Add the initial version of the Dawn article.
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Require Import Coq.Lists.List.
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From Ltac2 Require Import Ltac2.
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Inductive intrinsic :=
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| swap
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| clone
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| drop
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| quote
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| compose
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| apply.
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Inductive expr :=
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| e_int (i : intrinsic)
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| e_quote (e : expr)
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| e_comp (e1 e2 : expr).
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Definition e_compose (e : expr) (es : list expr) := fold_left e_comp es e.
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Inductive IsValue : expr -> Prop :=
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| Val_quote : forall {e : expr}, IsValue (e_quote e).
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Definition value := { v : expr & IsValue v }.
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Definition value_stack := list value.
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Definition v_quote (e : expr) := existT IsValue (e_quote e) Val_quote.
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Inductive Sem_int : value_stack -> intrinsic -> value_stack -> Prop :=
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| Sem_swap : forall (v v' : value) (vs : value_stack), Sem_int (v' :: v :: vs) swap (v :: v' :: vs)
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| Sem_clone : forall (v : value) (vs : value_stack), Sem_int (v :: vs) clone (v :: v :: vs)
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| Sem_drop : forall (v : value) (vs : value_stack), Sem_int (v :: vs) drop vs
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| Sem_quote : forall (v : value) (vs : value_stack), Sem_int (v :: vs) quote ((v_quote (projT1 v)) :: vs)
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| Sem_compose : forall (e e' : expr) (vs : value_stack), Sem_int (v_quote e' :: v_quote e :: vs) compose (v_quote (e_comp e e') :: vs)
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| Sem_apply : forall (e : expr) (vs vs': value_stack), Sem_expr vs e vs' -> Sem_int (v_quote e :: vs) apply vs'
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with Sem_expr : value_stack -> expr -> value_stack -> Prop :=
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| Sem_e_int : forall (i : intrinsic) (vs vs' : value_stack), Sem_int vs i vs' -> Sem_expr vs (e_int i) vs'
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| Sem_e_quote : forall (e : expr) (vs : value_stack), Sem_expr vs (e_quote e) (v_quote e :: vs)
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| Sem_e_comp : forall (e1 e2 : expr) (vs1 vs2 vs3 : value_stack),
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Sem_expr vs1 e1 vs2 -> Sem_expr vs2 e2 vs3 -> Sem_expr vs1 (e_comp e1 e2) vs3.
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Definition false : expr := e_quote (e_int drop).
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Definition false_v : value := v_quote (e_int drop).
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Definition true : expr := e_quote (e_comp (e_int swap) (e_int drop)).
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Definition true_v : value := v_quote (e_comp (e_int swap) (e_int drop)).
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Theorem false_correct : forall (v v' : value) (vs : value_stack), Sem_expr (v' :: v :: vs) (e_comp false (e_int apply)) (v :: vs).
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Proof.
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intros v v' vs.
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eapply Sem_e_comp.
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- apply Sem_e_quote.
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- apply Sem_e_int. apply Sem_apply. apply Sem_e_int. apply Sem_drop.
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Qed.
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Theorem true_correct : forall (v v' : value) (vs : value_stack), Sem_expr (v' :: v :: vs) (e_comp true (e_int apply)) (v' :: vs).
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Proof.
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intros v v' vs.
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eapply Sem_e_comp.
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- apply Sem_e_quote.
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- apply Sem_e_int. apply Sem_apply. eapply Sem_e_comp.
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* apply Sem_e_int. apply Sem_swap.
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* apply Sem_e_int. apply Sem_drop.
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Qed.
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Definition or : expr := e_comp (e_int clone) (e_int apply).
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Theorem or_false_v : forall (v : value) (vs : value_stack), Sem_expr (false_v :: v :: vs) or (v :: vs).
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Proof with apply Sem_e_int.
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intros v vs.
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eapply Sem_e_comp...
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- apply Sem_clone.
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- apply Sem_apply... apply Sem_drop.
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Qed.
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Theorem or_true : forall (v : value) (vs : value_stack), Sem_expr (true_v :: v :: vs) or (true_v :: vs).
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Proof with apply Sem_e_int.
