305 lines
13 KiB
JavaScript
305 lines
13 KiB
JavaScript
"use strict";
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var Syndicate = require('../../src/main.js');
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/// -->
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/// # {{ page.title }}
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///
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/// This is an extended two-buyer book-purchase protocol, based
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/// loosely on an example given in:
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///
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/// > K. Honda, N. Yoshida, and M. Carbone, “Multiparty asynchronous
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/// > session types,” POPL 2008.
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/// ## The Scenario
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///
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/// A book-seller responds to requests for book prices when asked. A
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/// pair of prospective buyers run through a shopping list. For each
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/// book, the first buyer offers to split the cost of the book with
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/// the second. If the second has enough money left, it accepts;
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/// otherwise, it rejects the offer, and the first buyer tries a
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/// different split. If the second buyer agrees to a split, it then
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/// negotiates the purchase of the book with the book-seller.
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/// ## The Protocol
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/// ### Role: SELLER
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///
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/// When interest in `bookQuote($title, _)` appears, asserts
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/// `bookQuote(title, Maybe Float)`, `false` meaning not available,
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/// and otherwise an asking-price.
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///
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/// When interest in `order($title, $offer-price, _, _)` appears,
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/// asserts `order(title, offer-price, false, false)` for "no sale",
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/// otherwise `order(title, offer-price, PositiveInteger, String)`, an
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/// accepted sale.
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/// ### Role: BUYER
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///
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/// Observes `bookQuote(title, $price)` to learn prices.
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///
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/// Observes `order(title, offer-price, $id, $delivery-date)` to make orders.
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/// ### Role: SPLIT-PROPOSER
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///
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/// Observes `splitProposal(title, asking-price, contribution,
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/// $accepted)` to make a split-proposal and learn whether it was
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/// accepted or not.
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/// ### Role: SPLIT-DISPOSER
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///
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/// When interest in `splitProposal($title, $asking-price,
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/// $contribution, _)` appears, asserts `splitProposal(title,
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/// askingPrice, contribution, true)` to indicate they are willing to
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/// go through with the deal, in which case they then perform the role
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/// of BUYER for title/asking-price, or asserts `splitProposal(title,
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/// asking-price, contribution, false)` to indicate they are unwilling
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/// to go through with the deal.
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/// ## The Code
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/// ### Type Declarations
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/// First, we declare *assertion types* for our protocol.
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var bookQuote = Syndicate.Struct.makeConstructor("bookQuote", ["title","price"]);
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var order = Syndicate.Struct.makeConstructor("order", ["title","price","id","deliveryDate"]);
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var splitProposal = Syndicate.Struct.makeConstructor("splitProposal", ["title","price","contribution","accepted"]);
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/// ### Utilities
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/// This routine is under consideration for possible addition to the
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/// core library.
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///
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function whileRelevantAssert(P) {
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Syndicate.Actor.spawnActor(function() { Syndicate.Actor.Facet.build(function () { {
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Syndicate.Actor.Facet.current.addAssertion((function() { var _ = Syndicate.__; return Syndicate.Patch.assert(P, 0); }));
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Syndicate.Actor.Facet.current.onEvent(Syndicate.Actor.PRIORITY_NORMAL, true, "retracted", (function() { var _ = Syndicate.__; return Syndicate.Patch.sub(Syndicate.observe(P), 0); }), (function() { var _ = Syndicate.__; return { assertion: Syndicate.observe(P), metalevel: 0 }; }), (function() {}));
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} }); });
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}
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/// ### Implementation: SELLER
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function seller() {
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Syndicate.Actor.spawnActor(function() { Syndicate.Actor.Facet.build(function () { {
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/// We give our actor two state variables: a dictionary recording our
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/// inventory of books (mapping title to price), and a counter
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/// tracking the next order ID to be allocated.
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///
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/// We mark each property (entry) in the `books` table as a *field* so
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/// that dependency-tracking on a per-book basis is enabled. As a result,
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/// when a book is sold, any client still interested in its price will
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/// learn that the book is no longer available.
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///
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/// We do not enable dependency-tracking for either the `books` table
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/// itself or the `nextOrderId` field: nothing depends on tracking
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/// changes in their values.
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this.books = {};
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Syndicate.Actor.declareField(this.books, "The Wind in the Willows", 3.95);
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Syndicate.Actor.declareField(this.books, "Catch 22", 2.22);
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Syndicate.Actor.declareField(this.books, "Candide", 34.95);
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this.nextOrderId = 10001483;
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/// Looking up a price yields `false` if no such book is in our
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/// inventory.
