1. Isabelle / HOL Theorem Proving System
Quang M. Tran
Isabelle / HOL Theorem Proving System
Course: CAS760 Logic for Practical Use
Instructor: Dr. William M. Farmer
Department of Computing and Software
McMaster University, ON, Hamilton, Canada

2. Outline History overview
Isabelle / HOL first taste: screenshot + Prove: rev (rev list) = list
Isabelle / HOL: big picture + terminologies
Natural deduction: Prove P v Q => Q v P
Isabelle classical reasoner
References + Conclusion
Conclusion
Isabelle / HOL

3. History: All started with Robin Milner
British computer scientist.
1972: Milner developed proof checker for Scott’s “Logic for Computable Functions” (LCF) at Stanford (known as “Stanford LCF” ).
1973: Milner moved to Edinburgh and started the successor project “LCF Edinburgh”. ML language is born in this time.
1981: Mike Gordon joined Cambridge and HOL was born.
1990s: Larry Paulson developed Isabelle.
Milner
Paulson
Isabelle / HOL

4. Isabelle / HOL first taste : Fun with (Toy)List
(*ToyList.thy*) theory ToyList imports Datatype begin (*Datatype of list*) datatype 'a list = Nil ("[]") | Cons 'a "'a list" (infixr ":" 65) … (contd. next slide)
CAS760: list as inductive data type, remember?
Syntax annotation
Official syntax []
Nil
10 : []
Cons 10 Nil
15 : 10 : []
Cons 15 (Cons 10 Nil)
Isabelle / HOL

5. Isabelle / HOL first taste (contd.)
(*Functions on lists*) primrec concat :: "'a list => 'a list => 'a list" (infixr “++" 65) where "[] ++ ys = ys" | "(x : xs) ++ ys = x : (xs ++ ys)“ primrec rev :: "'a list => 'a list" where "rev [] = [] " | "rev (x : xs) = (rev xs) ++ (x : [])"
“++” is defined by primitive recursion.
Syntax annotation
Official syntax
(10 : []) ++ []
concat (Cons 10 Nil) Nil
(10 : []) ++ (5 : [])
concat (Cons 10 Nil) (5 : Nil)
Isabelle / HOL

6. Proof: rev (rev list) = list
Main goal:
rev(rev xs) = xs
Subgoal 1:
rev (rev []) = []
Simplifier can solve it:
rev (rev []) = rev [] = []
Done!
Subgoal 2:
Forall a list. rev (rev list) = list => rev (rev (a : list)) = a : list
Simplifier reduces to subgoal 2.1:
Forall a list. rev (rev list) = list => rev (rev list ++ a : []) = a : list
Generate subgoals
Simplified
Simplified …
theorem rev_rev [simp]: "rev(rev xs) = xs”
apply(induct_tac xs) (*Apply induction tactic*)
apply(auto) (*Try to solve “automatically” using simplifier)
Isabelle / HOL

7. Proof (contd.) We need a lemma
rev (xs ++ ys) = (rev ys) ++ (rev xs)
Subgoal 1:
rev ([] ++ ys) = rev ys ++ rev []
Subgoal 2:
Forall a list. rev (list ++ ys) = rev ys ++ rev list
=> rev ((a : list) ++ ys) = rev ys ++ rev (a : list)
a list. rev (list ++ ys) = rev ys ++ rev list
=> (rev ys ++ rev list) ++ a : [] = rev ys ++ rev list ++ a : []
Find subgoals …
lemma rev_app [simp]: "rev(xs ++ ys) = (rev ys) ++ (rev xs)"
apply(induct_tac xs) (*Apply induction tactic on xs*)
apply(auto) (*Try to solve “automatically” using simplifier)
Isabelle / HOL

