of ordinaryclassical logic (weakening, contraction, associativity, exchange. . . ).
Examples:
• Lambek calculus totally rejects weakening and contraction(commutativity and associativity are optional too);
• Linear logic permits weakening and contraction only forformulas prefixed with “exponential” modalities;
• Relevant logic replaces some of the standard ‘additive’connectives, which obey weakening and contraction, with‘multiplicative’ variants which do not;
• Bunched logic is like relevant logic, but retains the additiveconnectives which relevant logic throws away onphilosophical grounds (e.g. →).
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Motivation for bunched logic
• So, bunched logics are essentially obtained by “splicing” anadditive propositional logic with a multiplicative one.
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Motivation for bunched logic
• So, bunched logics are essentially obtained by “splicing” anadditive propositional logic with a multiplicative one.
• This gives a nice Kripke-style resource semantics:Additive connectives have their usual meaning, andmultiplicatives denote resource composition properties:
r |= F1 ∧ F2 ⇔ r |= F1 and r |= F2
r |= F1 ∗ F2 ⇔ r = r1 ◦ r2 and r1 |= F1 and r2 |= F2
(where ◦ is a binary monoid operation).
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Motivation for bunched logic
• So, bunched logics are essentially obtained by “splicing” anadditive propositional logic with a multiplicative one.
• This gives a nice Kripke-style resource semantics:Additive connectives have their usual meaning, andmultiplicatives denote resource composition properties:
r |= F1 ∧ F2 ⇔ r |= F1 and r |= F2
r |= F1 ∗ F2 ⇔ r = r1 ◦ r2 and r1 |= F1 and r2 |= F2
(where ◦ is a binary monoid operation).
• Taking particular models gives us separation logic andother spatial logics (used in program verification).
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The bunched logic family
Additives / multiplicatives can be classical or intuitionistic:
BI
(Heyting, Lambek)
BBI
(Boolean, Lambek)
CBI
(Boolean, de Morgan)
dMBI
(Heyting, de Morgan)
¬∼
∼¬
• Subtitles (X,Y) indicate the underlying algebras.
• Arrows denote addition of classical negations ¬ or ∼.
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Bunched logics via elementary logics
Additives: ⊤ ⊥ ¬ ∨ ∧ →Multiplicatives: ⊤∗ ⊥∗ ∼ ∗∨ ∗ —∗
• IL and CL are standard intuitionistic / classical logic overthe additives;
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Bunched logics via elementary logics
Additives: ⊤ ⊥ ¬ ∨ ∧ →Multiplicatives: ⊤∗ ⊥∗ ∼ ∗∨ ∗ —∗
• IL and CL are standard intuitionistic / classical logic overthe additives;
• LM and dMM are (commutative and associative) Lambek /de Morgan logic over the multiplicatives;
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Bunched logics via elementary logics
Additives: ⊤ ⊥ ¬ ∨ ∧ →Multiplicatives: ⊤∗ ⊥∗ ∼ ∗∨ ∗ —∗
• IL and CL are standard intuitionistic / classical logic overthe additives;
• LM and dMM are (commutative and associative) Lambek /de Morgan logic over the multiplicatives;
• Define:BI = IL + LM
BBI = CL + LMdMBI = IL + dMM
CBI = CL + dMM
where + is union of minimal proof systems for the logics.