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intros v vs.
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eapply Sem_e_comp...
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- apply Sem_clone...
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- apply Sem_apply. eapply Sem_e_comp...
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* apply Sem_swap.
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* apply Sem_drop.
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Qed.
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Definition or_false_false := or_false_v false_v.
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Definition or_false_true := or_false_v true_v.
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Definition or_true_false := or_true false_v.
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Definition or_true_true := or_true true_v.
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Fixpoint quote_n (n : nat) :=
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match n with
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| O => e_int quote
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| S n' => e_compose (quote_n n') (e_int swap :: e_int quote :: e_int swap :: e_int compose :: nil)
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end.
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Theorem quote_2_correct : forall (v1 v2 : value) (vs : value_stack),
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Sem_expr (v2 :: v1 :: vs) (quote_n 1) (v_quote (e_comp (projT1 v1) (projT1 v2)) :: vs).
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Proof with apply Sem_e_int.
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intros v1 v2 vs. simpl.
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repeat (eapply Sem_e_comp)...
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- apply Sem_quote.
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- apply Sem_swap.
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- apply Sem_quote.
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- apply Sem_swap.
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- apply Sem_compose.
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Qed.
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Theorem quote_3_correct : forall (v1 v2 v3 : value) (vs : value_stack),
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Sem_expr (v3 :: v2 :: v1 :: vs) (quote_n 2) (v_quote (e_comp (projT1 v1) (e_comp (projT1 v2) (projT1 v3))) :: vs).
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Proof with apply Sem_e_int.
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intros v1 v2 v3 vs. simpl.
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repeat (eapply Sem_e_comp)...
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- apply Sem_quote.
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- apply Sem_swap.
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- apply Sem_quote.
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- apply Sem_swap.
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- apply Sem_compose.
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- apply Sem_swap.
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- apply Sem_quote.
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- apply Sem_swap.
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- apply Sem_compose.
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Qed.
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Ltac2 rec solve_basic () := Control.enter (fun _ =>
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match! goal with
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| [|- Sem_int ?vs1 swap ?vs2] => apply Sem_swap
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| [|- Sem_int ?vs1 clone ?vs2] => apply Sem_clone
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| [|- Sem_int ?vs1 drop ?vs2] => apply Sem_drop
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| [|- Sem_int ?vs1 quote ?vs2] => apply Sem_quote
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| [|- Sem_int ?vs1 compose ?vs2] => apply Sem_compose
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| [|- Sem_int ?vs1 apply ?vs2] => apply Sem_apply
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| [|- Sem_expr ?vs1 (e_comp ?e1 ?e2) ?vs2] => eapply Sem_e_comp; solve_basic ()
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| [|- Sem_expr ?vs1 (e_int ?e) ?vs2] => apply Sem_e_int; solve_basic ()
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| [|- Sem_e_quote ?vs1 (e_quote ?e) ?vs2] => apply Sem_quote
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| [_ : _ |- _] => ()
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end).
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Theorem quote_2_correct' : forall (v1 v2 : value) (vs : value_stack),
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Sem_expr (v2 :: v1 :: vs) (quote_n 1) (v_quote (e_comp (projT1 v1) (projT1 v2)) :: vs).
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Proof. intros. simpl. solve_basic (). Qed.
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Theorem quote_3_correct' : forall (v1 v2 v3 : value) (vs : value_stack),
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Sem_expr (v3 :: v2 :: v1 :: vs) (quote_n 2) (v_quote (e_comp (projT1 v1) (e_comp (projT1 v2) (projT1 v3))) :: vs).
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Proof. intros. simpl. solve_basic (). Qed.
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Definition rotate_n (n : nat) := e_compose (quote_n n) (e_int swap :: e_int quote :: e_int compose :: e_int apply :: nil).
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Lemma eval_value : forall (v : value) (vs : value_stack),
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Sem_expr vs (projT1 v) (v :: vs).
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Proof.
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intros v vs.
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destruct v. destruct i.
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simpl. apply Sem_e_quote.
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Qed.
