Fun With Twelf

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Fun With Twelf

  1. 1. Fun with Twelf Jake Donham, Skydeck Inc.
  2. 2. What's Twelf? Twelf is a dependently-typed higher-order logic programming language and also a proof assistant useful for proofs about programming languages
  3. 3. Outline • Judgments, inference rules, derivations • Proof search, logic programming • Representing judgments, dependent types • Logic programs as proofs • Higher-order syntax, logic programming
  4. 4. What is a natural number? N nat N is a natural number • a judgment • defined by inference rules ____________ z nat zero is a natural number N nat if N is a natural number ____________ s(N) nat then N + 1 is a natural number
  5. 5. Inference rules ____________ z nat axiom N nat premise(s) ____________ s(N) nat conclusion capital N is implicitly for all
  6. 6. Examples zero is a nat 2 is a nat ___________ z nat z nat _____________ s(z) nat _____________ s(s(z)) nat derivations
  7. 7. Example? s(s(xyzzy)) nat?
  8. 8. Example? s(s(xyzzy)) nat? nope, there is no derivation (can we prove it?)
  9. 9. What is addition? ___________ sum z N N 0+N=N sum M N P if M + N = P _____________ sum s(M) N s(P) then M + 1 + N = P + 1
  10. 10. Examples sum 0 2 2 sum 0 3 3 __________ __________ sum 1 2 3 sum 1 3 4 __________ __________ sum 2 2 4 sum 2 3 5 __________ sum 3 3 6 __________ sum 4 3 7
  11. 11. Recap o judgments are things for which we have proof o judgments are defined by inference rules o derivations are proofs of a judgment o natural numbers and addition as judgments next: proof search and logic programming
  12. 12. Searching for proof __________ sum z N N sum M N P _____________ sum s(M) N s(P) does sum 2 3 5 hold? can we find a derivation?
  13. 13. Searching for proof __________ sum z N N sum M N P _____________ sum s(M) N s(P) does sum 2 3 5 hold? can we find a derivation? which rule(s) must the derivation end with?
  14. 14. Searching for proof __________ sum z N N sum M N P _____________ sum s(M) N s(P) does sum 2 3 5 hold? can we find a derivation? which rule(s) must the derivation end with? sum 1 3 4 ________ sum 2 3 5 ~ sum s(M) N s(P) [M = 1, N = 3, P = 4]
  15. 15. Searching for proof __________ sum z N N sum M N P _____________ sum s(M) N s(P) does sum 2 3 5 hold? can we find a derivation? which rule(s) must the derivation end with? sum 0 3 3 ________ sum 1 3 4 ~ sum s(M) N s(P) [M = 0, N = 3, P = 3] ________ sum 2 3 5 ~ sum s(M) N s(P) [M = 1, N = 3, P = 4]
  16. 16. Searching for proof __________ sum z N N sum M N P _____________ sum s(M) N s(P) does sum 2 3 5 hold? can we find a derivation? which rule(s) must the derivation end with? ________ sum 0 3 3 ~ sum z N N [N = 3] ________ sum 1 3 4 ~ sum s(M) N s(P) [M = 0, N = 3, P = 3] ________ sum 2 3 5 ~ sum s(M) N s(P) [M = 1, N = 3, P = 4]
  17. 17. Proof search with unification __________ sum z N N sum M N P _____________ sum s(M) N s(P) does sum 2 3 P hold for any P?
  18. 18. Proof search with unification __________ sum z N N sum M N P _____________ sum s(M) N s(P) does sum 2 3 P hold for any P? sum 1 3 P' _________ sum 2 3 P ~ sum s(M) N s(P') [M=1, N=3, P=s(P')] need to invent fresh variable P'
  19. 19. Proof search with unification __________ sum z N N sum M N P _____________ sum s(M) N s(P) does sum 2 3 P hold for any P? sum 0 3 P'' _________ sum 1 3 P' ~ sum s(M) N s(P'') [M=0, N=3, P'=s(P'')] _________ sum 2 3 P ~ sum s(M) N s(P') [M=1, N=3, P=s(P')]
  20. 20. Proof search with unification __________ sum z N N sum M N P _____________ sum s(M) N s(P) does sum 2 3 P hold for any P? _________ sum 0 3 P'' ~ sum z N N [N = 3 = P''] _________ sum 1 3 P' ~ sum s(M) N s(P'') [M=0, N=3, P'=s(P'')] _________ sum 2 3 P ~ sum s(M) N s(P') [M=1, N=3, P=s(P')]
  21. 21. Proof search with unification __________ sum z N N sum M N P _____________ sum s(M) N s(P) does sum 2 3 P hold for any P? _________ sum 0 3 P'' ~ sum z N N [N = 3 = P''] _________ sum 1 3 P' ~ sum s(M) N s(P'') [M=0, N=3, P'=s(P'')] _________ sum 2 3 P ~ sum s(M) N s(P') [M=1, N=3, P=s(P')] now substitute to find P = s(P') = s(s(P'')) = 5
  22. 22. Other modes __________ sum z N N sum M N P _____________ sum s(M) N s(P) sum 2 N 5?