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LBI: the BI sequent calculus
• Sequents are Γ ⊢ F where F a formula and Γ a bunch:
Γ ::= F | ∅ | ∅ | Γ ; Γ | Γ , Γ
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LBI: the BI sequent calculus
• Sequents are Γ ⊢ F where F a formula and Γ a bunch:
Γ ::= F | ∅ | ∅ | Γ ; Γ | Γ , Γ
• Rules for —∗ are:
∆ ⊢ F1 Γ(F2) ⊢ F(—∗L)
Γ(∆ , F1 —∗ F2) ⊢ F
Γ , F ⊢ G(—∗R)
Γ ⊢ F —∗ G
where Γ(∆) is bunch Γ with sub-bunch ∆;
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LBI: the BI sequent calculus
• Sequents are Γ ⊢ F where F a formula and Γ a bunch:
Γ ::= F | ∅ | ∅ | Γ ; Γ | Γ , Γ
• Rules for —∗ are:
∆ ⊢ F1 Γ(F2) ⊢ F(—∗L)
Γ(∆ , F1 —∗ F2) ⊢ F
Γ , F ⊢ G(—∗R)
Γ ⊢ F —∗ G
where Γ(∆) is bunch Γ with sub-bunch ∆;
• LBI satisfies cut-elimination (Pym ’02).
• Unfortunately cut-elimination breaks if we try to extendLBI to BBI, dMBI, CBI in the obvious way.
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Display calculus: an overview
• Display calculi manipulate consecutions X ⊢ Y , with left-and right-introduction rules for each logical connective.
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Display calculus: an overview
• Display calculi manipulate consecutions X ⊢ Y , with left-and right-introduction rules for each logical connective.
• Structures X and Y are built from formulas and structuralconnectives. Substructures of X ⊢ Y are classified asantecedent or consequent parts.
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Display calculus: an overview
• Display calculi manipulate consecutions X ⊢ Y , with left-and right-introduction rules for each logical connective.
• Structures X and Y are built from formulas and structuralconnectives. Substructures of X ⊢ Y are classified asantecedent or consequent parts.
• In display calculi, one can rearrange consecutions:
Definition
≡D is a display-equivalence if for any antecedent (consequent)part Z of X ⊢ Y we have X ⊢ Y ≡D Z ⊢ W (W ⊢ Z).
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Display calculus: an overview
• Display calculi manipulate consecutions X ⊢ Y , with left-and right-introduction rules for each logical connective.
• Structures X and Y are built from formulas and structuralconnectives. Substructures of X ⊢ Y are classified asantecedent or consequent parts.
• In display calculi, one can rearrange consecutions:
Definition
≡D is a display-equivalence if for any antecedent (consequent)part Z of X ⊢ Y we have X ⊢ Y ≡D Z ⊢ W (W ⊢ Z).
• Belnap ’82 gives a set of syntactic conditions for displaycalculi which guarantee cut-elimination.
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Display calculus: syntax
• Structures are constructed from formulas and structuralconnectives:
Additive Multiplicative Arity Antecedent Consequent∅ ∅ 0 truth falsity♯ ♭ 1 negation negation; , 2 conjunction disjunction⇒ ⊸ 2 − implication
• Antecedent / consequent parts of consecutions X ⊢ Y aresimilar to positive / negative occurrences in formulas.
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Display calculus: syntax
• Structures are constructed from formulas and structuralconnectives:
Additive Multiplicative Arity Antecedent Consequent∅ ∅ 0 truth falsity♯ ♭ 1 negation negation; , 2 conjunction disjunction⇒ ⊸ 2 − implication
• Antecedent / consequent parts of consecutions X ⊢ Y aresimilar to positive / negative occurrences in formulas.
• We give display calculi for IL,CL,LM and dMM. Form ofantecedent and consequent parts is restricted in each case.
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DLCL: a display calculus for CL
Antecedent connectives: ∅ ♯ ;Consequent connectives: ∅ ♯ ;
Display postulates: X ; Y ⊢ Z <>D X ⊢ ♯Y ; Z <>D Y ; X ⊢ Z
X ⊢ Y ; Z <>D X ; ♯Y ⊢ Z <>D X ⊢ Z ; Y
X ⊢ Y <>D ♯Y ⊢ ♯X <>D ♯♯X ⊢ Y
Logical rules:
F ⊢ X G ⊢ X(∨L)
F ∨ G ⊢ X
X ⊢ F1 ; F2
(∨R)X ⊢ F1 ∨ F2
(etc.)