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this.priceOf = function (title) {
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return (title in this.books) && Syndicate.Actor.referenceField(this.books, title);
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};
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/// The seller responds to interest in bookQuotes by asserting a
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/// responsive record, if one exists.
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Syndicate.Actor.Facet.current.onEvent(Syndicate.Actor.PRIORITY_NORMAL, false, "asserted", (function() { var _ = Syndicate.__; return Syndicate.Patch.sub(Syndicate.observe(bookQuote(_, _)), 0); }), (function() { var _ = Syndicate.__; return { assertion: Syndicate.observe(bookQuote((Syndicate._$("title")), _)), metalevel: 0 }; }), (function(title) {
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var _cachedAssertion1522142580703_0 = (function() { var _ = Syndicate.__; return Syndicate.observe(bookQuote(title, _)); }).call(this);
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{ Syndicate.Actor.Facet.build(function () { {
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Syndicate.Actor.Facet.current.addAssertion((function() { var _ = Syndicate.__; return Syndicate.Patch.assert(bookQuote(title, this.priceOf(title)), 0); }));
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}
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Syndicate.Actor.Facet.current.onEvent(Syndicate.Actor.PRIORITY_NORMAL, true, "retracted", (function() { var _ = Syndicate.__; return Syndicate.Patch.sub(_cachedAssertion1522142580703_0, 0); }), (function() { var _ = Syndicate.__; return { assertion: _cachedAssertion1522142580703_0, metalevel: 0 }; }), (function() {})); }); }}));
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/// It also responds to order requests.
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Syndicate.Actor.Facet.current.onEvent(Syndicate.Actor.PRIORITY_NORMAL, false, "asserted", (function() { var _ = Syndicate.__; return Syndicate.Patch.sub(Syndicate.observe(order(_, _, _, _)), 0); }), (function() { var _ = Syndicate.__; return { assertion: Syndicate.observe(order((Syndicate._$("title")), (Syndicate._$("offerPrice")), _, _)), metalevel: 0 }; }), (function(title, offerPrice) {
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/// We cannot sell a book we do not have, and we will not sell for
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/// less than our asking price.
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var askingPrice = this.priceOf(title);
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if ((askingPrice === false) || (offerPrice < askingPrice)) {
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whileRelevantAssert(
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order(title, offerPrice, false, false));
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} else {
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/// But if we can sell it, we do so by allocating an order ID and
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/// replying to the orderer.
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var orderId = this.nextOrderId++;
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Syndicate.Actor.deleteField(this.books, title);
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whileRelevantAssert(
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order(title, offerPrice, orderId, "March 9th"));
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}
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}));
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} }); });
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}
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/// ### Implementation: SPLIT-PROPOSER and book-quote-requestor
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function buyerA() {
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Syndicate.Actor.spawnActor(function() {
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var self = this;
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/// Our actor remembers which books remain on its shopping list, and
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/// tries to buy them one at a time, sharing costs with `buyerB`.
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self.titles = ["Catch 22",
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"Encyclopaedia Brittannica",
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"Candide",
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"The Wind in the Willows"];
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/// JavaScript's callback-oriented blocking means that we express our
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/// loop in almost a tail-recursive style, using helper functions
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/// `buyBooks` and `trySplit`.
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buyBooks();
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function buyBooks() {
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if (self.titles.length === 0) {
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console.log("A has bought everything they wanted!");
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return;
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}
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var title = self.titles.shift();
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/// First, retrieve a quote for the title, and analyze the result.
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(function () { Syndicate.Actor.Facet.build(function () { {
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Syndicate.Actor.Facet.current.onEvent(Syndicate.Actor.PRIORITY_NORMAL, true, "asserted", (function() { var _ = Syndicate.__; return Syndicate.Patch.sub(bookQuote(title, _), 0); }), (function() { var _ = Syndicate.__; return { assertion: bookQuote(title, (Syndicate._$("price"))), metalevel: 0 }; }), (function(price) {
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if (price === false) {
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console.log("A learns that "+title+" is out-of-stock.");
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buyBooks();
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} else {
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console.log("A learns that the price of "+title+
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" is "+price);
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/// Next, repeatedly make split offers to a SPLIT-DISPOSER until
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/// either one is accepted, or the contribution from the
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/// SPLIT-DISPOSER becomes pointlessly small. We start the process by
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/// offering to split the price of the book evenly.