  23. 23. Other modes __________ sum z N N sum M N P _____________ sum s(M) N s(P) sum 2 N 5? _________ sum 0 N 3 ~ sum z N N [N = 3] _________ sum 1 N 4 ~ sum s(M) N s(P) [M=0, P=3] _________ sum 2 N 5 ~ sum s(M) N s(P) [M=1, P=4] which args are inputs vs. outputs - mode
  24. 24. Branching, backtracking __________ sum z N N sum M N P _____________ sum s(M) N s(P) sum M N 2? unifies with both rules
  25. 25. Branching, backtracking __________ sum z N N sum M N P _____________ sum s(M) N s(P) sum M N 2? unifies with both rules [M=2, N=0] _________ [M=1, N=1] sum M'' N 0 ~ sum z N N _________ ________ sum M' N 1 ~ sum z N N | sum M' N 1 ~ sum s(M'') N s(P) [M=0, N=2] ________ ________ sum M N 2 ~ sum z N N | sum M N 2 ~ sum s(M') N s(P)
  26. 26. Recap o interpret judgments as logic programs o proof search gives rise to computation o unification of terms containing variables o viewing a judgment in different modes o branching, backtracking o sum as a logic program next: representing judgments in Twelf
  27. 27. Representing syntax ____________ z nat N nat ____________ s(N) nat in OCaml: type nat = Z | S of nat in Twelf: nat : type. z : nat. s : nat -> nat.
  28. 28. Representing derivations __________ sum z N N sum M N P _____________ sum s(M) N s(P) in OCaml: type sum = Sum_z of nat | Sum_s of nat * nat * nat * sum
  29. 29. Representing derivations __________ sum z N N sum M N P _____________ sum s(M) N s(P) in OCaml: type sum = Sum_z of nat | Sum_s of nat * nat * nat * sum but what derivation does Sum_s(1, 2, 3, Sum_z 4) represent? type sum is not adequate
  30. 30. Dependent types __________ sum z N N sum M N P _____________ sum s(M) N s(P) in Twelf: sum : nat -> nat -> nat -> type. sum_z : {N:nat} sum z N N. sum_s : {M:nat} {N:nat} {P:nat} sum M N P -> sum (s M) N (s P).
  31. 31. Dependent types __________ sum z N N sum M N P _____________ sum s(M) N s(P) in Twelf: sum : nat -> nat -> nat -> type. sum_z : {N:nat} sum z N N. sum_s : {M:nat} {N:nat} {P:nat} sum M N P -> sum (s M) N (s P). • type sum M N P is indexed by M N P • dependent type (depends on terms) • indices let us express invariant • no inadequate terms
  32. 32. Dependent types __________ sum z N N sum M N P _____________ sum s(M) N s(P) in Twelf: sum : nat -> nat -> nat -> type. sum_z : {N:nat} sum z N N. sum_s : {M:nat} {N:nat} {P:nat} sum M N P -> sum (s M) N (s P). or (with implicit arguments): sum_z : sum z N N. sum_s : sum M N P -> sum (s M) N (s P).
  33. 33. Twelf types as logic programs in Twelf: sum : nat -> nat -> nat -> type. sum_z : sum z N N. sum_s : sum s(M) N s(P) <- sum M N P. in OCaml: let rec sum : (nat * nat) -> nat = function | Z, n -> n | S m, n -> S (sum (m, n))
  34. 34. Recap o represent syntax by datatypes o represent derivations by datatypes o want adequate representation o OCaml type system is not rich enough o need dependent types o interpret Twelf datatypes as logic programs next: logic programs as proofs
  35. 35. A theorem about addition addition is commutative M+N=N+M
  36. 36. A theorem about addition addition is commutative M+N=N+M but we have not said: o sum is a function if sum M N P and sum M N P' then P = P' o sum is total for all M, N there exists P where sum M N P o we haven't even defined equality
  37. 37. A theorem about addition addition is commutative M+N=N+M
  38. 38. A theorem about addition addition is commutative M+N=N+M if sum M N P then sum N M P
  39. 39. A theorem about addition addition is commutative M+N=N+M if sum M N P then sum N M P if you give me a derivation of sum M N P I will give you a derivation of sum N M P
  40. 40. A theorem about addition addition is commutative M+N=N+M if sum M N P then sum N M P if you give me a derivation of sum M N P I will give you a derivation of sum N M P function of type sum M N P -> sum N M P Curry-Howard correspondence
  41. 41. Addition is commutative proof outline: • prove "right-handed" versions of sum rules for all N, sum N z N if sum M N P then sum M (s N) (s P) • recurse down derivation of sum M N P • build it back up using right-handed rules
  42. 42. sum : nat -> nat -> nat -> type. Addition is commutative sum_z : sum z N N. sum_s : sum M N P -> sum (s M) N (s P). sum_z' : {N} sum N z N -> type. - : sum_z' z sum_z. - : sum_z' (s N') (sum_s D) <- sum_z' N' D. sum_s' : sum M N P -> sum M (s N) (s P) -> type. - : sum_s' sum_z sum_z. - : sum_s' (sum_s D1) (sum_s D2) <- sum_s' D1 D2.