Structural rules:
∅ ; X ⊢ Y======= (∅L)
X ⊢ Y
X ⊢ Z(WkL)
X ; Y ⊢ Z(etc.)
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DLLM: a display calculus for LM
Antecedent connectives: ∅ ,
Consequent connectives: ⊸
Display postulates: X , Y ⊢ Z <>D X ⊢ Y ⊸ Z <>D Y , X ⊢ Z
Logical rules:
X ⊢ F G ⊢ Y(—∗L)
F —∗ G ⊢ X ⊸ Y
X ⊢ F ⊸ G(—∗R)
X ⊢ F —∗ G(etc.)
Structural rules:
∅ , X ⊢ Y======== (∅L)
X ⊢ Y
W , (X , Y ) ⊢ Z============= (MAL)(W , X) , Y ⊢ Z
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Display calculi for bunched logicsWe obtain display calculi DLL for L ∈ {BI,BBI,dMBI,CBI} by:
DLL1+L2= DLL1
+ DLL2
where + is component-wise union of specifications.The following hold for all our calculi:
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Display calculi for bunched logicsWe obtain display calculi DLL for L ∈ {BI,BBI,dMBI,CBI} by:
DLL1+L2= DLL1
+ DLL2
where + is component-wise union of specifications.The following hold for all our calculi:
Proposition (Display)
≡D, given by the display postulates of DLL, is indeed adisplay-equivalence for DLL.
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Display calculi for bunched logicsWe obtain display calculi DLL for L ∈ {BI,BBI,dMBI,CBI} by:
DLL1+L2= DLL1
+ DLL2
where + is component-wise union of specifications.The following hold for all our calculi:
Proposition (Display)
≡D, given by the display postulates of DLL, is indeed adisplay-equivalence for DLL.
Theorem (Soundness / Completeness)
X ⊢ Y is DLL-provable iff its formula translation is provable inthe minimal proof system for L.
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Display calculi for bunched logicsWe obtain display calculi DLL for L ∈ {BI,BBI,dMBI,CBI} by:
DLL1+L2= DLL1
+ DLL2
where + is component-wise union of specifications.The following hold for all our calculi:
Proposition (Display)
≡D, given by the display postulates of DLL, is indeed adisplay-equivalence for DLL.
Theorem (Soundness / Completeness)
X ⊢ Y is DLL-provable iff its formula translation is provable inthe minimal proof system for L.
Theorem (Cut-elimination)
Any DLL proof of X ⊢ Y can be algorithmically transformedinto a cut-free DLL proof of X ⊢ Y .
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Translating LBI into DLBI
Recall the LBI rules for —∗:
∆ ⊢ F1 Γ(F2) ⊢ F(—∗L)
Γ(∆ , F1 —∗ F2) ⊢ F
Γ , F ⊢ G(—∗R)
Γ ⊢ F —∗ G
(—∗R) has a direct equivalent in DLBI, while (—∗L) can bederived in DLBI as follows:
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Translating LBI into DLBI
Recall the LBI rules for —∗:
∆ ⊢ F1 Γ(F2) ⊢ F(—∗L)
Γ(∆ , F1 —∗ F2) ⊢ F
Γ , F ⊢ G(—∗R)
Γ ⊢ F —∗ G
(—∗R) has a direct equivalent in DLBI, while (—∗L) can bederived in DLBI as follows:
∆ ⊢ F1
Γ(F2) ⊢ F(D≡)
F2 ⊢ X(—∗L)
∆ , F1 —∗ F2 ⊢ X(D≡)
Γ(∆ , F1 —∗ F2) ⊢ F
Translation preserves cut-freeness of proofs.
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Translating DLBI into LBI
For any DLBI consecution X ⊢ Y define pX ⊢ Y q as the resultof maximally applying transformations:
X ⊢ Y ⇒ Z 7→ X ; Y ⊢ Z
X ⊢ Y ⊸ Z 7→ X , Y ⊢ Z
Note pX ⊢ Y q is always an LBI sequent.