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trySplit(title, price, price / 2);
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}
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}));
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} }); }).call(this);
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}
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function trySplit(title, price, contribution) {
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console.log("A makes an offer to split the price of "+title+
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" contributing "+contribution);
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/// If we are about to offer to split the price, but the other buyer
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/// would contribute less than 10c, then it's not worth bothering; we
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/// may as well buy it ourselves. Another version of the program could
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/// perform the BUYER role here.
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if (contribution > (price - 0.10)) {
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console.log("A gives up on "+title+".");
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buyBooks();
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} else {
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/// Make our proposal, and wait for a response.
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(function () { Syndicate.Actor.Facet.build(function () { {
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Syndicate.Actor.Facet.current.onEvent(Syndicate.Actor.PRIORITY_NORMAL, true, "asserted", (function() { var _ = Syndicate.__; return Syndicate.Patch.sub(splitProposal(title, price, contribution, true), 0); }), (function() { var _ = Syndicate.__; return { assertion: splitProposal(title, price, contribution, true), metalevel: 0 }; }), (function() {
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console.log("A learns that the split-proposal for "+
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title+" was accepted");
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buyBooks();
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}));
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Syndicate.Actor.Facet.current.onEvent(Syndicate.Actor.PRIORITY_NORMAL, true, "asserted", (function() { var _ = Syndicate.__; return Syndicate.Patch.sub(splitProposal(title, price, contribution, false), 0); }), (function() { var _ = Syndicate.__; return { assertion: splitProposal(title, price, contribution, false), metalevel: 0 }; }), (function() {
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console.log("A learns that the split-proposal for "+
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title+" was rejected");
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trySplit(title,
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price,
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contribution + ((price - contribution) / 2));
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}));
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} }); }).call(this);
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}
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}
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});
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}
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/// ### Implementation: SPLIT-DISPOSER and BUYER
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function buyerB() {
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Syndicate.Actor.spawnActor(function() { Syndicate.Actor.Facet.build(function () { {
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/// This actor maintains a record of the amount of money it has left
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/// to spend.
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this.funds = 5.00;
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/// It spends its time waiting for a SPLIT-PROPOSER to offer a
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/// `splitProposal`.
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Syndicate.Actor.Facet.current.onEvent(Syndicate.Actor.PRIORITY_NORMAL, false, "asserted", (function() { var _ = Syndicate.__; return Syndicate.Patch.sub(Syndicate.observe(splitProposal(_,
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_,
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_,
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_)), 0); }), (function() { var _ = Syndicate.__; return { assertion: Syndicate.observe(splitProposal((Syndicate._$("title")),
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(Syndicate._$("price")),
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(Syndicate._$("theirContribution")),
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_)), metalevel: 0 }; }), (function(title, price, theirContribution) {
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var myContribution = price - theirContribution;
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console.log("B is being asked to contribute "+myContribution+
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" toward "+title+" at price "+price);
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/// We may not be able to afford contributing this much.
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if (myContribution > this.funds) {
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console.log("B hasn't enough funds ("+this.funds+
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" remaining)");
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whileRelevantAssert(
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splitProposal(title, price, theirContribution, false));
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} else {
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/// But if we *can* afford it, update our remaining funds and spawn a
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/// small actor to handle the actual purchase now that we have agreed
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/// on a split.
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var remainingFunds = this.funds - myContribution;
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console.log("B accepts the offer, leaving them with "+
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remainingFunds+" remaining funds");
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this.funds = remainingFunds;
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Syndicate.Actor.spawnActor(function() { Syndicate.Actor.Facet.build(function () { {
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/// While waiting for order confirmation, take the opportunity to
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/// signal to our SPLIT-PROPOSER that we accepted their proposal.
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Syndicate.Actor.Facet.current.addAssertion((function() { var _ = Syndicate.__; return Syndicate.Patch.assert(splitProposal(title,
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price,
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theirContribution,
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true), 0); }));
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/// When order confirmation arrives, this purchase is completed.
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Syndicate.Actor.Facet.current.onEvent(Syndicate.Actor.PRIORITY_NORMAL, true, "asserted", (function() { var _ = Syndicate.__; return Syndicate.Patch.sub(order(title, price, _, _), 0); }), (function() { var _ = Syndicate.__; return { assertion: order(title, price, (Syndicate._$("id")), (Syndicate._$("date"))), metalevel: 0 }; }), (function(id, date) {
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console.log("The order for "+title+" has id "+id+
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", and will be delivered on "+date);
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}));
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} }); });
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}
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}));
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} }); });
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}
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/// ### Starting Configuration
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new Syndicate.Ground(function () {
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seller();
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buyerA();
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buyerB();
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}).startStepping();
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