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Theorem rotate_3_correct : forall (v1 v2 v3 : value) (vs : value_stack),
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Sem_expr (v3 :: v2 :: v1 :: vs) (rotate_n 1) (v1 :: v3 :: v2 :: vs).
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Proof.
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intros. unfold rotate_n. simpl. solve_basic ().
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repeat (eapply Sem_e_comp); apply eval_value.
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Qed.
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Theorem rotate_4_correct : forall (v1 v2 v3 v4 : value) (vs : value_stack),
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Sem_expr (v4 :: v3 :: v2 :: v1 :: vs) (rotate_n 2) (v1 :: v4 :: v3 :: v2 :: vs).
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Proof.
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intros. unfold rotate_n. simpl. solve_basic ().
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repeat (eapply Sem_e_comp); apply eval_value.
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Qed.
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Theorem e_comp_assoc : forall (e1 e2 e3 : expr) (vs vs' : value_stack),
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Sem_expr vs (e_comp e1 (e_comp e2 e3)) vs' <-> Sem_expr vs (e_comp (e_comp e1 e2) e3) vs'.
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Proof.
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intros e1 e2 e3 vs vs'.
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split; intros Heval.
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- inversion Heval; subst. inversion H4; subst.
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eapply Sem_e_comp. eapply Sem_e_comp. apply H2. apply H3. apply H6.
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- inversion Heval; subst. inversion H2; subst.
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eapply Sem_e_comp. apply H3. eapply Sem_e_comp. apply H6. apply H4.
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Qed.
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---
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title: "Formalizing Dawn in Coq"
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date: 2021-11-20T19:04:57-08:00
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tags: ["Coq", "Dawn"]
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draft: true
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---
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The [_Foundations of Dawn_](https://www.dawn-lang.org/posts/foundations-ucc/) article came up
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on [Lobsters](https://lobste.rs/s/clatuv/foundations_dawn_untyped_concatenative) recently.
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In this article, the author of Dawn defines a core calculus for the language, and provides its
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semantics. The definitions seemed so clean and straightforward that I wanted to try my hand at
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translating them into machine-checked code. I am most familiar with [Coq](https://coq.inria.fr/),
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and that's what I reached for when making this attempt.
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### Defining the Syntax
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#### Expressions and Intrinsics
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For the most part, this is the easy part. A Dawn expression is one of three things:
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* An "intrinsic", written \\(i\\), which is akin to a built-in function or command.
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* A "quote", written \\([e]\\), which takes a Dawn expression \\(e\\) and moves it onto the stack (Dawn is stack-based).
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* A composition of several expressions, written \\(e_1\\ e_2\\ \\ldots\\ e_n\\), which effectively evaluates them in order.
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This is straightforward to define in Coq, but I'm going to make a little simplifying change.
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Instead of making "composition of \\(n\\) expressions" a core language feature, I'll only
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allow "composition of \\(e_1\\) and \\(e_2\\)", written \\(e_1\\ e_2\\). This change does not
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in any way reduce the power of the language; we can still
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{{< sidenote "right" "assoc-note" "write \(e_1\ e_2\ \ldots\ e_n\) as \((e_1\ e_2)\ \ldots\ e_n\)." >}}
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The same expression can, of course, be written as \(e_1\ \ldots\ (e_{n-1}\ e_n)\).
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So, which way should we <em>really</em> use when translating the many-expression composition
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from the Dawn article into the two-expression composition I am using here? Well, the answer is,
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it doesn't matter; expression composition is <em>associative</em>, so both ways effectively mean
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the same thing.<br>
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<br>
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This is quite similar to what we do in algebra: the regular old addition operator, \(+\) is formally
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only defined for pairs of numbers, like \(a+b\). However, no one really bats an eye when we
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write \(1+2+3\), because we can just insert parentheses any way we like, and get the same result:
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\((1+2)+3\) is the same as \(1+(2+3)\).
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{{< /sidenote >}}
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With that in mind, we can translate each of the three types of expressions in Dawn into cases
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of an inductive data type in Coq.