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Translating DLBI into LBI
For any DLBI consecution X ⊢ Y define pX ⊢ Y q as the resultof maximally applying transformations:
X ⊢ Y ⇒ Z 7→ X ; Y ⊢ Z
X ⊢ Y ⊸ Z 7→ X , Y ⊢ Z
Note pX ⊢ Y q is always an LBI sequent.
Then the rules of DLBI are LBI-derivable under p−q, e.g.:
pX ⊢ Fq pG ⊢ Y q
pX , F —∗ G ⊢ Y q
=X ⊢ F Γ(G) ⊢ H
Γ(X , F —∗ G) ⊢ H
Translation again preserves cut-freeness of proofs.
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Display calculi vs. sequent calculi
• By the two previous translations we have:
Proposition
There is a one-to-many correspondence between cut-free proofsin LBI and cut-free proofs in DLBI.
So LBI can be seen as an optimised DLBI.
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Display calculi vs. sequent calculi
• By the two previous translations we have:
Proposition
There is a one-to-many correspondence between cut-free proofsin LBI and cut-free proofs in DLBI.
So LBI can be seen as an optimised DLBI.
• However, display proofs for BBI, dMBI, CBI do not easilytranslate to sequent proofs in the same way. E.g., it is notobvious how to translate the DLBBI consecutionF , ♯G ⊢ H into a sequent without the unary ♯.
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Display calculi vs. sequent calculi
• By the two previous translations we have:
Proposition
There is a one-to-many correspondence between cut-free proofsin LBI and cut-free proofs in DLBI.
So LBI can be seen as an optimised DLBI.
• However, display proofs for BBI, dMBI, CBI do not easilytranslate to sequent proofs in the same way. E.g., it is notobvious how to translate the DLBBI consecutionF , ♯G ⊢ H into a sequent without the unary ♯.
• Thus we claim that our display calculi really are canonicalproof systems for the bunched logics.
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Applications
• Cut-free proof search is still very difficult (display rules,structural rules).
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Applications
• Cut-free proof search is still very difficult (display rules,structural rules).
• In general, for both display and sequent calculi:
cut-elimination 6⇒ (semi)decidability(cf. linear logic, relevant logic, arithmetic . . . )
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Applications
• Cut-free proof search is still very difficult (display rules,structural rules).
• In general, for both display and sequent calculi:
cut-elimination 6⇒ (semi)decidability(cf. linear logic, relevant logic, arithmetic . . . )
• Indeed, while BI is known decidable (Galmiche et al. ’05),BBI and CBI are known undecidable (Brotherston andKanovich ’10, Larchey-Wendling and Galmiche ’10).
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Applications
• Cut-free proof search is still very difficult (display rules,structural rules).
• In general, for both display and sequent calculi:
cut-elimination 6⇒ (semi)decidability(cf. linear logic, relevant logic, arithmetic . . . )
• Indeed, while BI is known decidable (Galmiche et al. ’05),BBI and CBI are known undecidable (Brotherston andKanovich ’10, Larchey-Wendling and Galmiche ’10).
• Cut-elimination provides structure and removes infinitebranching points from the proof search space.
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Applications
• Cut-free proof search is still very difficult (display rules,structural rules).
• In general, for both display and sequent calculi:
cut-elimination 6⇒ (semi)decidability(cf. linear logic, relevant logic, arithmetic . . . )
• Indeed, while BI is known decidable (Galmiche et al. ’05),BBI and CBI are known undecidable (Brotherston andKanovich ’10, Larchey-Wendling and Galmiche ’10).
• Cut-elimination provides structure and removes infinitebranching points from the proof search space.
• Our calculi could be potentially be used in interactivetheorem proving tools (proof-by-pointing) or to definepartial search strategies.