exercises.custom_ra

From iris.algebra Require Import cmra.
From iris.heap_lang.lib Require Export par.
From iris.heap_lang Require Import proofmode notation.

(* ################################ *)

Custom Resource Algebra

(* ================================================================= *)

A Motivating Example

In this chapter, we will define a new resource algebra from scratch. To motivate the resource algebra, we will look at the following program:

Definition prog : expr :=
  let: "l" := ref #4 in
  ("l" <- !"l" + #1) ||| ("l" <- !"l" + #1);;
  !"l".

We wish to show that the program terminates with a number that is at least 5. To do this, we will use an invariant separated into the different abstract states of our program. At the outset, the location will map to 4. During the execution of the threads, the location will be changed to either 5 or 6. However, we just care about it becoming greater than 4. As such, we will have the following states.

Inductive state :=

The starting state.

| Start

The state after the location has been increased.

| Final

An invalid state we will use to restrict the operations on the state.

| Invalid.

The state data structure will be our carrier for the RA. To use state as a resource algebra, we need to turn it into a resource algebra, meaning we need it to adhere to RAMixin. As such, we must define an equivalence relation, composition, the core, and valid elements, and prove that these definitions satisfy the fields in RAMixin.

(* ================================================================= *)

Defining the State RA

Section state.

First, we define an equivalence relation on the state. We just want states to be equivalent exactly when they are equal.

Global Instance state_equiv_instance : Equiv state := eq.
Global Instance state_equiv_equivalence : Equivalence (≡@{state}) := _.

To help convert between equivalence and equality, we can tell Iris that they coincide, which in this case is trivial.

Global Instance state_leibniz_equiv : LeibnizEquiv state := _.

Recall that resource algebras are discrete cameras and that cameras build on OFEs (Ordered family of equivalences). As such, to define a resources algebra, we must first define its OFE. An OFE encodes a kind of time dependence, but when the camera is discrete and hence a resource algebra, it does not depend on time. To quickly define a discrete OFE, we can use discrete_ofe_mixin. This will produce a warning because we are reusing a definition in a canonical projection. However, this warning can be safely ignored in this case.

Canonical stateO := Ofe state (discrete_ofe_mixin _).

Next, we define how elements of state can be combined. We define Final ⋅ Final as Final, and make every other combination Invalid. In particular, this means that Start will be exclusive, while Final can be shared.

Local Instance state_op_instance : Op state := λ s1 s2 ,
  match s1, s2 with
  | Final, Final => Final
  | _, _ => Invalid
  end.

The core then simply reflects that Start is exclusive.

Local Instance state_pcore_instance : PCore state := λ s ,
  match s with
  | Start => None
  | _ => Some s
  end.

Finally, we have to define which elements of state are valid. Naturally, we define everything except for Invalid as valid.

Local Instance state_valid_instance : Valid state := λ s ,
  match s with
  | Start | Final => True
  | Invalid => False
  end.

With everything defined, we can move on to prove RAMixin for state.

Lemma state_ra_mixin : RAMixin state.
Proof.
  split.
  - solve_proper.
  - naive_solver.
  - solve_proper.
  - by intros [] [] [].
  - by intros [] [].
  - by intros [] [].
  - by intros [] [].
  - intros [] _ [] [[] ->] e; try done.
    all: eexists; split; first done.
    all: try by exists Invalid.
    by exists Final.
  - by intros [] [].
Qed.

The final step is to package this into a CMRA structure, allowing us to use the resource algebra in proofs (using Coq's Context mechanism).

Canonical Structure stateR := discreteR state state_ra_mixin.

To help the Iris Proof Mode, we will register stateR as a discrete CMRA.

Global Instance state_cmra_discrete : CmraDiscrete state.
Proof. apply discrete_cmra_discrete. Qed.

End state.

(* ================================================================= *)

Properties of State

To alleviate working with the State RA, we here show some useful facts about the resource algebra. Firstly, Start is exclusive and Final is shareable.

Global Instance Start_exclusive : Exclusive Start.
Proof. by intros []. Qed.

Global Instance Final_core_id : CoreId Final.
Proof. red. done. Qed.

We want the ability to change the state from Start to Final. However, we will instead prove a more general fact: That any state can update to Final.

Lemma state_update s : s ~~> Final.
Proof.

As we are working with a discrete CMRA, we can simplify this statement as follows.

  rewrite cmra_discrete_update.
  intros mz H.
  by destruct s, mz as [[| |]|].
Qed.

Next, we give lemmas that help working with the State RA in Iris.

Section properties.
Context `{!inG Σ stateR}.

Our first lemma shows how a new State resource is created. We can create new resources via the basic update modality |==> P. This operation states that P holds after an update of resources. To work with the update modality, we can use two facts:

P ⊢ |==> P
(P ⊢ |==> Q) ⊢ (|==> P ⊢ |==> Q)

Rather than working with these facts directly, we can use iModIntro and iMod respectively.

Lemma alloc_Start : ⊢ |==> ∃ γ, own γ Start.
Proof.

To allocate a new resource, we use own_alloc.

iApply own_alloc.

We are naturally only allowed to allocate valid resources, but since Start is valid, this is no problem.

done.
Qed.

Likewise, owning a resource means that it is valid. A quick note: ✓ _ and ⌜✓ _⌝ are different – they only coincide when the CMRA is discrete.

Lemma state_valid γ (s : state) : own γ s ⊢ ⌜✓ s⌝.
Proof.
  iIntros "H".
  iPoseProof (own_valid with "H") as "%H".
  done.
Qed.

Next, we can lift state_update to ownership using own_update. As a shorthand for _ -∗ |==> _ we write _ ==∗ _.

Lemma state_bupd γ (s : state) : own γ s ==∗ own γ Final.
Proof.
iApply own_update.
apply state_update.
Qed.

End properties.

(* ================================================================= *)

Using the State RA in Proofs

Let us return to the motivating example from the first section of this chapter. We will be using the State RA to prove that prog terminates with a value that is at least 5.

Section proofs.

Context `{!heapGS Σ, !spawnG Σ, !inG Σ stateR}.

Let N := nroot .@ "state".

We can now define the invariant we hinted at in the beginning.

Definition state_inv γ (l : loc) (x : Z) : iProp Σ :=
∃ y : Z, l ↦ #y ∗ (own γ Start ∗ ⌜x = y⌝ ∨ own γ Final ∗ ⌜x < y⌝%Z).

Rather than proving the entire program at once, we will split it into three pieces, starting with the two threads.

Note that WP contains an update modality, meaning we can update resources in between steps.

Lemma thread_spec γ l (x : Z) : {{{inv N (state_inv γ l x)}}} #l <- !#l + #1 {{{RET #(); own γ Final}}}.
Proof.
  (* exercise *)
Admitted.

Lemma body_spec l (x : Z) : {{{l ↦ #x}}} (#l <- !#l + #1) ||| (#l <- !#l + #1);; !#l {{{(y : Z), RET #y; ⌜x < y⌝%Z}}}.
Proof.
  (* exercise *)
Admitted.

Lemma prog_spec : {{{True}}} prog {{{(y : Z), RET #y; ⌜5 ≤ y⌝%Z}}}.
Proof.
  (* exercise *)
Admitted.

End proofs.

Iris ships with a bunch of different CMRAs that you can use out of the box. Rather than creating new resources for every new situation, we can instead compose existing CMRAs. For instance, our State resource algebra corresponds to csum (excl ()) (agree ()).

Some of the available CMRAs can be found here:

https://gitlab.mpi-sws.org/iris/iris/-/tree/master/iris/algebra