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{{< codelines "Coq" "dawn/Dawn.v" 12 15 >}}
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Why do we need `e_int`? We do because a token like \\(\\text{swap}\\) can be viewed
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as belonging to the set of intrinsics \\(i\\), or the set of expressions, \\(e\\). While writing
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down the rules in mathematical notation, what exactly the token means is inferred from context - clearly
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\\(\\text{swap}\\ \\text{drop}\\) is an expression built from two other expressions. In statically-typed
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functional languages (like Coq or Haskell), however, the same expression can't belong to two different,
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arbitrary types. Thus, to turn an intrinsic into an expression, we need to wrap it up in a constructor,
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which we called `e_int` here. Other than that, `e_quote` accepts as argument another expression, `e` (the
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thing being quoted), and `e_comp` accepts two expressions, `e1` and `e2` (the two sub-expressions being composed).
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The definition for intrinsics themselves is even simpler:
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{{< codelines "Coq" "dawn/Dawn.v" 4 10 >}}
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We simply define a constructor for each of the six intrinsics. Since none of the intrinsic
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names are reserved in Coq, we can just call our constructors exactly the same as their names
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in the written formalization.
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#### Values and Value Stacks
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Values are up next. My initial temptation was to define a value much like
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I defined an intrinsic expression: by wrapping an expression in a constructor for a new data
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type. Something like:
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```Coq
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Inductive value :=
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| v_quot (e : expr).
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```
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Then, `v_quot (e_int swap)` would be the Coq translation of the expression \\([\\text{swap}]\\).
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However, I didn't decide on this approach for two reasons:
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* There are now two ways to write a quoted expression: either `v_quote e` to represent
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a quoted expression that is a value, or `e_quote e` to represent a quoted expression
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that is just an expression. In the extreme case, the value \\([[e]]\\) would
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be represented by `v_quote (e_quote e)` - two different constructors for the same concept,
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in the same expression!
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* When formalizing the lambda calculus,
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[Programming Language Foundations](https://softwarefoundations.cis.upenn.edu/plf-current/Stlc.html)
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uses an inductively-defined property to indicate values. In the simply typed lambda calculus,
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much like in Dawn, values are a subset of expressions.
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I took instead the approach from Programming Language Foundations: a value is merely an expression
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for which some predicate, `IsValue`, holds. We will define this such that `IsValue (e_quote e)` is provable,
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but also such that here is no way to prove `IsValue (e_int swap)`, since _that_ expression is not
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a value. But what does "provable" mean, here?
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By the [Curry-Howard correspondence](https://en.wikipedia.org/wiki/Curry%E2%80%93Howard_correspondence),
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a predicate is just a function that takes _something_ and returns a type. Thus, if \\(\\text{Even}\\)
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is a predicate, then \\(\\text{Even}\\ 3\\) is actually a type. Since \\(\\text{Even}\\) takes
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numbers in, it is a predicate on numbers. Our \\(\\text{IsValue}\\) predicate will be a predicate
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on expressions, instead. In Coq, we can write this as:
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{{< codelines "Coq" "dawn/Dawn.v" 19 19 >}}
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You might be thinking,
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> Huh, `Prop`? But you just said that predicates return types!
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This is a good observation; In Coq, `Prop` is a special sort of type that corresponds to logical
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propositions. It's special for a few reasons, but those reasons are beyond the scope of this post;
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for our purposes, it's sufficient to think of `IsValue e` as a type.
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Alright, so what good is this new `IsValue e` type? Well, we will define `IsValue` such that
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this type is only _inhabited_ if `e` is a value according to the Dawn specification. A type
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is inhabited if and only if we can find a value of that type. For instance, the type of natural
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numbers, `nat`, is inhabited, because any number, like `0`, has this type. Uninhabited types
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are harder to come by, but take as an example the type `3 = 4`, the type of proofs that three is equal
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to four. Three is _not_ equal to four, so we can never find a proof of equality, and thus, `3 = 4` is
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uninhabited. As I said, `IsValue e` will only be inhabited if `e` is a value per the formal
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specification of Dawn; specifically, this means that `e` is a quoted expression, like `e_quote e'`.
