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Proofs and Programs Wei Hu 11/01/2007

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Outline Motivation Theory Lambda calculus Curry-Howard Isomorphism Dependent types Practice Coq Wei Hu2

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Motivation - Why Learn Coq Helps understand PL theory better Good for CS615 (sadly, not quals) Coq is becoming popular People rethinking trusted software Functional programming is gaining attention Has been used to Prove mathematical theorems Specify hardware Recently, papers being published for Operating systems Compilers 3Wei Hu

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Coq is an Interactive Theorem Prover Two types of theorem provers Automated TP Simplify, CVC, … Interactive TP (aka, Proof Assistants) Coq, PVS, ACL2, HOL, Isabelle, Twelf, NuPRL, Agda Coq’s highlights Higher-order intuitionistic logic Higher-order type theory Embedded functional programming language Goal transformation through tactics Mainly works interactively, with limited support of automation to discharge trivial propositions 4Wei Hu

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Coq as a Programming Language ML-like Fixpoint is_even (n:nat) : bool := match n with | 0 => true | 1 => false | S (S n') => is_even n’ end. Eval compute in is_even 3. Let’s try it out! Restrictions No side effects No non-terminating programs (to avoid inconsistency) 5Wei Hu

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Program Extraction Fixpoint is_even (n:nat) : bool := match n with | 0 => true | 1 => false | S (S n') => is_even n’ end. let rec is_even = function | O -> True | S n0 -> (match n0 with | O -> False | S n' -> is_even n') is_even n = case n of O -> True S n0 -> (case n0 of O -> False S n' -> is_even n') OCaml Coq Haskell 6Wei Hu

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Nothing Is Built-in! Inductive bool : Set := | true : bool | false : bool. Inductive nat : Set := | O : nat | S : nat -> nat. type bool = | True | False type nat = | O | S of nat data Bool = True | False data Nat = O | S Nat OCaml Coq (theories/Init/Datatypes.v) Haskell 7Wei Hu

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Simply Typed Lambda Calculus Type Base types: nat, bool, … Function types: nat->nat, nat->bool, … Term Variables: x, y, z Abstractions: x: T1. t2 Applications: M N Environment ( ) Gives typing for variables 8Wei Hu

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Typing Rules , x: T1 |- t2 : T2 |- x: T1. t2 : T1 -> T2 (Abs) |- x : T (Var) x : T |- t2 : T11 |- t1 t2 : T12 (App) |- t1 : T11 -> T12 The typing rules work equally well in logic! 9Wei Hu

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Curry-Howard Isomorphism Propositions = Types Proofs (inhabit Props) = Programs (inhabit Sets) Proof tactics = Program constructs So what? Proof checking now becomes type checking (can be undecidable if non-termination is allowed) Programs and proofs are organically unified Why is it hard to prove something? Unlike general programming, we are doing type-level programming In other words, we are constructing programs for a given type Tactics help with the construction 10Wei Hu

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C-H at Work Section example1. Hypothesis A B : Prop. Hypothesis C : B -> A. Lemma Var : B -> A. exact C. Qed. Lemma Abs : A -> A. intros. exact H. Qed. Lemma App : A -> (A -> B) -> B. intros. apply H0. exact H. Qed. , x: T1 |- t2 : T2 |- x: T1. t2 : T1 -> T2 |- x : T x : T |- t2 : T11 |- t1 t2 : T12 |- t1 : T11 -> T12 function or implication? 11Wei Hu

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Type-centric View -> is primitive, others not theories/Init/Logic.v Inductive True : Prop := I : True. Inductive False : Prop :=. Definition not (A:Prop) := A -> False. Inductive and (A B:Prop) : Prop := conj : A -> B -> A /\ B. Inductive or (A B:Prop) : Prop := | or_introl : A -> A \/ B | or_intror : B -> A \/ B. 12Wei Hu BHK Interpretation (Brouwer-Heyting- Kolmogorov)

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Intuitionistic (Constructive) Logic vs. Classical Logic A set of equivalent axioms missing in IL Law of excluded middle: P ∨ ~P Double negation rule: ~~P → P Peirce’s Law: ((P → Q) → P) → P The correspondence between call/cc and Pierce’s Law See Coq FAQ #30 “What axioms can be safely added to Coq?” CL takes a denotational view Assigns to every variable a value ( true or false ) Builds truth tables for primitive operations /\ ∨ ~ (P -> Q) ≡ (~P ∨ Q ) Wei Hu13

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Polymorphism and Universal Quantification Parametric polymorphism You have been using it all the time! Polymorphic functions Definition id := fun (X : Set) (x : X) => x. Check id. id : forall X : Set, X -> X. Polymorphic data types (e.g., lists) Universal quantification Lemma refl : forall P : Prop, P -> P. intros. apply H. Qed. Print refl. refl = fun (P : Prop) (H : P) => H : forall P : Prop, P -> P 14Wei Hu

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Dependent Types Type systems in ML-like languages are just not expressive enough We want to specify propositions like ∀ n:nat, n ≤ n The result type ( n ≤ n ) depends on arguments ( n ) We generalize arrows (A -> B, or λx:A. B) to dependent products (∏x:A. B) where B is dependent on x. In Coq: forall n:nat, n<=n forall m n: nat, m + n = n + m 15Wei Hu

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Dependent Types Compatibility with non-dependent product The type of a function that builds a pair: forall (A B:Set) (a:A) (b:B), A*B forall A B : Set, A -> B -> A * B Defining existential ∀ x, (P x -> ∃ x, P x) Dependent sums Dependent types in programming Epigram (http://www.e-pig.org/)http://www.e-pig.org/ Ynot (http://www.eecs.harvard.edu/~greg/ynot.html)http://www.eecs.harvard.edu/~greg/ynot.html Concoqtion (http://www.metaocaml.org/concoqtion/)http://www.metaocaml.org/concoqtion/ Omega (http://web.cecs.pdx.edu/~sheard/)http://web.cecs.pdx.edu/~sheard/ ATS (http://www.cs.bu.edu/~hwxi/ATS/)http://www.cs.bu.edu/~hwxi/ATS/ Wei Hu16

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Polymorphism vs. Dependent Types Polymorphism Terms can take types as arguments Dependent types Types can take terms as arguments Inductive types and predicates Generalization of conventional user-defined data types We are talking about parameterized data types (e.g. vector n ) and propositions (e.g. m <= n ) Wei Hu17

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Conclusions We talked about type theory Curry-Howard correspondence is cool Dependent types matter * We did not cover common proof techniques Proof by induction Use of tactics and tacticals * T. Altenkirch, C. McBride, and J. McKinna, “Why Dependent Types Matter” Wei Hu18

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How to Learn Coq? Resources (for beginners) Hardest parts (IMO) Design/Formalization Combination of tactics Tools are getting better, but still hard What’s the next breakthrough? 19Wei Hu

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