  43. 43. sum : nat -> nat -> nat -> type. Addition is commutative sum_z : sum z N N. sum_s : sum M N P -> sum (s M) N (s P). sum_z' : {N} sum N z N -> type. sum_s' : sum M N P -> sum M (s N) (s P) -> type. sum_comm : sum M N P -> sum N M P -> type. - : sum_comm sum_z D <- sum_z' _ D. - : sum_comm (sum_s D1) D3 <- sum_comm D1 D2 <- sum_s' D2 D3.
  44. 44. sum : nat -> nat -> nat -> type. Totality sum_z : sum z N N. sum_s : sum M N P -> sum (s M) N (s P). only a proof if function is total - succeeds on all inputs totality = coverage + termination sum_comm : sum M N P -> sum N M P -> type. %mode sum_comm +D1 -D2. - : sum_comm sum_z ... - : sum_comm (sum_s D1) D3 <- sum_comm D1 D2 ... %total (D1) (sum_comm D1 D2).
  45. 45. Recap o programs are proofs o functions from derivations to derivations o need adequacy of representation o need totality o write proofs as Twelf logic programs o we proved that sum is commutative next: proofs about programming languages
  46. 46. Programming languages a tiny programming language: N nat E1 exp E2 exp ________ ______________ nat(N) exp let x = E1 in E2 exp E1 exp E2 exp ___________ x is a bound variable in E2 E1 + E2 exp
  47. 47. Representing PLs N nat ________ nat(N) exp E1 exp E2 exp ___________ E1 + E2 exp in OCaml: E1 exp E2 exp type exp = ______________ | Var of string let x = E1 in E2 exp | Nat of nat | Plus of exp * exp | Let of exp * string * exp • no alpha-equivalence; choice of name matters • must implement scope, substitution manually • inadequate: what does Var "x" w/o Let represent?
  48. 48. Representing PLs N nat ________ nat(N) exp E1 exp E2 exp ___________ E1 + E2 exp another try in OCaml: E1 exp E2 exp type exp = ______________ | Nat of nat let x = E1 in E2 exp | Plus of exp * exp | Let of exp * (exp -> exp) • body of let is function that does substitution: let x = 1 in x + 2 == Let (Nat 1, (fun x -> Plus (x, Nat 2)) • unbound var inadequacy goes away • functions that branch on arg, or raise exception?
  49. 49. Representing PLs N nat ________ nat(N) exp E1 exp E2 exp ___________ E1 + E2 exp in Twelf: E1 exp E2 exp exp : type. ______________ nat : nat -> exp. let x = E1 in E2 exp plus : exp -> exp -> exp. let : exp -> (exp -> exp) -> exp. • body of let is function that does substitution: let x = 1 in x + 2 == (let (nat 1) ([x] plus x (nat 2))) • Twelf functions are very weak, just templates • adequate, alpha-equivalence
  50. 50. Higher-order logic programming count variables uses - e.g. count (let x = 1 in x + x) 2 count : exp -> nat -> type. count_nat : count (nat _) z. count_plus : count (plus E1 E2) C <- count E1 C1 <- count E2 C2 <- sum C1 C2 C.
  51. 51. Higher-order logic programming count : exp -> nat -> type. count_let : count (let E1 E2) C <- count E1 C1 <- ({x:exp}{d:count x (s z)} count (E2 x) C2) <- sum C1 C2 C. • { } indicates scoped axioms (just for enclosed goal) • x is a fresh variable • substitute E2's bound variable with x • when we find x in E2 it gets a count of 1
  52. 52. Recap • programming languages have scope and binding • want a convenient way to work with it • higher-order syntax representation • higher-order logic programming • only works because Twelf functions are weak
  53. 53. Twelf this approach scales to realistic programming lanaguages: semantics and type safety proof for Standard ML: Lee, Crary, Harper Toward a Mechanized Metatheory of Standard ML http://www.cs.cmu.edu/~rwh/papers/tslf/full.pdf formalized x86 arch. and type-safe assembly language: Crary, Sarkar Foundational Certified Code in a Metalogical Framework http://www.cs.cmu.edu/~crary/papers/2005/mafcc.pdf
  54. 54. Pointers Pfenning, Logic Progamming course notes http://www.cs.cmu.edu/~fp/courses/lp/ Pfenning, Computation and Deduction course notes http://www.cs.cmu.edu/~fp/courses/comp-ded/
  55. 55. Thanks for listening questions?

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