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To this end, we define `IsValue` as follows:
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{{< codelines "Coq" "dawn/Dawn.v" 19 20 >}}
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Now, `IsValue` is a new data type with only only constructor, `ValQuote`. For any expression `e`,
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this constructor creates a value of type `IsValue (e_quote e)`. Two things are true here:
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* Since `Val_quote` accepts any expression `e` to be put inside `e_quote`, we can use
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`Val_quote` to create an `IsValue` instance for any quoted expression.
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* Because `Val_quote` is the _only_ constructor, and because it always returns `IsValue (e_quote e)`,
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there's no way to get `IsValue (e_int i)`, or anything else.
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Thus, `IsValue e` is inhabited if and only if `e` is a Dawn value, as we intended.
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Just one more thing. A value is just an expression, but Coq only knows about this as long
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as there's an `IsValue` instance around to vouch for it. To be able to reason about values, then,
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we will need both the expression and its `IsValue` proof. Thus, we define the type `value` to mean
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a pair of two things: an expression `v` and a proof that it's a value, `IsValue v`:
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{{< codelines "Coq" "dawn/Dawn.v" 22 22 >}}
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A value stack is just a list of values:
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{{< codelines "Coq" "dawn/Dawn.v" 23 23 >}}
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### Semantics
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Remember our `IsValue` predicate? Well, it's not just any predicate, it's a _unary_ predicate.
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_Unary_ means that it's a predicate that only takes one argument, an expression in our case. However,
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this is far from the only type of predicate. Here are some examples:
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* Equality, `=`, is a binary predicate in Coq. It takes two arguments, say `x` and `y`, and builds
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a type `x = y` that is only inhabited if `x` and `y` are equal.
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* The mathematical "less than" relation is also a binary predicate, and it's called `le` in Coq.
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It takes two numbers `n` and `m` and returns a type `le n m` that is only inhabited if `n` is less
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than or equal to `m`.
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* The evaluation relation in Dawn is a ternary predicate. It takes two stacks, `vs` and `vs'`,
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and an expression, `e`, and creates a type that's inhabited if and only if evaluating
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`e` starting at a stack `vs` results in the stack `vs'`.
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Binary predicates are just functions of two inputs that return types. For instance, here's what Coq has
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to say about the type of `eq`:
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```
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eq : ?A -> ?A -> Prop
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```
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By a similar logic, ternary predicates, much like Dawn's evaluation relation, are functions
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of three inputs. We can thus write the type of our evaluation relation as follows:
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{{< codelines "Coq" "dawn/Dawn.v" 35 35 >}}
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We define the constructors just like we did in our `IsValue` predicate. For each evaluation
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rule in Dawn, such as:
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{{< latex >}}
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\langle V, v, v'\rangle\ \text{swap}\ \rightarrow\ \langle V, v', v \rangle
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{{< /latex >}}
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We introduce a constructor. For the `swap` rule mentioned above, the constructor looks like this:
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{{< codelines "Coq" "dawn/Dawn.v" 28 28 >}}
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Although the stacks are written in reverse order (which is just a consequence of Coq's list notation),
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I hope that the correspondence is fairly clear. If it's not, try reading this rule out loud:
|
||||
|
||||
> The rule `Sem_swap` says that for every two values `v` and `v'`, and for any stack `vs`,
|
||||
evaluating `swap` in the original stack `v' :: v :: vs`, aka \\(\\langle V, v, v'\\rangle\\),
|
||||
results in a final stack `v :: v' :: vs`, aka \\(\\langle V, v', v\\rangle\\).
|
||||
|
||||
With that in mind, here's a definition of a predicate `Sem_int`, the evaluation predicate
|
||||
for intrinsics:
|
||||
|
||||
{{< codelines "Coq" "dawn/Dawn.v" 27 33 >}}
|
||||
|
||||
Hey, what's all this with `v_quote` and `projT1`? It's just a little bit of bookkeeping.
|
||||
Given a value -- a pair of an expression `e` and a proof `IsValue e` -- the function `projT1`
|
||||
just returns the expression `e`. That is, it's basically a way of converting a value back into
|
||||
an expression. The function `v_quote` takes us in the other direction: given an expression \\(e\\),
|
||||
it constructs a quoted expression \\([e]\\), and combines it with a proof that the newly constructed
|
||||
quote is a value.