8. Complete Proof: rev(rev list) = list
… lemma app_assoc [simp]: "(xs ++ ys) ++ zs = xs ++ (ys ++ zs)" apply(induct_tac xs) apply(auto) done lemma app_Nil2 [simp]: "xs ++ [] = xs" lemma rev_app [simp]: "rev(xs ++ ys) = (rev ys) ++ (rev xs)" theorem rev_rev [simp]: "rev(rev xs) = xs"
We need to prove 3 supporting lemmas , i.e. “Backward” proof
This is the theorem what we want to prove
Isabelle / HOL

9. Isabelle: big picture Proof General For Isabelle Isabelle / HOL
(X)Emacs GUI for theorem provers
A concrete Isabelle instance for Higher- Order-Logic (HOL)
Proof General For Isabelle
Isabelle / ZF
Isabelle / HOL
Isabelle / Your Logic Here
Provides a generic infrastructure to develop theorem provers.
Isabelle
Isabelle / HOL

10. Theorem proving terminologies
Meaning
Examples
Theorem
The formula we want to prove
rev (rev list) = list
Lemma
Supporting (sub)theorems for proving the target theorem
rev (xs ++ ys) = (rev ys) ++ (rev xs)
Tactic
Produces subgoals from a goal
apply(induct_tac)
Simplification
The process of simplifying a term / formula by repeated application of rewrite rules
rev (rev []) = rev [] = []
Isabelle / HOL

11. Natural deduction By the German mathematician and logician Gentzen.
Motivation: Logical formalism that occurs “naturally” (closely to human reasoning).
Assume: “If pigs can fly, then there are green men on Mars” is true.
You see a pig flies in Hamilton? Then there are green men on Mars!
Gentzen
Modus Ponens. This is true for arbitrary P, Q
Isabelle / HOL

12. Natural deduction: Inference rules
Introduction (intro.)
Elimination (elim.)
Conjunction elim.
Conjunction intro.
Disjunction intro.
Disjunction elim.
Implication elim. (modus pones !)
Implication intro.
Isabelle / HOL

13. Natural deduction (contd.)
Introduction (intro.)
Elimination (elim.)
Existential quantifier intro.
Existential quantifier elim.
Universal quantifier intro.
Universal quantifier elim.
Isabelle / HOL

14. Proof: P v Q => Q v P lemma disj_swap: "P v Q => Q v P"
Applies disjunction elim. rule:
lemma disj_swap: "P v Q => Q v P"
apply (erule disjE)
Subgoals:
P => Q v P (1)
Q => Q v P (2)
apply (rule disjI2)
Subgoal:
P => P
apply assumption
(*Likewise for (2)*)
apply (rule disjI1)
done
Applies disjunction intro. rule (2) :
Isabelle / HOL

15. Isabelle’s classical reasoner
Working with primitive rules like before are tedious.
Classical reasoner = a family of tools that perform proofs automatically.
Examples: blast method.
lemma disj_swap2: "P v Q => Q v P" apply (blast) No subgoals! Done!
“blast“ can solve this automatically
Isabelle / HOL

16. Proof: P v Q => Q v P
Demo
Isabelle / HOL

17. References
Isabelle newcomers: A Proof Assistant for Higher-Order Logic, written by Isabelle authors e.g. C. Paulson, online PDF available.
Historical development: From LCF to HOL: a short history , Mike Gordon and The next 700 Theorem Provers, C. Paulson.
Theorem prover design techniques: Design a Theorem Prover, C. Paulson.
Isabelle / HOL

18. Conclusion: Should I bother with Isabelle?
If you need computer-aided proofs, e.g. formal verification.
If you want to deepen your knowledge in logics / mathematics / functional programming.
If you have interest in mechanizing mathematics.
… then the answer is Yes.
Isabelle can be used as a tool to get work done or simply a platform to experiment and study.
Isabelle / HOL

19. Create your first workbook
Acknowledgements
The author is grateful to Tian Zhang, Eden Burton and Bojan Nokovic (ITB 206) for their very useful feedbacks while preparing this presentation.
Create your first workbook

20. The End
Comments?
Questions?
Isabelle / HOL