|
||||
|
||||
The above two function in combination help us define the `quote` intrinsic, which
|
||||
wraps a value on the stack in an additional layer of quotes. When we create a new quote, we
|
||||
need to push it onto the value stack, so it needs to be a value; we thus use `v_quote`. However,
|
||||
`v_quote` needs an expression to wrap in a quote, so we use `projT1` to extract the expression from
|
||||
the value on top of the stack.
|
||||
|
||||
In addition to intrinsics, we also define the evaluation relation for actual expressions.
|
||||
|
||||
{{< codelines "Coq" "dawn/Dawn.v" 35 39 >}}
|
||||
|
||||
Here, we may as well go through the three constructors to explain what they mean:
|
||||
|
||||
* `Sem_e_int` says that if the expression being evaluated is an intrinsic, and if the
|
||||
intrinsic has an effect on the stack as described by `Sem_int` above, then the effect
|
||||
of the expression itself is the same.
|
||||
* `Sem_e_quote` says that if the expression is a quote, then a corresponding quoted
|
||||
value is placed on top of the stack.
|
||||
* `Sem_e_comp` says that if one expression `e1` changes the stack from `vs1` to `vs2`,
|
||||
and if another expression `e2` takes this new stack `vs2` and changes it into `vs3`,
|
||||
then running the two expressions one after another (i.e. composing them) means starting
|
||||
at stack `vs1` and ending in stack `vs3`.
|
||||
|
||||
### \\(\\text{true}\\), \\(\\text{false}\\), \\(\\text{or}\\) and Proofs
|
||||
Now it's time for some fun! The Dawn language specification starts by defining two values:
|
||||
true and false. Why don't we do the same thing?
|
||||
|
||||
|Dawn Spec| Coq encoding |
|
||||
|---|----|
|
||||
|\\(\\text{false}\\)=\\([\\text{drop}]\\)| {{< codelines "Coq" "dawn/Dawn.v" 41 42 >}}
|
||||
|\\(\\text{true}\\)=\\([\\text{swap} \\ \\text{drop}]\\)| {{< codelines "Coq" "dawn/Dawn.v" 44 45 >}}
|
||||
|
||||
Let's try prove that these two work as intended.
|
||||
|
||||
{{< codelines "Coq" "dawn/Dawn.v" 47 53 >}}
|
||||
|
||||
This is the first real proof in this article. Rather than getting into the technical details,
|
||||
I invite you to take a look at the "shape" of the proof:
|
||||
|
||||
* After the initial use of `intros`, which brings the variables `v`, `v`, and `vs` into
|
||||
scope, we start by applying `Sem_e_comp`. Intuitively, this makes sense - at the top level,
|
||||
our expression, \\(\\text{false}\\ \\text{apply}\\),
|
||||
is a composition of two other expressions, \\(\\text{false}\\) and \\(\\text{apply}\\).
|
||||
Because of this, we need to use the rule from our semantics that corresponds to composition.
|
||||
* The composition rule requires that we describe the individual effects on the stack of the
|
||||
two constituent expressions (recall that the first expression takes us from the initial stack `v1`
|
||||
to some intermediate stack `v2`, and the second expression takes us from that stack `v2` to the
|
||||
final stack `v3`). Thus, we have two "bullet points":
|
||||
* The first expression, \\(\\text{false}\\), is just a quoted expression. Thus, the rule
|
||||
`Sem_e_quote` applies, and the contents of the quote are puhsed onto the stack.
|
||||
* The second expression, \\(\\text{apply}\\), is an intrinsic, so we need to use the rule
|
||||
`Sem_e_int`, which handles the intrinsic case. This, in turn, requires that we show
|
||||
the effect of the intrinsic itself; the `apply` intrinsic evaluates the quoted expression
|
||||
on the stack.
|
||||
The quoted expression is contains the body of false, or \\(\\text{drop}\\). This is
|
||||
once again an intrinsic, so we use `Sem_e_int`; the intrinsic in question is \\(\\text{drop}\\),
|
||||
so the `Sem_drop` rule takes care of that.
|
||||
|
||||
Following these steps, we arrive at the fact that evaluating `false` on the stack simply drops the top
|
||||
element, as specified. The proof for \\(\\text{true}\\) is very similar in spirit:
|
||||
|
||||
{{< codelines "Coq" "dawn/Dawn.v" 55 63 >}}
|
||||
|
||||
We can also formalize the \\(\\text{or}\\) operator:
|
||||
|
||||
|Dawn Spec| Coq encoding |
|
||||
|---|----|
|
||||
|\\(\\text{or}\\)=\\(\\text{clone}\\ \\text{apply}\\)| {{< codelines "Coq" "dawn/Dawn.v" 65 65 >}}
|
||||
|
||||
We can write two top-level proofs about how this works: the first says that \\(\\text{or}\\),
|
||||
when the first argument is \\(\\text{false}\\), just returns the second argument (this is in agreement
|
||||
with the truth table, since \\(\\text{false}\\) is the identity element of \\(\\text{or}\\)).
|
||||
The proof proceeds much like before:
|
||||
|
||||
{{< codelines "Coq" "dawn/Dawn.v" 67 73 >}}
|
||||
|
||||
To shorten the proof a little bit, I used the `Proof with` construct from Coq, which runs
|
||||
an additional _tactic_ (like `apply`) whenever `...` is used.
|
||||
Because of this, in this proof writing `apply Sem_apply...` is the same
|
||||
as `apply Sem_apply. apply Sem_e_int`. Since the `Sem_e_int` rule is used a lot, this makes for a
|
||||
very convenient shorthand.
|
||||
|
||||
Similarly, we prove that \\(\\text{or}\\) applied to \\(\\text{true}\\) always returns \\(\\text{true}\\).
|
||||
|
||||
{{< codelines "Coq" "dawn/Dawn.v" 75 83 >}}
|
||||
|
||||
Finally, the specific facts (like \\(\\text{false}\\ \\text{or} \\text{false}\\) evaluating to \\(\\text{false}\\))
|
||||
can be expressed using our two new proofs, `or_false_v` and `or_true`.
|
||||
|
||||
{{< codelines "Coq" "dawn/Dawn.v" 85 88 >}}
|
||||
|
||||
### Derived Expressions
|
||||
#### Quotes
|
||||
The Dawn specification defines \\(\\text{quote}_n\\) to make it more convenient to quote
|
||||
multiple terms. For example, \\(\\text{quote}_2\\) composes and quotes the first two values
|
||||
on the stack. This is defined in terms of other Dawn expressions as follows:
|
||||
|
||||
{{< latex >}}
|
||||
\text{quote}_n = \text{quote}_{n-1}\ \text{swap}\ \text{quote}\ \text{swap}\ \text{compose}
|
||||
{{< /latex >}}
|
||||
|
||||
We can define this in Coq as follows:
|
||||
|
||||
{{< codelines "Coq" "dawn/Dawn.v" 90 94 >}}
|
||||
|
||||
This definition diverges slightly from the one given in the Dawn specification; particularly,
|
||||
Dawn's spec mentions that \\(\\text{quote}_n\\) is only defined for \\(n \\geq 1\\).However,
|
||||
this means that in our code, we'd have to somehow handle the error that would arise if the
|
||||
term \\(\\text{quote}\_0\\) is used. Instead, I defined `quote_n n` to simply mean
|
||||
\\(\\text{quote}\_{n+1}\\); thus, in Coq, no matter what `n` we use, we will have a valid
|
||||
expression, since `quote_n 0` will simply correspond to \\(\\text{quote}_1 = \\text{quote}\\).
|
||||
|
||||
We can now attempt to prove that this definition is correct by ensuring that the examples given
|
||||
in the specification are valid. We may thus write,
|
||||
|
||||
{{< codelines "Coq" "dawn/Dawn.v" 96 106 >}}
|
||||
|
||||
We used a new tactic here, `repeat`, but overall, the structure of the proof is pretty straightforward:
|
||||
the definition of `quote_n` consists of many intrinsics, and we apply the corresponding rules
|
||||
one-by-one until we arrive at the final stack. Writing this proof was kind of boring, since
|
||||
I just had to see which intrinsic is being used in each step, and then write a line of `apply`
|
||||
code to handle that intrinsic. This gets worse for \\(\\text{quote}_3\\):
|
||||
|
||||
{{< codelines "Coq" "dawn/Dawn.v" 108 122 >}}
|
||||
|
||||
It's so long! Instead, I decided to try out Coq's `Ltac2` mechanism to teach Coq how
|
||||
to write proofs like this itself. Here's what I came up with:
|
||||
|
||||
{{< codelines "Coq" "dawn/Dawn.v" 124 136 >}}
|
||||
|
||||
You don't have to understand the details, but in brief, this checks what kind of proof
|
||||
we're asking Coq to do (for instance, if we're trying to prove that a \\(\\text{swap}\\)
|
||||
instruction has a particular effect), and tries to apply a corresponding semantic rule.
|
||||
Thus, it will try `Sem_swap` if the expression is \\(\\text{swap}\\),
|
||||
`Sem_clone` if the expression is \\(\\text{clone}\\), and so on. Then, the two proofs become:
|
||||
|
||||
{{< codelines "Coq" "dawn/Dawn.v" 138 144 >}}
|
||||
|
||||
#### Rotations
|
||||
There's a little trick to formalizing rotations. There's an important property of values:
|
||||
when a value is run against a stack, all it does is place itself on a stack. We can state
|
||||
this as follows:
|
||||
|
||||
{{< latex >}}
|
||||
\langle V \rangle\ v = \langle V\ v \rangle
|
||||
{{< /latex >}}
|
||||
|
||||
Or, in Coq,
|
||||
|
||||
{{< codelines "Coq" "dawn/Dawn.v" 148 149 >}}
|
||||
|
||||
This is the trick to how \\(\\text{rotate}_n\\) works: it creates a quote of \\(n\\) reordered and composed
|
||||
values on the stack, and then evaluates that quote. Since evaluating each value
|
||||
just places it on the stack, these values end up back on the stack, in the same order that they
|
||||
were in the quote. When writing the proof, `solve_basic ()` gets us almost all the way to the
|
||||
end (evaluating a list of values against a stack). Then, we simply apply the composition
|
||||
rule over and over, following it up with `eval_value` to prove that the each value is just being
|
||||
placed back on the stack.
|
||||
|
||||
{{< codelines "Coq" "dawn/Dawn.v" 156 168 >}}
|
||||
|
||||
### `e_comp` is Associative
|
||||
When composing three expressions, which way of pressing parentheses is correct?
|
||||
Is it \\((e_1\\ e_2)\\ e_3\\)? Or is it \\(e_1\\ (e_2\\ e_3)\\)? Well, neither!
|
||||
Expression composition is associative, which means that the order of the parentheses
|
||||
doesn't matter. We state this in the following theorem, which says that the two
|
||||
ways of writing the composition, if they evaluate to anything, evaluate to the same thing.
|
||||
|
||||
{{< codelines "Coq" "dawn/Dawn.v" 170 171 >}}
|
||||
|
||||
That's all I've got in me for today. However, we got pretty far! The Dawn specification
|
||||
says:
|
||||
|
||||
> One of my long term goals for Dawn is to democratize formal software verification in order to make it much more feasible and realistic to write perfect software.
|
||||
|
||||
I think that Dawn is definitely getting there: formally defining the semantics outlined
|
||||
on the page was quite straightforward! We can now have complete confidence in the behavior
|
||||
of \\(\\text{true}\\), \\(\\text{false}\\), \\(\\text{or}\\), \\(\\text{quote}_n\\) and
|
||||
\\(\\text{rotate}_n\\). The proof of associativity is also enough to possibly argue for simplifying
|
||||
the core calculus' syntax even more. All of this we got from an official source, with only
|
||||
a little bit of tweaking to get from the written description of the language to code! I'm
|
||||
looking forward to reading the next post about the _multistack_ concatenative calculus.
|
||||
Loading…
Reference in New Issue